Personally I would split this into two aspects: the tone that the argument is expressed in, and the meat of the argument. People who tell you not to contradict with elders (which I take to mean not just older relatives but also just people in a more senior position) usually only care about the tone of your voice, and the fact that your tone displays respect towards elders. And this is usually good advice: you want to avoid hurting their ego, because they are in a position of power and they can inflict harm and misery on you. On the other hand, once you speak in a sufficiently respectful tone these people don't really mind changing their mind in an argument. They don't like to be confronted and be forced to change their mind; they just like to feel respected while changing their mind. It's an art to speak in a way that protects the fragile ego of the counterparty while still contradicting them.

If the old person is right, you're going to lose what they know when they die. Let them get everything out while they still have time. If it sounds like they're full of shit, which they often are, consider the alternative that you may be an idiot. You don't have to make a final decision between the two possibilities now (only provisional ones), but maybe by the time you're old you'll have figured it all out.

When gray, take both reasoning & conclusions.

When white, just conclusions.

~Taleb

But it seems a little optimistic to call it an asset. Experience can be a powerful asset; but if it isn't backed up by a reasonable argument it is going to drive stupid behaviour. There are far too many cases of destructive behaviour because of what amounts to tradition and senior people who have no interest in optimising behaviour to achieve outcomes.

Regarding the second point, correcting this required decades of experiences where I felt absolutely confident about some technical point, only to find out that I was wrong. This teaches you humility, which is an incredibly valuable asset. I wish I could carry my current brain back in time to my younger self: I would be incredibly productive. You will learn these lessons in the future, and then you'll be sad that you also face aging and loss of that hard-won knowledge.

A big part of the (ongoing?) failure vs the limits of the possible is the apparently non-negotiable instincts we all have to determine truth based on number of believers, good looks, tradition, power, guesswork or status games rather than evidence and good arguments.

In that line of thinking a culture that sees age as a good in itself is at a disadvantage to one that sees age as correlating with valuable things.

ACOUP disagrees:

https://acoup.blog/2022/08/26/collections-why-no-roman-indus...

It is unlikely that the Romans could have had an industrial revolution, given that they were limited by being human and the conditions that caused the revolution in the Europe weren't present in Rome. But there was nothing actually stopping the Romans.

It means both "to treat someone like an authority" and "to treat someone like a human".

Everyone deserves to be treated with the latter sort.

The former sort must be earned.

[0] The context in which I read it was about marginalized people getting the short end of the stick on both counts—privileged people, especially those who are relatively ignorant or for whom that privilege is all they have going for them, have a tendency to express sentiments along the lines of "I won't respect them if they don't respect me!" But what they mean is "I won't treat them like a human if they don't treat me like an authority."

Thanks for underlining my point.

They were addressing someone else. Any chance you shot your load over the wrong target there?

I also was explained a definition of respect that forever changed my relationship to the word and it’s lived embodiment.

The Latin meaning of the word: Re-Spect - to see again. That’s it.

The second definition you gave- to treat someone like a human is way too low a bar. Because one might think that humans are generally POS. Also a person who has a problem with self-respect, does not lack seeing themselves as human per se.

If we elevate it a bit - from treating someone as human, to something like, treating someone with basic dignity, i think that’s an improvement. However it’s still basically passive and allows room for a lot of laziness and prejudice.

This is why i found the Latin meaning so helpful. Give that person a 2nd look. With each interaction try to see them again and again.

It’s much more active, it improves others in our eyes over time (usually, but not always). And I find it improves me over time, as i continue to remember to use my sight.

This definition helped me capture the more elevated aspect of respect, the one i felt the word already connotes, but had such a vague definition. A high virtue indeed.

1. I don't care what you think is right or wrong; you need to do what I say

2. I don't care if you do what I say or not, but if you do not you will be doing the wrong thing

call them political and scientific authority for short. the other interesting thing is that religious authority tries to combine both forms into one.

A common example is a speed limit. It aught to be an engineered number, but can be determined politically. Regardless, the authority enforcement is the same regardless of the basis of the law.

It's more like there's two different basis for laws and only one body enforcing both laws, which makes it awkward.

now looking at the religious case, a priest would say "this is what you need to do because the gods said so; I can enforce that because I'm an authority in the church, but also I am right because I'm an authority on what the gods want". so that combines both uses of the word into one person in the way that more secular authorities mostly do not.

These sort of abstract truisms are things I believe in, but there's so much value in just not being annoying to people around you. So many arguments short circuited by having a "default" level of respect for the person on the other side.

Even in older age, many have fragile egos and may throw their weight around to punish you just for the insolence of contradicting them. It's worth knowing what's at stake, what's the upside. It's not enough to be right. That applies not only to subverting authority, but also popular conventional wisdom, as beautifully exemplified in anecdotes from the book the Scout Mindset.

If you're talking to those who value the pursuit of truth then not being a dick can be sufficiently disarming.

Regardless of tone, leaders being corrected in front of others carries an undercurrent of antagonism, and can develop into outright hostility. This happens with leaders of animals and alphas as well.

Challenging their assertions is a step away from challenging their leadership and authority, especially if you do so repeatedly.

Leaders are often hard to get ahold of privately, because of how many people they deal with, and usually sycophants and well-connected supporters are prioritized for these meetings rather than someone who disagrees on substance

The best you’re gonna get is if you’re a sponsor or a fellow leader who offers them to come on your show, or you come on theirs, and debate.

No matter how respectful your tone is, you can’t challenge them unless they’ve agreed to debate: https://youtu.be/Mwe91SXEyaM?si=sI8dzC9YTQzAPbmu

I think that Bob should understand that while the senior engineer may be incorrect, it's best to ask questions and seek to understand first. It's more likely that Bob is missing something. Even if not, this would come out in a better way through polite questions than direct contradiction.

Life is like this too. It's a skill that we should be improving over time.

People who have been doing it longer that us probably have insight that we don't. I think it's arrogant to assume otherwise.

Avoiding contradiction shows respect for the experience and knowledge of the other person, while directly contradicting someone encourages conflict and argument.

Whenever a conversation turns into an argument, both people lose. You've caused conflict that can have significant consequences.

This was a costly lesson for me to learn.

Manners are the lubricant of civilization. Without them the whole machine fails.

But what I think was great about my parents was that we were encouraged to find out the answers to any questions we had.

It was pre-internet time, so it basically was looking something up on a dictionary or on the Encyclopedia Brittanica (a Spanish language version my parents still have today), many times it was rather hard to figure something out.

No question left unanswered, at least as best as we could.

Even if it meant that my dad went out and bought me a book about basic electricity (Ohms law and such) just to give to me because I wanted to know something.

That care about knowledge and logic (my mom was great at logical arguments and she knows a lot of crap, seriously) helped me succeed.

I hope I'm doing as good a job with my kids (ideally even better).

Raising people is weird and can be so fulfilling.

It was probably easier raising an inquisitive kid in such an environment when you would refer to books and issues of the day, I'd imagine myself today going down internet rabbitholes and thinking myself an expert on a new thing every week. Which would probably often resort to the parent's exhaustion and just resorting to "that's great, son".

That belief implies that:

1) All relevant knowledge about something can be expressed as arguments.

2) There is a canonical way to interpret arguments

Most of the important things worth arguing about cannot be expressed a pure logical syllogisms. Assumptions and relative weights are very subjective.

There is also the existence of sophistry, where logic is used not to enlighten but to hide.

For example, I may have some disagreements with Anders Hejlsberg about language design. Even if I present a better argument about something, you would probably be better off taking into consideration Anders' view over mine.

3) Children never argue in bad faith (e.g. as a hourslong stalling tactic at bedtime)

4) Children never argue during time sensitive disagreements (e.g. why do I have to get out of the street!?)

5) Children understand probability and risk-benefit analysis (e.g. why shouldn't I play the knife game, I'm not going to cut myself and it's fun)

I've also discovered leaving everything open to debate and having to convince kids of everything leaves them unprepared for life around authority figures. It is not appropriate to argue with an on-duty lifeguard about basic water safety, but a kid who has never had to follow a single command or order will try.

(Not making this assertion out of some "respect your elders" orthodoxy but out of empirical observations of the typical knowledge level of 10 year olds.)

> Let the wookie win

To Friedman, I suppose, his childhood self is simply a rational actor who is arguing from first principles - as all children of rationalists and libertarians clearly would!

The possibility that "maybe I was an irrational child (like every other child) and my parents simply gave up trying to correct me because I was a terror" doesn't appear to cross Friedman's mind, which is incredibly amusing to me.

I've intuitively believed this to be true for a long time. But the article made me think about it, and the textbook is quite wrong.

It's a test similar to if you're in a train in motion, and you drop a ball in the aisle, does it fall towards the front or the back of the train?

The momentum vectors cancel out, but the relative velocity is identical and the total kinetic energy is the same.

The damage isn't the same because car/car has twice the crumple zone. More of the energy will be dissipated through elastic deformation, so the deceleration will be slightly gentler. (Although almost certainly still fatal.)

That aside - the driver's ed book is basically saying that head-ons are much more dangerous and deadly than most people understand, even at low speeds. All of which is true.

To see this, you can imagine the car/car case as having an invisible wall between them. The wall has no net force so it doesn't move and acts the same as the brick wall despite not being either rigid, massive, or fixed to the ground.

Then assuming it requires X energy to crumple a car and assuming the wall is strong enough to not yield at all (so no work is done on it), it requires 2X energy to crumple 2 cars, so there's X less energy available to destroy the passenger in the 2 car scenario vs the wall. So the 2 car scenario is slightly better, but probably not meaningfully so (X will be much less than the kinetic energy of a 100 mph car).

The correct symmetry principles to invoke here are conservation of energy and invariance of Newtonian physics under Galilean transformations.

Do you know how crazy that is? the collision doesn't dissipate more or less kinetic energy depending on the reference plane you pick.

If you pick a 100/0 reference frame, the combined cars are moving 50 in one direction after. Therefore, they both only expereince a 50mph change. one from 100 to 50, the other from 0 to -50.

your frame doesnt change the collision energy. If that was the case, you could pick a 100,000mph reference frame of an astronaut flying by, and when the cars collide there would be an atomic explosion.

to put this another way, the energy of the collision comes from the deltaV before and after. The deltaV is the same in EVERY reference frame you can construct.

50/50 is not the same as 100/0 between car and an ideal wall: an immovable barrier of practically infinite inertial mass that suffers no damage.

car/car 50/50 is the same as car/ideal-barrier 50.

Is it though? Genuinely wondering if I'm missing something in my ponderings...

I would assume that a collision between two identical cars traveling at the same velocity would each decelerate to zero within the distance of one crumple zone. ie due to the other one also decelerating the same way. Assuming that they don't end up in some sort of intermingled mashup (spherical cows and all that) which is maybe what you're describing?

And that a car at the same speed hitting a practically immovable object would also decelerate the same amount in the same single crumple zone distance.

Anyway as an irrelevant aside, hitting a brick wall isn't the same as hitting an immovable object - a car will mostly go right through a fairly standard unreinforced brick wall. Which unless a brick coming through the windscreen hits you directly, is probably going to be a lot safer than the deceleration of hitting an oncoming car or immovable object.

Imagine a car hitting a car with the wheels magically glued to the road at 100 km/h.

But if the car just busts straight through a wall of equal mass (and a comparable crumple zone), then it will be just like the car/car case energetically: the car and wall both continue at 50 mph following the collision.

And yeah "immovable" isn't literal, just shorthand for practically immovable compared to the car.

Together, they have the energy of two cars at 50 each hitting a wall, and this energy is split between the two cars.

If somehow ALL of the kinetic energy was absorbed only by one car , it still wouldnt be equal to 100mph, do the the square.

Together, two cars traveling at 50mph have the same energy as one car traveling at 70mph.

The only case when two objects colliding at 50mph is like 100mph is if the final reference frame is still traveling at 50mph. a Car going 50pmh hitting a train going 50mph would be like the car hitting a wall at 100mph (net velocity change of 100mph.

The mass of the wall is required to determine which system has more energy.

Or, take it from the perspective of car B or the wall. In both cases, car A is approaching with a relative speed of 100mph.

It goes from sitting still to going backwards at 50pmh, for a net change of 50mph.

The difference of velocity between cars 100mph in every reference frame you can pick. Each of the cars always experience a change of 50mph from the collision, no matter what reference frame you pick.

People are confused because the pick a reference frame for car A, but don't hold it constant for their calculations, as required by physics.

The wall approaches at 100mph and, in a totally elastic collision, it recedes at 100mph, as does car B.

>No, if you’re taking car A as a reference point it behaves as if stationary before and after collision - it’s everything else that moves.

If that were true, I could set a reference plan as a bullet in a gun, and when I fired it everyone in the world dies from rapid acceleration as their kinetic energy goes from 0 to 3000mph.

you change 50 if you hit a unmoving wall.

IF you start at 0 or 100mph, you also change 50.

Each car approaches the impact point at 50mph, each car recedes at 50 in the opposite direction after a perfect collision, so each has a dv of 100.

And if my initial reference frame is car A, travelling at 50, then before impact car A appears to be travelling at 0 while car A is approaching at 100. After collision both cars experience a dv of 100, so car A appears to be receding at 100mph while B appears stationary.

For the wall, the car approaches at 100 and recedes at 100 for a delta of 200mph. But in any case the velocity delta doesn’t affect the energy, it just has to be conserved.

As I’ve said elsewhere we are trying to find the collision energy in the moment of impact from the perspective of car A (the question is whether you driving into a wall the same as you driving into another opposite car at half the speed). For that you have to consider a frame of reference relevant to car A, and not a stationary reference that is not following either car.

I believe the correct approach is to look at it from the perspective of the object that you in car A are striking. Taken from the perspective of the wall, car A strikes it at 100mph. From the perspective of car B, car A also strikes it with the same speed (and kinetic energy).

IF the two cars have equal mass, this will also be the impact energy on car A. And that’s what we’re trying to find for both cases.

The most convenient reference frame for colliding objects is the combined center of mass of those objects. It isn't affected by the collision, and for perfectly inelastic collisions the objects will come to rest with respect to that frame. This makes the math a lot simpler that working in some other frame.

Twice the crumple zone divided by two cars is the same as one car and one crumple zone.

The parked car will move away, providing cushion.

When you have two cars in opposite directions, they cant both give way for each other.

The trampoline doesn't deflect, and acts exactly like a concrete wall.

The trampoline cant simultaneously deflect in opposite directions.

Same amount of momentum, two collisions. In one, there is one crumple zone, we will assume the brick wall is completely undamaged, for the sake of simplicity. In the other, there are two crumple zones.

All of the crumpling is dissipated energy. What doesn't get crumpled, goes into the driver's face.

Clearly, the situation with two crumple zones is superior. There's a limit to how much energy can be absorbed by crumpling. With two crumple zones, that limit is 2x, with one, 1x.

Therefore, damage to the driver or drivers is much worse in the brick wall scenario.

For another analogy, picture a go-kart, with a half-meter block of styrofoam, hitting a brick wall. Now, picture the same go-kart, hitting at the same speed, but this time, there's a meter of styrofoam.

That's the one-crumple and two-crumple scenarios.

If you're standing next to the stationary brick wall, it falls straight down relative to the wall.

To me this is a clear demonstration that motion is relative, therefore the two collision scenarios ARE equivalent.

Imagine that the two cars are in space. Are they both moving at 50MPH towards each other? Or is one stationary and one moving at 100MPH, like the wall scenario? Or perhaps moving the same direction, but one at 100MPH and the other at 200MPH, about to catch up to the first one? The answer is that it doesn't matter, because the only difference between these scenarios is the reference frame. The collision is the same in all of them.

But I maintain that whatever the mathematical procedure, it is absolutely impossible for the stationary-ness of one of the objects to affect the outcome of the collision, assuming we're ignoring things like static friction, air resistance, etc. This is a simple matter of reference frames. If you disagree, put the two objects in space as I suggested and tell me which one is "stationary" and how that could possibly affect the outcome of the collision.

And if you think about it further, the difference is not that you're choosing one object as still and one as moving but that in the car vs. car case, the second car takes on some of the energy from the collision, whereas the wall does not. This makes some intuitive sense if you imagine this collision happening. Forget all the crumple zone stuff - a car hitting a stationary car is going to make that car move - and if you ignore friction and assume the cars stick together the two cars will be moving in the same direction as the initially moving car, just slower. This means not all the initial kinetic energy from the moving car was "used up" in the collision.

[1] https://en.wikipedia.org/wiki/Inertial_frame_of_reference

For a truly immovable wall, the ground has to exert a lot of force to keep it in place, which goes into the car, causing the entire discrepancy.

You're right that if you match your reference frame to one car, that car becomes "stationary" until the collision, but after the collision it is moving relative to the reference frame. But that's the key--it now has kinetic energy relative to the reference frame, but not to the other car. As far as the pair of cars is concerned, all of the kinetic energy between them has been expended, because they are now moving together.

Of course this is assuming a scenario where the cars crumple into a single mass, i.e. they don't "bounce". I think part of the confusion in these comments stems from the problem being poorly defined. Some people are considering crumple zones, others are considering whether a brick wall is movable and still others replace it with another car for simplicity.

I do think the driver's ed manual is correct in some Spherical Cow sense, but I don't think its authors ever intended for the problem to stand up to Hacker News scrutiny. It's quite funny to me how innocuous things like this can generate these "unladen swallow"-type discussions on here, even though I'm participating.

I think people dont realize they are changing their reference frames, and getting insane results like the amount of kinnetic energy release in the collision depend on what frame you pick.

two crumple zones don't matter for head on collision, which is another thing that trips people up.

As per your "stationary" objections, the issue is that this is modeled as a very sturdy wall anchored to the Earth. Yes, the Earth will "recoil" some, but for all intents and purposes it can be modelled as completely stationary because its mass is so much greater than the car.

> the issue is that this is modeled as a very sturdy wall anchored to the Earth.

That's not what you modeled in the comment you linked. Your math (incorrectly) attempts to describe a car colliding with another car of equal mass. But if you're considering a "very sturdy wall anchored to the earth", then you effectively have a car colliding with the earth.

Either one is fine with me, but pick one. It grinds my gears when you authoritatively call me "absolutely incorrect", "correct" me with an incorrect answer of your own, and then try to claim you were modeling something entirely different all along.

The link I posted, https://news.ycombinator.com/item?id=40628932, clearly showed the kinetic energy math for both a car hitting a stationary wall at speed 2X, and 2 cars hitting each other each with speed X. This is basic high school math of inelastic collisions.

Imagine compressing two springs in a line vs one against a wall.

If the wall is immovable and perfectly fixed to the ground, then the car/wall collision will indeed have a greater effect than the car/car collision.

But if the wall's connection to the ground is negligibly weak (e.g., an actual brick wall), then after the collision, both the car wreckage and destroyed wall will continue forwards at 50 mph, until some other force slows them down. In this case, the effect (change in kinetic energy) is the same as the car/car collision, not counting the impact of crumple zones.

In the former case, the kinetic energy of the car relative to the ground is relevant only because the ground is exerting a massive force to keep the wall in place.

In the Newtonian inertial frame of the "stationary" Earth, it accelerates downwards at 9.81m/s² until it hits the floor and comes to a stop and is no longer accelerating ("gravity", which is a thing here, and the normal force now summing to zero).

This is fairly academic for sporting goods, as if working with these two frames results in substantially different outcomes for you or your ball, your day is probably going badly, or at least fairly energetically, and baseball isn't your priority right now.

https://www.youtube.com/watch?v=r8E5dUnLmh4

But yes, the textbook is wrong. Unless the brick wall (and only the wall) is 100% inelastic so it "catches" the 100 MPH car, slowing it to 0. Quite unrealistic.

If the kinetic energy for a car at 50 miles per hour is 1x, at 100 miles per hour, it’s 4x. After the collision, the total kinetic energy is 2x, and the other 2x was absorbed by damaging both cars. That’s different from hitting a wall where all the kinetic energy goes into damaging the car.

The damage from hitting the parked car matches the head-on collision, which starts with 2x kinetic energy and afterwards, it’s zero. (If they are stopped afterwards.)

If the cars were rigid, they would bounce off and go in opposite directions, like balls on a pool table. That would be very bad for anyone inside. Crumple zones absorb energy for good reasons.

I expect that guard rails absorb more energy than a brick wall?

Once people are done with that they might be amused by try this one:

• You are on a stationary bus, holding a helium balloon on a string. The balloon is floating directly above you. The bus starts accelerating forward. Does the balloon:

1. Move toward the front of the bus relative to you,

2. Continue to float directly above you, or

3. Move toward the back of the bus relative to you.

Alternatively, imagine one car is parked (in neutral, with its brake off) and the other car hits it at 100. The center of mass of the two cars is moving at 50 both before and after the collision (conservation of momentum); after it, the cars will be moving at that average speed. The impact will again be equivalent to the original scenario.

The original claim probably results from a conflation of these two scenarios.

ETA: So what student drivers should be told is that hitting another car head-on is like hitting a brick wall at the same speed. For this to be exactly true, the momenta (mass * velocity) of the two vehicles have to be equal and opposite, but to communicate the general idea, I don't think we have to go into that.

Assume two perfectly elastic cars. When they collide at 50mph, each car will bounce backwards at 50mph (due to conservation of momentum), representing a change in velocity of 100mph — identical to the brick wall case.

I feel that introducing deformation or other energy dissipation to the equation kind of takes it out of the “high school physics” realm, right? What else am I missing?

Edit: ah I see, the car will bounce off the brick wall at 100mph as well, resulting in a 200mph change in velocity. I guess you could explain it then that the effect of the impact is felt entirely in one car in the brick wall case, and it’s spread out over two cars in the head-on case?

This is the incorrect part. They would both go to zero velocity/momentum.

Momentum is a vector quantity, so has a direction and magnitude. Two identical cars with the same speed going opposite directions would have the same magnitude of momentum, but opposite sign. After colliding, their sum would be zero.

If you watch billiards you would see kinda the same thing going on.

Edit: completely messed this up. Other comments are more correct

In a perfectly elastic collision, both kinetic energy and momentum are conserved. In a perfectly inelastic collision, kinetic energy is not conserved (because it is converted to heat), but momentum is always conserved.

So lets say you have 2 objects of the same mass traveling toward each other at the same speed. In a perfectly elastic collision, the balls objects will "bounce" off each other, going back in the opposite directions. In that case momentum is conserved (as you note, it's a vectored metric, so before and after the the total momentum of the system is 0), but so is kinetic energy, because you still have 2 masses traveling at the same speeds (think about if you have a Newton's cradle and pull both end balls up and drop them at the same time - they'll both bounce back).

In a perfectly inelastic collision, both masses will essentially crush and come to a complete stop where they collide. Again, momentum is conserved (it's still 0 before and after the collision) but kinetic energy is not conserved because it's all converted to heat of the 2 objects.

Thanks for the correction.

(Also, how does a “throwaway” account get that much karma??)

I don't think so. In my high school physics class I learned about both fully elastic and fully inelastic collisions. The math basically works out the same in the fully inelastic collision case, which I did here, https://news.ycombinator.com/item?id=40628932

Perfectly elastic objects bounce apart with mirrored velocities. No energy is lost.

Perfectly inelastic objects just stop. All of the energy is dissipated through noise, heat, and deformation.

Momentum is conserved in both.

For drivers ed, cars are almost perfectly inelastic.

With two cars hitting head on, the energy to be dissipated is twice as much, but it is equally distributed to each car, and so the energy dissipated in each car is the same as if each independently hit a brick wall.

In other words, conservation of energy.

Assuming the cars are of equal mass. If not, the heavier one will drive the other car backwards faster, and the energy will not be divided equally between them. But the sum will remain the same.

I think it's simpler just to directly look at the equation for kinetic energy: KE = 1/2mv^2. In other words, kinetic energy is directly proportional to the mass, and proportional to the square of the velocity. Also, let's assume that in both cases the car comes to a complete stop at the wall, or both cars come to a complete stop where they collide (a perfectly inelastic collision), so that all of that kinetic energy is essentially released as heat (i.e. the cars crush).

In the case of 2 cars hitting each other, if the cars both have equal mass X and are traveling at velocity Y, then the total kinetic energy dissipated in the collision is 2 * 1/2XY^2, but then of course for each car it's just 1/2XY^2.

In the case of a car hitting a wall but traveling twice as fast, the kinetic energy dissipated in the collision is 1/2X(2Y)^2, or 2XY^2 - so that's essentially 4 times what the car experienced in the head on collision (of course in the real world the wall would dissipate some of the heat, but in this "spherical cow" example I think it's OK to assume the vast majority of the crushing occurs in the car).

It's this assumption that causes the entire discrepancy. For a wall to be truly immovable, the ground must exert a great force into the wall to keep it in place, which goes into the car.

But if the wall isn't anchored very securely (or backed by the ground), then the car will bust through it and continue moving forward. In the limiting case where the wall isn't attached to the ground at all, both the car and wall move at 50 mph following the collision. The dissipated kinetic energy comes out the same as in the car/car case.

If we consider the force experienced by a passenger in any car, then you also have to take into account the crumple zone (larger in modern cars). Here we imagine the cars have perfectly inelastic collisions coming to a complete rest as they crumple, first calculating the KE in each scenario:

100 mph (44 m/s) into a brick wall -> KE = 1/2 m (44)^2 = m(968)

2 50 mph (22 m/s) into each other -> KE = 2 * 1/2 m (22)^2 = m(484)

Thus the car in the brick wall scenario has to absorb twice the energy of the overall collision in the two-car scenario, in which each car only has to absorb 1/4 of the brick wall total KE (assuming the brick wall absorbs nothing, maybe a giant granite boulder is a better model). Thus in modern cars it's not just the KE but the crumple zone that reduces the force felt by the two-car passengers.

(Incidentally I used Chat GPT4 to help sort this out and it tried to claim that (x/2)^2 == (x^2)/2 so use with care - it did catch the error but only when I said double check that calculation in a step-by-step manner. User beware!)

Imagine the two cars hitting each other in a (unrealistically) symmetric, mirror image way. Where they hit is a plane that neither car crosses. That is your immovable wall, it’s just both cars experience it.

This is an odd assumption.

If someone asked me what kind of wall cars are most likely to run into at 100 mph, I'd have assumed something like a freeway barrier wall that are fully anchored to the ground or the reinforced concrete walls they use for crash tests - something with a mass substantially greater than a vehicle.

Some people in this thread think that you can take two cars and double the kinetic energy acting on both of them. If this were true, you could create infinite energy by repeatedly colliding cars.

Perhaps it isn't surprising that people who think this is true also disagree with his economics.

https://news.ycombinator.com/item?id=40631089

Although I still think Chicago school/libertarian economics is "confidently wrong".

Of course I don't expect anyone to refute an entire school of thought in an HN post.

He definitely isn’t

https://news.ycombinator.com/item?id=40631089

Kinetic energy scales as velocity squared, so a car at 100 MPH has 4x the energy of a car at 50 MPH. But in the 50 MPH scenario there are two cars, so the total energy dissipated in the 50 MPH head-on collision is half that of the 100 MPH brick-wall collision. In the brick-wall case, presumably all the energy is available to demolish the one car, but in the head-on case, the energy is spread out demolishing two cars.

Impact force is maybe trickier, it depends on the acceleration, but if the two 50 MPH cars end up with zero momentum, then they have 50 MPH of delta-v each over some collision time. The single car of course has 100 MPH of delta-v. If the collision times are the same (arguably a reasonable approximation if the head-on is a highly symmetric), then the impact force in the head-on case is half that of the brick-wall case.

Maybe half the multiple choice questions were phrased to trick you into giving a wrong answer that broke newtons 3rd law.

But I think the reason nobody likes economists isn’t because they have unpopular opinions. It’s because, despite being wrong almost constantly, they have no humility at all. I’ve never read anything by an economist that didn’t absolutely drip with ego and posturing and this piece is not different at all.

That would only be the case if Friedman, even after getting his PhD in physics, still continued to disagree with that teacher and that textbook.

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Each car crumples against the dotted line, not passing through it at all.

(In our idealized example with a perfectly mirror-like collision in which both cars crumple identically, it behaves like an ideal brick wall: the worst kind.)

Hitting an ideal brick wall at 50 is far less severe than doing so at 100.

If the collision is perfectly inelastic, then in the two car case, the energy dissipated is 2mv² where v is 50 mph. In in the one car case where it is going double (2v) it the energy is m(2v)² which is 4mv². We assume the brick wall has no kinetic energy to contribute. We also assume the brick wall is not damaged so the 4mv² is absorbed by one car. Whereas in the two car case 2mv² energy is absorbed by two cars, so mv² each.

Thus, from the point of view of one car, the 100 mph brick wall collision is 4 times more energetic than the 50 mph head-on collision. From that alone you can intuit that head-on is same as brick wall, since the damage scales 4 times with a doubling in velocity.

(Now yes, hitting an identical parked car at 100 mph would me more similar to the head-on collision. A parked car is not a brick wall. It will move in the direction of the collision in addition to crumpling. Books for drivers should probably not talk about either brick walls or parked cars, but just convey that kinetic energy follows a square law, quadrupling when speed doubles.)

Since in a head on, they are both changing speed then the deceleration must be half the combined relative deceleration.

The way I was imagining it was as if the car collides with another car going 50mph in a head on collision and the other car doesn't slow down due to the collision.

So it appears that it is I who am confidently wrong about physics (although I still hold that Milton Friedman is confidently wrong about economics!).

Two approximately round space rocks of exactly equal size and density have a center of mass that is halfway between them. If one is moving 40 mph (according to the observer's frame of reference) and the other is catching up to it in exactly the same direction at 50 mph, then what it means is that the center of mass between the two rocks is moving at 45 mph. If the rocks collide in such a way that they clump together (inelastic collision) and no pieces fly off, the combined double rock will thereafter be moving at 45 mph.

Vehicle collisions are more complicated because of cars skidding and rolling. In the milliseconds after the main impact though, those effects don't matter that much, I think. E.g. if you hit a parked car that is in neutral with no parking brake, versus one that has its transmission and brakes locked down, and this happens at 100 mph, I suspect those factors hardly make a difference.

Ummmm ... what? I mean, sure if you assume that the dynamics of a brick wall and a car are different, then they won't be identical but if you just take a parked car and a brick wall to both be "immovable objects at rest" which for all intents and purposes is true for the driver, then I don't see how this statement could possibly be true.

If you have 1m of hood to work with, you're decelerating 100km/h to 0km/h over 1m in the wall case.

In the hood-to-hood case, depending on reference frame, you're either decelerating 50km/h to 0km/h over 1m or 100km/h to 0km/h over 2m (both hoods).

Either way it should be trivially obvious that running into a wall is going to be a much more violent experience. Likely the wall crumpling up the front of your car won't be enough to bring you to a stop before you personally impact something.

Then you also have the force exerted by their car attempting to accelerate you backwards.

For each car, there are 2 forces: the force that you exert due to your trying to accelerate them in your direction of travel and the opposing force.

What you're saying is the same as saying that if you fall from the roof of an elevator at rest, the effect is the same as if you fall from the roof of an elevator that travels upwards at the same time as you fall. That's obviously not true, but the physics you need to prove it are F = ma and Newton's 3rd law.

For Car A:

- Force of Car B accelerating Car A opposite to direction of travel

- Force of Car B opposing force of Car A trying to accelerate Car B

And vice versa for Car B.

EDIT: Like if you hit a wall, the wall feels the force of your car. The force that crushes the car is the force opposing that, which is the wall acting on the car.

EDIT2 (because I can't reply): the force acting on the driver will be the sum of the force of Car B acting on Car A opposing the force of Car A acting on Car B plus the force of Car B acting on Car A. There are 2 forces in each direction of travel that are exerted by the cars. The force felt by the driver is the sum of these 2 forces, exerted by the dashboard on the driver's face. If the car hits a brick wall, the only force opposing the direction of travel of the car is the force of the wall acting on the car, which opposes the force of the car acting on the wall. As such the force felt by the driver of the dashboard acting on their face is halved.

EDIT3 (still because I can’t reply): the dynamics of the materials is not the issue. Yes bricks behave differently than cars, but that’s not the basis of the author’s argument.

> EDIT2 (because I can't reply): the force acting on the driver will be the sum of the force of Car B acting on Car A opposing the force of Car A acting on Car B plus the force of Car B acting on Car A. There are 2 forces in each direction of travel that are exerted by the cars. The force felt by the driver is the sum of these 2 forces, exerted by the dashboard on the driver's face. If the car hits a brick wall, the only force opposing the direction of travel of the car is the force of the wall acting on the car, which opposes the force of the car acting on the wall. As such the force felt by the driver of the dashboard acting on their face is halved.

If this was how it worked, crumple zones would do nothing. Next you're going to model the airbag as a rigid body too? Ouch.

Correct me if I'm wrong, but I think you've added non existent extra forces.

In the car/car situation, there are still only two forces - each the reaction of the other depending on which cars point of view you're taking. There aren't two separate forces to sum.

Each car experiences a decelerating force - the reaction of which is the decelerating force for the other car and the reaction of that is the original deceleration force of the original car.

If the wall is also moving towards the car then the force felt by the car is the sum of the reaction force it would have felt if the wall were stationary plus the force due to the wall trying to accelerate the car in the opposite direction.

In the author’s example is says that 2 cars crashing into each other is the equivalent of two cars crashing into a wall, and it’s false.

A parked car will move when struck. The combination of the two cars continues to have kinetic energy, which is lost by friction.

https://news.ycombinator.com/item?id=40629971

It is entirely unassailable; all you can argue with is the realism of my assumptions. (Real head-on collisions won't be precise mirror images.)

When two identical objects are moving toward each other with a relative speed v, the collision is like hitting an ideal barrier at speed v/2.

Okay yep I get it now. The 4mv2 did it for me mathematically but I still couldn't get it intuitively until I imagined a car going 50mph rear ending a car going 40mph. If they wind up going the same speed as a result of the collision then the deceleration felt by the car in the back would only be 10mph if the car in front didn't change speed because it was like, an ocean liner. If the cars are the same size and the collision is perfect and all that then the car in front would gain 5mph and the car behind would lose 5mph.

Since in a head on, they are both changing speed then the deceleration must be half the combined relative deceleration.

The way I was imagining it was as if the car collides with another car going 50mph in a head on collision and the other car doesn't slow down due to the collision.

So it appears that it is I who am confidently wrong about physics (although I still hold that Milton Friedman is confidently wrong about economics!).

As a driver I'd rather run into another car than a brick wall. No amount of simplified textbook physics is going to convince me otherwise.

On a related school anecdote: A test problem once asked us to calculate the length/angle a ladder would have to be extended from a firefighting vehicle to reach a window, given height of the window and distance of the firefighting vehicle to the house. I had about twenty minutes left and was a bit of a cheeky student, so I explained how I could not answer that question since I did not know the height at which the ladder originates from the vehicle. That got me full points since I had a great teacher.

That said, you have to be a bit of a jackass to intentionally miss the point in either case.

Smarts kids probably often have resentment when they first discover their teachers/elders whom they hold up to a naturally realistic high regard are less inquisitive or intellectually rigorous than they are (or hope to be). That sort of thing has a way of sticking in your brain.

(An "attendee" is someone being attended, e.g., King Charles is attended by his attendants — they are the attenders, he is the attendee.)

</rant>

This is very emblematic of the sort of thinking you also get from other modernist Enlightenment advocates like Steven Pinker or 20th century positivists but it's simply an extremely cartoonish position. For no good reason it assumes that everything worth doing is legible to individuals, even children in this case, and if you can't present an explicit "rational" argument for it, there's no good reason for it to be around.

Of course the fundamental problem is that there's no reason to believe that everything of value in the world is legible, in particular to individuals, and throwing long traditions away just because you can't defend them at any given point doesn't mean that in them isn't encoded some institutional knowledge that nobody can directly articulate. If an entire community acted like this household of eccentric libertarian economists it'd look like an insane asylum.

In many ways a good analogy for what's wrong with this worldview is the whole old vs new AI debate where you had the Chomsky rationalist, let's explicitly construct all knowledge camp vs the learning approaches. The philosophical insight for why the latter won out is because most valuable knowledge in the world can't be articulated, it's just embedded in practices.

Learning incrementally is not the same as following an unjustifiable tradition, nor is it in opposition to wanting to provide arguments based on what we know now. A tradition is in opposition to learning incrementally, because when it is wrong there will be unjustifiable resistance to correction.

I mean Milton Friedman (the author's father) was quite influential in the late 20th century, and arguably his ideas are responsible for much of the insanity of modern capitalism.

Believing this statement certainly makes one more libertarian. I wonder how many people have internally locus'd their way into their dad's business, or out of a cancer diagnosis.

Social mobility is not the same thing. Internal-vs-external locus of control is probably pretty correlated between parents and kids for reasons of both nature and nurture. The point is that what successful parents pass down to their kids is not cash, but attitudes that are highly adaptive to making fat stacks.

What I said was: get a job at dad's business. Some similar situational advantages: buy parent's not-a-lemon old car at a sweetheart rate, get a little help with tuition, live for a few years low rent or no rent with family, get a job at a friend of the family's business, get a referral from a connected family member or friend of the family, grow up in a neighborhood with lots of green space, play sports and hang out with kids whose parents are comparatively rich, be white and have had orthodontics and health care for your childhood, have the time and energy to do extracurriculars as a teenager, have a decent school music program, or a good local library, take time to do a "travel year", have the security to know if you fail at a business, you can 'just move home', etc.

The point here is, the list of "legs up" some people get that they don't necessarily recognize, let alone control, is very, very long, and every one of these is a mild additive push toward relative life success.

Having an internal locus of control is certainly beneficial in dealing with the things life throws at you and in seizing opportunities, but I contend that one's surface area of luck is not made up solely or even mostly by attitude.

I didn't even list any of the negative things that might totally derail an otherwise focused, high potential kid above, and that list is probably much longer.

The profession breakdown gives a clue: teachers being highly represented suggests the population is largely people at retirement age who had stable employment and invested in tax advantaged retirement accounts for their entire adulthood.

Being set up for carefree old age is the American dream, but it’s not what we imagine as wealthy.

I have no idea why economists are so guilty of thinking that their layman knowledge of various topics is somehow better than real experts.

> This would be a serious problem if we were facing rapid change, but we are not.

My understanding is that, geologically speaking, climate change is happening extremely quickly. But he goes on to say,

> Other species can [adapt] by evolution or by changing their range, but that could be a problem for species such as trees that evolve slowly and shift their range slowly

So he incorrectly claims that climate change is “slow” because it is happening over a period of decades or even centuries, and he does not seem to understand evolution at all, which I understand happens on a timescale several orders of magnitude longer.

What a pile of easily refutable nonsense, spewed out as fact. It’s just the usual blather, someone influential who thinks they understand everything, failing to accept even the basic, provable tenets of something inconvenient for their personal world view.

If that’s the quality of the “arguments” he had with his parents, little wonder we’re all fucked.

Current climate change is slow by human experience, but rapid by geologic terms. There is almost no precedent in the geological record for climate change happening at the current rate outside of mass extinction events.

When you are dealing with risk management for something that can affect billions of lives and manny trillions of dollars in assets the tolerance for risk must be very low. Policy makers are currently exposing us to far too much risk for very little upside. There has been a lot of collaboration between climate scientists and economists to map this out the last decade or so.

Really what is happening now is an unplanned, out of control experiment with the thermal and chemical properties of our atmosphere and oceans. The burden of proof lies with those who wish to continue the mad scientist experiment that is greenhouse gas emissions to prove it's safe. Not on the rest of us to prove it's unsafe. Despite the fact that it has been pretty well proven to be unsafe.

So it's not so much a question of ideology, as a question of where the money is coming from ("Cui bono?").

In those pre-internet times, it also surfaced a problem with that environment: a sibling who vehemently averred to facts they did not have. They have since matured to be sufficiently tethered to reality to be an excellent parent and employee, but now cherry pick the internet for justification of social policy preferences.

I am not sure the author isn't conflating "libertarian economists" with "good parents".

- A supreme allergy to competition law.

- Externality denialism as a nervous tic.

As an example of the latter:

>When climate change replaced population growth as the looming catastrophe that all of the authorities insisted something should be done about, I took the same approach to that issue and reached the same conclusion.

I'll give this guy the benefit of the doubt solely because overpopulation was, as far as I can tell, veiled Nazi shit. On the other hand, the argument against climate change here boils down to "well humans will adapt", which... I mean... technically yes?

The biggest adaptation is going to be people moving out of their home countries, which isn't an option for the vast majority of humanity. Immigration is not recognized by most countries as a right. If we bake the Middle East or Africa and it becomes unlivably hot, the answer countries are going to choose isn't going to be "admit hundreds of millions of climate refugees". I'm absolutely one of those "migration is a human right" people, and if David Friedman is anything close to consistent, he'll be too. But most governments are run by people who vehemently disagree with that and you're not going to sell people on open borders as an alternative to climate change mitigation.

Also, some of the actual arguments this guy uses are just... oh my god:

- Sea level rise isn't a problem because most land will not be flooded. Land ISN'T FUNGIBLE, improvements on land CAN'T BE MOVED, and the people getting flooded can't sell their homes to Aquaman. As an economist, it's your literal job to know this.

- Warming is Good Actually because more people die of extreme cold than extreme warm. How are you THIS sure global warming will only result in local warming?! Global warming is an average, and so far, we've seen local increases in both warming and cooling (e.g. the polar vortex weakening).

Yes, humans can adapt, but that adaptation isn't anywhere close to free, and that cost is not going to be borne by the people emitting.

[0] Capital L, or right-libertarianism specifically. And of course excluding all the authoritarians cosplaying as libertarians because being the Supreme Leader feels a lot like freedom if you squint.

[1] See also: The Structurelessness of Tyranny, which was targeted at left-libertarians

What is your reason to expect warming to increase temperature related mortality? Are you assuming that it makes hot times and places hotter and cold colder? That is inconsistent with the IPCC projections, which show both minimal and maximal temperatures rising.

It's true that adjusting to changed circumstances costs something but warming so far has been about a tenth of a degree a decade, may be rising to two tenths, pretty slow change compared to all the other changes people are adjusting to.

That aside, why do you think the temperature of a century ago was optimal? It wasn't designed for us, we weren't designed for it, and it varied much more over parts of the world where people lived successfully than it is going to change in the next century.

If you, or any reader, is interested in my arguments, which are not mostly about adaptation, you can find my climate posts at: http://www.daviddfriedman.com/Sorted_Posts.html#Climate

Agreed. I expect to live to grieve the loss of New Orleans, and I'm not young.

-a resident

I've talked to David Friedman about migration, and he absolutely does support open borders/free migration. It's quite possible that you and he don't have exactly the same understanding of what it means, but basically, yes.

> [1] See also: The Structurelessness of Tyranny, which was targeted at left-libertarians

I think you mean "The Tyranny of Structurelessness" (by Jo Freeman)?

I have the opposite experience.

Like every libertarian group I was in shit their pants and lit their hair on fire when covid hit.

Most established libertarian thinkers (at least the ones I largely identify with) looked at it both A rationally and B as old vulnerable people and actually appraised the situation with a modicum of sense.

Heck I got banned from an economic historians page, because I was on the friendslist of someone who was screeching at him about covid being 5g or something. He just cleaned house.

>The biggest adaptation is going to be people moving out of their home countries, which isn't an option for the vast majority of humanity. Immigration is not recognized by most countries as a right. If we bake the Middle East or Africa and it becomes unlivably hot, the answer countries are going to choose isn't going to be "admit hundreds of millions of climate refugees". I'm absolutely one of those "migration is a human right" people, and if David Friedman is anything close to consistent, he'll be too. But most governments are run by people who vehemently disagree with that and you're not going to sell people on open borders as an alternative to climate change mitigation.

Yeah this is true. Even within a single country, if a certain area becomes unihabitable, people will (likely by force) migrate to where it remains habitable. I put this as an argument to some people claiming Australia is more mighty than Indonesia.

Indonesias fishing fleet, is mostly 100% capable of coming to Australia. (people have crossed that water in jetskis, stopping to refuel) They have roughly 600,000 ships. Like counting everything above a canoe that has to register with their authorities. If they coordinate even a little bit, theres nothing 3 frigates and 4 growlers could meaningfully do to slow that down. They will make landfall and probably take over north australia. Heck they could make multiple trips. And thats not including Indonesias military or shipping fleets. Thats just fishing. The average capacity of these boats is like 8 people. 16 if overloaded. Thats heaps of people. Lots of people will die in the attempt but if they are driven from their homes and hey happen to be adjacent to the ocean its game over.

I've met several academics / ideological celebrities who were indeed insufferable. It's hard to determine this solely from someone's writing.

> My wife likes to quote the advice “never go to bed with an argument unsettled”

I've never found that to be true. Sleep it off, cooler heads will prevail tomorrow.

A new day brings new responsibilities, and excuses to avoid returning to the topic. It's an easy way to let resentment build up, especially if you're each carrying on the argument in your heads with a fictional version of your partner.

You can't simply resolve all kinds of arguments and disagreements before going to bed. Sure, if you have a minor disagreements you might be able to make amends but other issues will take time to be processed and discussed at a more opportune time.

More to it, one might be tempted to quickly attempt to solve the issue with this line of thinking but failing to address the root cause, thus ending up even causing more damage later on as recurring disagreements appear.

Emotions aren't binary.

But I think the real meat of the saying isn’t really about going to bed angry or whatever. It’s about trying to resolve the issue (going to bed is just a forcing function). If you’re a couple who can’t amenably resolve issues, you’re not going to last very long.

But not everyone is so confident in the partner to assume they see themselves as an equal in partnership.

My experience was neither was impacted by any different resolution strategy. Also - with 5 kids, getting sleep was always the point.

My aside is that I haven't found the advice to apply well to quarrels either.

Sleeping on either quarrels or arguments could be good.