3 ms·
If you do that then you're considering 2 collisions, ie. you're considering 2 cars, both hitting a brick wall at 50mph. The assumption therefore is that the for
by dools 2y ago
If you do that then you're considering 2 collisions, ie. you're considering 2 cars, both hitting a brick wall at 50mph. The assumption therefore is that the force felt by the wall is not great enough to overcome the inertia of the wall, therefore each vehicle does not exert any force on the opposing vehicle.
It's the same as saying that if you drive 2 cars into the same wall at 50mph the force felt by each driver is not the same as driving a single car into a wall at 100mph.
That is true obviously.
However if there isn't a perfectly immovable "piece of paper" in between the 2 cars, then you can't just ignore the force acting on each car, from the other car.
F = ma
Also, Newton's 3rd law states that forces generate equal and opposite forces.
So if you punch a wall, you are exerting a force on the wall, and the wall is exerting an equal and opposing force on your hand.
When car A hits car B going 50mph, the force is the mass of the car times the acceleration required to stop the car (ie. deceleration, acceleration in the opposite direction).
However that force is matched by a force in the other direction PLUS the force of each car attempting to decelerate the other car.
You have 2 forces for each car: the force felt by car A as car B tries to accelerate it in the direction car B was travelling, and the force felt by car B from car A that opposes that force, and vice versa.
So if we assume the cars are driving in an Elon Musk hyperloop tunnel under Vegas in a straight line then yeah, the forces felt by the drivers in each vehicle is the same for a car driving 100mph into a "brick wall" (or more accurately a big pile of bricks the same size and shape as a car with equivalent mass).
- indigo0086 2y agoPeak Hackernews reply thread
- dools 2y agoWell that’s what you get when Milton Friedman’s son comes out as being as confidently wrong about physics as his dad was about economics
- vundercind 2y agoYeah this is more on-brand than I could have imagined. I’m not sure I’d have dreamed up something this perfect if asked to parody the guy.
- deleted 2y ago[deleted]
- s1artibartfast 2y agoThe irony is that he is correct about the physics. 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.
- dools 2y ago> The irony is that he is correct about the physics. He definitely isn’t
- s1artibartfast 2y agoHe absolutely is, which just makes it all more funny.
- dools 2y agoYep okay I retract my "confidently wrong about physics" statement, someone else explained it and I get it: https://news.ycombinator.com/item?id=40631089 https://news.ycombinator.com/item?id=40631089
- quern 2y agoI agree he's correct about the physics but I disagree with his father's economics -- my thinking on economics is more in line with David's.
- Aloisius 2y ago> or more accurately a big pile of bricks the same size and shape as a car with equivalent mass 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.
- deleted 2y ago[deleted]