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Comment by dalek_cannes | Hacker News Reader

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dalek_cannes·

If physicists ruled the world, speed limit signs would be replaced with momentum limit signs.

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Cute, but momentum and kinetic energy aren't comparable.

Momentum: p = mv

Kinetic Energy: E = 1/2 mv^2

IOW E is the first integral of p with respect to velocity.

Your suggestion has something important going for it, though -- more massive vehicles would have to slow down.

Given one, the other is implied though. Only the limit will have to be set accordingly.

You're wrong. The relationship is different, the graph of the speed limit vs velocity would also be different, for the same masses/cars.

This would make some sense if all vehicles had the same braking force. Though, even then, I think you'd want a kinetic energy limit. (A stopping distance limit would probably be the most direct.)

So Smart cars and motorcycles should be allowed to drive at 100+ mph?

Momentum (or kinetic energy) as modified by human reaction times, which in turn are a function of the road design and other local conditions. That implies signs which are not only tailored to their location, but can automatically adjust based on weather and human activity patterns. Streetlights come the closest to approximating this, but I'm not aware of streetlights that react to weather.

(Road design is a fascinating field; the intersection of physics, materials science, automobile engineering, and human psychology. It's possible to design a road to allow the average person to drive a modern midsize sedan in relative safety given good weather conditions, but you have to make the road a closed system (controlled access) and become zealous about maintaining it and the cars that use it. The Autobahn, in other words.)

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