Calculating a Car Crash - Numberphile
Why Hacker News recommends it
Used as a short explainer when people need to illustrate why speed matters so much in braking and crash severity.
What it is
Numberphile's Ben Sparks works through the physics of a car crash, showing how kinetic energy grows with the square of speed and why stopping distance climbs so quickly as velocity increases.
Recommended by 1 HN commenters across 1 different months, 2018 to 2026.
What Hacker News said
Recommends nox101 · 2024-06 · on “The right not to be subjected to AI profiling based on publicly available data”
Lol, I think they should be low and stay low and there should be more enforcement For me, I live in a quiet neighborhood. The residential street outside my apartment has speed limit signs of 20mph. People are trying to walk their dogs and kids. 9 out of 10 cars go 40-50mph. Part of that is road design. The road is new, straight, barrier and speed bump free. Worse, if you try to obey the law and go 20 the person behind you will get road rage and then illegally pass on the left (going into the opposite lane) or pass on the right and swerve around to get to the left turn lane. That said, a street a block over is 25mph with speed bump and at least 1 of 3 cars races to each bump. And 1 of 20 just
Cites Balgair · 2018-03 · on “Self-driving cars will cause sprawl”
Numberphile had a recent video on car crashes here: [this video] In short: Since kinetic energy goes as (velocity)^2, doubling the speed will quadruple the stopping distance (MANY other factors are significant, this is a rough estimate). 3X of the speed is ~9X the stopping distance, etc. NHTSB guidelines for 'proper' following distance are nearly never seen in LA and NYC, it's typically much shorter. If anything, L5 AVs will probably have longer gaps between AVs than what is currently on the road. Likely, the major improvement of L5 AVs will be in safety, not congestion. It turns out (I think), you can't technology your way out of a political problem. We need higher density housing, not magic car
Cites leipert · 2020-11 · on “For the first time in Europe, registrations for EVs overtook diesel”
If it's your hobby and like driving 200 km/h, sure please do, on a separate track where it is as safe as possible. But driving it on _public_ roads is inherently dangerous to others. Our society has all kind of rules around protection of others. There are laws and rules around food safety, gun safety, distribution of drugs, hunting, using heavy equipment, operating boats and planes. Often you need a license to do these things. Noone trains for driving at high speeds in driving school. Also a lot of folks do not understand the relationship between velocity and the time you need to come to a stop. So 120km/h is way better than 200km/h. [this video]
Cites throw0101a · 2026-06 · on “Why does kinetic energy increase quadratically, not linearly, with speed? (2011)”
Fun little anecdote: A blue care is travelling along at 70 units, and a red car (exact same make and model) is catching up to it going 100. When they're both right beside each other a bend in the road reveals an obstacle blocking both lanes, so both cars brake at the same intensity and deceleration. The blue care stops right before the obstacle. Since the red car was going at a faster speed, and braked at the same rate, it doesn't managae to stop: but what speed is it going when it hits the obstacle? The blue car, using ½mv², shed (~70²=) 4900 units of energy (we'll hand wave away the constants). So the red car, which had (100²=) 10000 units of kinetic energy to start, also shed 4900 units,