Calculation a car crash Numberphile is wrong

Just making a correction to a video I watch. A guy compares two vehicles, at differnt speed to 0 MPH for one, but not for the other. His math was not right and to help those misled I am making this video. If you lost kinetic enery to 0, then both objects must lost their energy to 0! If you deduct and have 5100 units of energy left, then his 0 MPH vehicle would be -5100 in Ke kinetic energy. Best Wishes.

[jans]

The video you're debunking is a simplification. He even says at the end that the final number is not exactly that because of the wind. For the model he uses it's correct

[jonneiswender]

Your calculation in this video are incorrect and what you're saying is incorrect.

1. you state heavier vehicles slow down at a slower rate because of the coefficient of friction. This is incorrect. Coefficient of friction changes based off road type (asphalt, concrete, dirt road, etc) and type and quality of tire. The reason heavier vehicles take longer to stop is simply due to kinetic energy (KE = 1/2mv^2). Doubling the mass will double the KE and KE is directly proportional to BRAKING distance. Double the speed will QUADRUPLE the KE and thus QUADRUPLE the braking distance.

2. the video you're debating is relating SPEED to Kinetic Energy - it is obvious to see from the KE formula, that KE quadruples when velocity doubles.