The Physics of Stopping: Mercedes Commercial 80 mph in 4 Sec

[krehbester]

There's a Mercedes commercial that claims one of their cars can stop from 80 mph in 4 seconds. My question is: if you stop any object moving at 80 mph in 4 seconds will it always take the same distance to stop no matter the size or weight? If so how far?

 

[russ_watters]

If they give you the time and speed, the size and weight are already accounted for.

80mph is 117 fps. Assuming constant deceleration, the average speed is 58.7fps and therefore the distance is 235 ft.

The primary factor in stopping distance is the friction coefficient of the tires/ground. In this case, to decelerate by 20mph every second, you need a friction coefficient of .95, as g is 21 mph/s.