How long does it take to stop at 20 miles per hour?

Answer 1

You have to know the decelerating force acting on the car (say the brakes).

time taken = velocity change divided by deceleration (negative acceleration).

so

if you can measure the distance it took to stop you can find the negative acceleration a:

a = velocity change squared divided by 2x distance measured.

Your velocity change = 20 mph - 0 mph = 20 mph = 29.333 feet per second.

Units must all be consistant i.e. feet and seconds.