Q & A: Braking distance

Q:
I want to calculate the distance my car travels from 70MPH to 0MPH by measuring the time it takes to do this. I am doing a brake upgrade and will like to calculate the difference in distance before and after. Rather than hitting the brakes at a given point on the road and getting out of the car to measure the distance traveled to the complete stop mark, I would prefer to have a stopwatch on board and calculate the time it took to come to a complete stop from inside the car and then calculate the distance it travelled. Thank you for your help, Robert :)
- Robert (age 51)
Tampa, FL, USA
A:

Hi Robert,

You can do it IF your de-acceleration is a consant.  In that case you can invert the usual formula for distance  x = x0 + v0t + 1/2 at2 .  The acceleration will be a =  - v0/t and the the result is x = 1/2 v0t.    If your de-acceleration is not constant then you will need some device that will record it as a function of time.  You then need to do some numerical integration to get the answer.

Leeh

(published on 05/03/2015)