The brakes on a car permit it to decelerate at the rate of -8.0m/s^2. How much distance is required to stop this car when it is travelling 60.0km/hr?

Question

The brakes on a car permit it to decelerate at the rate of -8.0m/s^2.  How much distance is required to stop this car when it is travelling 60.0km/hr?

The answer given is 173.6m

I solved it this way
60km/h = 16.667m/s
A=V/T  16.667m/s / -.80m/s^2 = 20.834 seconds
D=RT  16.667m/s x 20.834seconds = 347.24m

This is wrong and I do not understand why.  Am I doing it right or is the answer wrong?

bobpursley · Answer

The last equation.... distance= rate/time. You need the average velocity, not the initial. Average here is 1/2 the initial.

Binh-An · Answer

Hi, I'm very familiar with math. I'm 11, and I'm the king of math of my class. Your answer is twice as much as the given answer, David. Hey there, you got the second equation wrong. For the divisor you are supposed to have the time, not the acceleration. Try solving for the time by knowing about integrals (F(a)=v; F(v)=t; this is tricky, I hope it works).