Expert: Ahmed Salami - 4/23/2009

Question
The driver of an automobile applies the brakes at time t=0 and comes to a halt after traveling 500ft. Find the automobile's velocity at t=0 assuming that the rate of deacceleration was a constant -10ft/s^2.

Answer
Hi Becca,
For constant acceleration (or deceleration)
v^2 = u^2 + 2as
where v is the final velocity
u is the initial velocity
a is the acceleration
and s is the distance covered
In this problem,
v = 0  (since the automobile comes to a halt)
a = -10ft/s^2
s = 500ft
velocity at t = 0 is simply referring to the initial velocity u, and so
0^2 = u^2 + 2(-10)(500)
0 = u^2 - 10000
u^2 = 10000
u = 100ft/s

Regards