physics help?

Question

the velocity of a car is reduced by 40.0 m/s for every second that brakes are applid. If the car i initially moving at the rate of 60.0 m/s how long will it take the car to stop?

what formula do i use ?

thanks =)

Answer 1

If the car slows 40 m/s for each second of braking then the acceleration is a =- 40 m/s^2 since the car is slowing down. Now you can use the realtionship:

v = v0 +at where v = initial speed, t = time

Since the car stops, v = 0 so

0 = v0 +at ---> t =-v0/a = -60/(-40) = 1.5 seconds