Breaking distance of a car!

1. Oct 30, 2012

rokr32

1. The problem statement, all variables and given/known data

The driver applies the brakes and the car comes to rest in 15m. Calculate the average braking force exerted on the car in coming to rest.

The average resistive force acting on the car is 2.4 x 103N

2. Relevant equations
F=ma
SUVAT maybe?

3. The attempt at a solution
When using KE = 1/2mv2, the problem is easy to solve. However, I'm not allowed to use this. Therefore, I have NO IDEA how to solve this.

One approach I took is, given that the average resistive force is 2400N, I did:
15F + 2400 = 0
F = -160N.

Then, given that the signs are irrelevant, I can add 160N to 2400N and get 2560N which rounds to 2.6 x 103N.

By the way, the answer should be 2.6 x 103N

Thanks :D

2. Oct 30, 2012

Staff: Mentor

Um, where does the 15F come from?
Signs are never irrelevant! They tell you which direction (with respect to your chosen coordinate system) things are acting or moving.
Can you list all the forces acting on the car and their directions? Draw a free body diagram.