# Simple coefficient of friction problem

1. Jun 30, 2010

### jsel21

Did my teacher incorrectly write this problem? I looked at it for a good 20 minutes straight before posting this on here.. It's so easy but I think there is something wrong.

1.) The Wheels of a 1.450 x 10^3 Kg car car are locked so that it can only slide on wet concrete at a constant speed of 3.00 m/s when pulled by a horizontal force of 3.45 x 10^3 N.
(a) Find the coefficient of sliding friction for rubber on when concrete.
(b) what is the acceleration of the car after 3.50s?

Coefficient of kinetic friction of rubber on wet concrete - 0.5 (given in book)
Coefficient of Static friction of rubber on wet concrete - 0.7 (given in book)

]Normal Force = mg
Friction Force = Friction Coefficient x Normal Force

What's strange about this is that it seems that 3.45 x 10^3 N is not enough either to get the locked wheels to slide when from rest or already in motion. So i don't see how it can be sliding at 3.00 m/s in the first place.

2. Jul 1, 2010

### Uku

A body that is still is more difficult to move than a moving body (static and kinetic friction). So, assume that the body in your exercise is already moving.

3. Jul 1, 2010

### inky

Read the problem again. Constant speed!!! Equilibrium condition!!

4. Jul 1, 2010

### PhanthomJay

Do not assume this. You are asked to find the coefficient of sliding friction, per above replies.

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