Homework Help: 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.