# Car on an incline, kinetic/static friction when break applied confussion.

1. Mar 29, 2009

### alba_ei

I've been doing some problems relative to car on an incline (and also boxes on an incline).
Let's suppose we are given a problem with kinetic or static coefficient.
I'm confusing when should or should not apply kinetic friction to a car, for example in some problems if brakes are applied then friction is involved (as if it were a box), however if brakes are not applied friction is not taken into account(Am I right? I'm not sure, if it is true then why kinetic friction is not taken into account despite the car is moving).
Does this mean that the maximum brake force is (kinetic friction)*(normal)?

What really confused me was the other day a problem in which impending slipping was involved.

1. The problem statement, all variables and given/known data
Data is not important, the problem is concepts not the problem itself.
A car of weigh W is on an incline it has certain initial velocity then brakes are applied, knowing that slipping is impending find static coefficient.

3. The attempt at a solution
I'm not looking for an anwser to this problem, I just want to understand. why is it that when brakes are applied static friction coefficient is involved. Shouldn't kinetic friction be involved as the car is moving with respect to the ground? Why in other problems with horizontal surface surface kinetic friction was involvved and not static friction as in this problem? Does this happen only when impending slipping is involved?

2. Mar 29, 2009

### LowlyPion

You have 2 basic kinds of friction represented by the static and kinetic coefficients.

When you are applying the brakes, presumably they aren't locked and you aren't skidding, for if you were you would be dealing with kinetic as opposed to static friction considerations.

When you step on the brakes of a car the tire - even though rolling is not slipping - the crucial phrase. So the maximum force the driver can apply will be based on static friction coefficient. Usually the static coefficient is larger than than the kinetic and offers the better braking opportunity.