# Homework Help: Calculating Static Frictional Force on an incline

1. Feb 28, 2014

### Shadow236

1. The problem statement, all variables and given/known data
A car (m = 1680 kg) is parked on a road that rises 17° above the horizontal. What are the magnitudes of (a) the normal force and (b) the static frictional force that the ground exerts on the tires?

2. Relevant equations
FsMAX = Mu*FN or in this case: FsMAX = Mu*mg*cos(17)

3. The attempt at a solution
I found (a), the normal force by finding mg (1680 * 9.8) and then multiplying that by cos(17) because of the incline. My problem is finding the static frictional force because there is no coefficient given. I've tried to find the coefficient by using the normal force, but I still end up with two unknowns... 16464 (FN) = Fs/Mu.

2. Feb 28, 2014

### Staff: Mentor

Good.

You are asked to find the actual static friction force, not the maximum value between the surfaces. Hint: You don't need the coefficient, just the conditions for static equilibrium.

3. Feb 28, 2014

### voko

Because the car is parked, it is stationary, that is to say, in equilibrium. So the force of static friction is balanced by some other force. What is this force? Can you find it?

4. Feb 28, 2014

### Shadow236

Are they the Normal Force and the weight of the car?

5. Feb 28, 2014

### voko

Isn't the normal force perpendicular to the force of friction? How can they balance each other then?

It is probably best to list all the forces, choose a coordinate system, and write down the equations of equilibrium.

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