Car parked on 4.5° icy hill: friction force and static friction coefficient

[pokeefer]

Homework Statement

A car with a mass of 560 kg is parked on a hill inclined at 4.5 degrees to the horizontal.

a.) What is the force of fricton necessary to hold the car on the slope?

b.) If there is ice on the hill and the car is about to slip, what is the coefficient of static friction?

Homework Equations

Ff = u x Fn = u x mg(costheta)

The Attempt at a Solution

The answer I got for part a

a =
0 = - (9.8sin4.5 + U9.8cos4.5)
u = 0.078
Ff = (0.078)(560)(9.8)(cos4.5) Ff = 427N


I'm stuck on Part B

Thanks.

 

[tiny-tim]

hi pokeefer!

a.) What is the force of fricton necessary to hold the car on the slope?

b.) If there is ice on the hill and the car is about to slip, what is the coefficient of static friction?
…
I'm stuck on Part B

it's just asking for the relation between friction force and the coefficient of friction