# Homework Help: Rotational dynamics and coefficient of friction

1. Jan 4, 2010

### squintyeyes

How do you find the minimum coefficient of friction needed to prevent slipping for part a and part b.

A hollow spherical shell of mass 4.00 kg rolls without slipping down a 38.0° slope.

(a) Find the acceleration.
3.62 m/s2
Find the friction force.
9.64 N
Find the minimum coefficient of friction needed to prevent slipping.

(b) How would your answers to part (a) change if the mass were doubled to 8.00 kg?
Acceleration
3.62 m/s2
Friction force
19.31 N
Coefficient of friction

for part a I already tried setting mgsinθ-μmgcosθ = 0
and got 0.781 but it was wrong.

2. Jan 4, 2010

### jegues

If you're looking for a coefficient of friction that will prevent slipping then the object won't move at all. This sounds alot to me like static friction.

EDIT: I think I'm confused with what you mean by slipping, can you elaborate a litte bit?

3. Jan 4, 2010

### JaWiB

If you've already found the frictional force, you just need to find the normal force between the shell and the surface and then solve the formula:
$$F_{friction,max}={\mu}N$$