# Homework Help: Moment of inertia rolling down a hill

1. Oct 27, 2012

### lo31415926535

1. The problem statement, all variables and given/known data

Two solid spheres -- a large, massive sphere and a small sphere with low mass -- are rolled down a hill. Which one reaches the bottom of the hill first?

2. Relevant equations

ICM= (2/5)MR2

3. The attempt at a solution

I thought that this would be the smaller sphere because it has a lower moment of inertia… but this isn't right. Can someone please explain?

2. Oct 27, 2012

### Simon Bridge

You have to do the physics ... what are the forces on the cylinders and the constraints?
Have you just done a lesson or so on rolling without slipping?

3. Oct 27, 2012

### lo31415926535

What physics do I have to do? My teacher talked about it a little… but honestly he is not very good at explaining things so I am having trouble understanding this.

4. Oct 27, 2012

### lo31415926535

Just gravity is acting on them right? So the larger, more massive one would reach the bottom first?

5. Oct 27, 2012

### Simon Bridge

The wheel has to roll as well - so it will have an angular velocity.
Have a look at how other's do it.
http://faculty.wwu.edu/vawter/PhysicsNet/Topics/RotationalKinematics/RollingWithoutSlipping.html [Broken]
... there are also video tutorials on youtube.

Last edited by a moderator: May 6, 2017
6. Oct 28, 2012

### lo31415926535

okay so since the masses and radii drop out, they reach the bottom at the same time?

7. Oct 28, 2012

### Simon Bridge

why would the masses and the radii "drop out" - did you do any math?

8. Oct 28, 2012

### lo31415926535

that's what it was on your second example… What math am I supposed to do??

9. Oct 28, 2012

### Simon Bridge

Have you had a look at the links I have given you?