# Homework Help: Rotational Motion (Inclined Plane)

1. Jan 5, 2008

### Izzybee

[SOLVED] Rotational Motion (Inclined Plane)

Hi Everyone!
My question is: A solid cylinder and a solid sphere are resting on top of a inclined plane. Using mathematics, which object would reach the ground first. Given: I (cylinder) = 1/2mr ^2 and I (sphere) = 2/5 mr ^2.

I know that the sphere would reach the ground first because of its mass distribution, but I am not quite sure how to show it mathematically. My teacher said a problem like this will show up on my test Monday!

2. Jan 5, 2008

### Kurdt

Staff Emeritus
What is the energy of a rotating solid?

3. Jan 5, 2008

### foxjwill

I think what kurdt's suggesting is to try using energy methods.

p.s. Are you taking AP Physics C?

4. Jan 6, 2008

### physixguru

CALCULATE THE KINETIC ENERGY OF THE CYLINDER AND THE SPHERE AT THE BOTTOM OF THE PLANE...AND THEN FIND WHICH OF THE TWO HAS THE GREATER VELOCITY....

THE ONE WITH GREATER VELOCITY IS THE ONE WHICH COMES DOWN EARLY..

USE THE FORMULA..

mgh=1/2mv^2(1+k^2/r^2)

5. Jan 6, 2008

### Izzybee

Thanks, but I figured it out!
P.S: What is AP Physics C??

6. Jan 6, 2008

### Kurdt

Staff Emeritus
Advanced Placement college board course. Its an American thing I believe.

7. Jan 6, 2008

No I am not.