# Homework Help: HELP - Energy of a rolling sphere (no radius?)

1. Oct 24, 2007

### BlueSkyy

HELP - Energy of a rolling sphere (no radius??)

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

A solid sphere of mass 0.602 kg rolls without slipping along a horizontal surface with a translational speed of 5.18 m/s. It comes to an incline that makes an angle of 34degrees with the horizontal surface. Neglecting energy losses due to friction,

(a) what is the total energy of the rolling sphere?
(b) to what vertical height above the horizontal surface does the sphere rise on the incline?

2. Relevant equations

KE = 1/2 m (v^2) + 1/2 I (w^2) where I = 2/5 m (r^2)
PE = mgh

3. The attempt at a solution

I'm not given a radius so I can't use the KE equation...where do I go from here?
Once I have a radius it will be much easier to solve the problem...

2. Oct 24, 2007

### Staff: Mentor

Just call the radius "R" and keep going.

3. Oct 24, 2007

### BlueSkyy

i kept "r" as a variable and came up with:
8.0766 + 0.1204 (r^2) (w^2)

but the problem asks for a specific number ( _____ J) so I can't keep the variable "r" there...

also, when they say total energy, do they mean total KE, since the ball is still in motion? i'm trying to use PE to solve for the height once I have KE solved...

4. Oct 24, 2007

### Staff: Mentor

When a sphere rolls without slipping, what's the relationship between the translational speed (v) and the angular speed (w)? (Express the full KE in terms of v and you won't see an "r".)

Total energy means include everything: translational KE, rotational KE, and PE. (When it's on the horizontal surface, I would just call that level PE = 0.)

5. Oct 24, 2007

### BlueSkyy

AH! I forgot!

KE(rotational) = B 1/2 m (v^2) where B = 2/5

Thank you, I figured it out now~
:)

6. Apr 10, 2009

### Whome

Re: HELP - Energy of a rolling sphere (no radius??)

E initial = E final
K initial translational + K initial rotational + PE initial = Same except final
is this what you mean? then solve for h on the final side?

7. Apr 10, 2009

### Staff: Mentor

Re: HELP - Energy of a rolling sphere (no radius??)

That's right.

8. Apr 10, 2009

### Whome

Re: HELP - Energy of a rolling sphere (no radius??)

Thank you.