# Homework Help: Sphere rolling down a ramp linear acceleration

1. Jan 10, 2007

### velvetymoogle

1. The problem statement, all variables and given/known data
"A solid sphere of radius R and mass M is initially at rest at the top of a ramp. The lowest point of the sphere is a vertical h above the base of the ramp. It is released and rolls without slipping down the ramp. Determine the linear acceleration while the sphere is anywhere on the ramp.

M (mass), R (radius), h (height), g (gravity), theta

2. Relevant equations
conservation of momentum
I = 2/5MR^2
w = v/r

3. The attempt at a solution

I ended up finding the linear velocity anywhere on the ramp to be square root of 10gh/7. How would I be able to use that though to find acceleration? I seriously don't know what else to do.

2. Jan 10, 2007

### pmp!

Apply Newton 2nd Law twice, for linear and rotational motion. Friction is providing a net torque.

3. Jan 10, 2007

### velvetymoogle

That requires that I find the frictional force, but that's the next question so there has to be some way I can do it without knowing the friction force.

4. Jan 10, 2007

### pmp!

You don't need to calculate the friction force. Just call it F and get the value from one of the equations and substitute in the other equation.

5. Jan 10, 2007

### pmp!

The right answer is $\frac{5}{7}g\sin\theta$. Did you find it?

6. Jan 10, 2007

### velvetymoogle

Yes, I did. Thanks for your help. Would you be able to explain how I would get the frictional force then? Is it just F = m times that or is it -mgsin(theta) because it's on a ramp?

7. Jan 11, 2007

### pmp!

You got two equations before. Solve one to get the value of F.

8. Jan 11, 2007

### AngeloG

pmp! is correct.

It'd be helpful to draw a force body diagram to really grasp the idea of this =).

There's a normal force, a force of a friction, a force of gravity pulling it down in the x and y direction. There's an incline on the ramp (theta).

We know in the Y direction it's not accelerating; thus we can set that summination in the Y axis to 0. However in the X axis we do have a mass that's accelerating; thus we can set that summination in the X direction to mass * acceleration.

After you setup your forces you can then figure it all out mathematically.

Last edited: Jan 11, 2007
9. Jan 11, 2007

### velvetymoogle

Oooh okay. That really helps. (I didn't originally use force equations to find the linear acceleration). I used vf^2 = vi^2 + 2ad. Is the correct answer:
(2mgsin(theta))/7?