# Homework Help: Free body diagram of atwood machine

1. Jun 21, 2011

### Shukie

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

I need to find the forces on this modified atwood machine:

http://imageshack.us/photo/my-images/36/atwood.jpg/

3. The attempt at a solution

What I think:

- On mass M there are two forces, Mg and -T.
- On the mass above the spring there are two more forces, -2mg and T.
- On the mass below the spring there are also two forces, -mg and kx.

Is any of this correct?

2. Jun 22, 2011

### soothsayer

You seem to have got it right.

3. Jun 22, 2011

### Shukie

Thanks.

4. Jun 22, 2011

### Staff: Mentor

Careful: The only force of gravity on that mass is mg. (But there's a spring force on it.) So there are three forces acting on that mass.

5. Jun 22, 2011

### soothsayer

Ooh, you're right, I was thinking there might be a 2mg plus a kx after you mentioned the spring force, but I think it's only mg and kx.

I knew the spring was going to make it trickier than I had thought.

6. Jun 22, 2011

### Shukie

Are the forces on the mass above the spring then T and -mg and - kx?

7. Jun 22, 2011

### Staff: Mentor

Yes.

8. Jun 22, 2011

Thanks