# Ball swinging around a post

1. Nov 17, 2011

### jjd101

1. The problem statement, all variables and given/known data
A ball on a 6.5 m long string swings down and wraps around a post that is 4.0 m below the post the string is attached to. If the ball is released level with the post to which it is attached, what will be the velocity of the ball when it is at its highest point over the lower post?

2. Relevant equations
circular momentum? Inertia of a ball on a string

3. The attempt at a solution
I really have no idea where to start on this problem, i was thinking you start with the initial acceleration of 9.8 due to gravity but that doesnt translate to an initial angular velocity.

2. Nov 17, 2011

### Staff: Mentor

Use conservation of energy! What forms of energy are involved here? Where's the energy coming from to give the ball motion?

3. Nov 17, 2011

### jjd101

gravitiational potential energy and kinetic energy??

4. Nov 17, 2011

### Staff: Mentor

Yes. So determine the change in energies for the various locations.

5. Nov 17, 2011

### jjd101

??? do i split it up into sections like pie to 3pie/2, 3pie/2 to 2pie, 2pie to pie/2?

6. Nov 17, 2011

### Staff: Mentor

You can split it up any way you like, as long as you get the information you need in order to find the kinetic and potential energies at the various locations. Personally, since gravitational potential energy is involved, I'd be more concerned about the change in heights of the object at the various points, rather than any angle. Why not draw a picture of the situation at each critical point and see if you can label the changes in height.

7. Nov 17, 2011

### jjd101

the change in height goes from 4m above the post to 2.5m above the post, aka 1.5m lesss than the drop point, but i dont know how to do this energy problem without mass?

8. Nov 17, 2011

### Staff: Mentor

Leave it in as a variable if you wish; you'll find that it cancels out anyways.

9. Nov 17, 2011

### jjd101

i left m in as a variable and ended up with velocity is equal to 5.42m/s. Is this correct?

10. Nov 17, 2011

### Staff: Mentor

It looks fine.

11. Nov 17, 2011

thanks