# Homework Help: Circular movement

1. Jul 11, 2008

### asi123

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

Everything is in the pictures.
I did part A, I have a problem with part B.

2. Relevant equations

3. The attempt at a solution

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2. Jul 11, 2008

### Staff: Mentor

You need to find the tension as a function of angle. (Forget the tension at the top.)

You have the correct equation. Hint: Express v^2 as a function of angle.

3. Jul 11, 2008

### asi123

I know this equation (in the pic) but I'm doubtful if this what you meant...

Should I use trigo? can I get another tip ?

10x.

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4. Jul 11, 2008

### Staff: Mentor

The equation I had in mind was: mv^2/r = T + mg cos(theta).

Another hint: What's conserved as the ball continues on its path?

5. Jul 11, 2008

### asi123

Ok, I already wrote that equation at the beginning in the solution part I uploaded and got stuck there.

I got stuck because I dont know the velocity at this point, I mean, the velocity is not constant, right? there is a mgsin(theta) that keep changing it...I think.

The velocity at the top is not the velocity at the bottom, right?

10x.

6. Jul 11, 2008

### Staff: Mentor

The velocity is definitely not constant. Reread my hint in post #4.

Given the velocity at the top, you should be able to find the velocity at any point as a function of angle.

7. Jul 11, 2008

### asi123

I'm totally stuck, should I use trigo, or there is some equation for the velocity that I dont know about...

Oh, maybe u mean to use energy calculation?

8. Jul 11, 2008

### Staff: Mentor

Getting warmer!

9. Jul 11, 2008

### asi123

The potential energy turns into kinetic energy, no?

10. Jul 11, 2008

### Staff: Mentor

You got it. Keep going.

11. Jul 11, 2008

### asi123

Ok, I haven't really learned Conservation of energy so I'm not really familiar with it...

I wrote this equation about the energy, is it correct?

And another thing, What about T? it's not a Conservative force, right? Do I just ignore it in the the energy equation?

10x.

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12. Jul 11, 2008

### Staff: Mentor

Looks good to me.
Excellent question! Ask yourself: Does the tension force do any work on the ball?

13. Jul 11, 2008

### asi123

Oh, right, it's vertical to his movement.

10x a lot.

14. Jul 12, 2008

### Staff: Mentor

The ball's movement is always perpendicular to the tension, thus the tension does no work on the ball.