# Having trouble with a centripetal force problem

1. Jan 10, 2010

### jap129

1. The problem statement, all variables and given/known data
A 2.0 kg stone is whirled in a horizontal circle on the end of a 2.1 m long rope. The stone travels around the circle 50 times each minute. What centripetal force is exerted by the rope on the stone?

2. Relevant equations

3. The attempt at a solution
I've tried to work put the variables into the formula: m*v^2/r. But it didn't work. I've tried anything I could figure, and this is the only problem I have been stumped on from the last 50 problems I've done.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 10, 2010

### jegues

Can you show us your work? I'm fairly certain it should work...

3. Jan 10, 2010

### rock.freak667

so then how many oscillations are in one second?

4. Jan 10, 2010

### jap129

50/60= 0.83

2*0.83^2/1.05 = 1.3 <That answer did not look correct to me. I probably did something wrong. But don't remember what.

5. Jan 10, 2010

### jegues

Where did you get that radius from? Isn't the rope 2.1m?

6. Jan 10, 2010

### jap129

Woops. I was looking at a different problem when I wrote down the radius. So...
2*0.83^2/2.1 = 0.65
The answer is still wrong though.

7. Jan 10, 2010

### jegues

You can compute its circumference and with it solve for a linear velocity. Then think about the relationship between linear and centripetal velocity.

8. Jan 10, 2010

### BoldKnight399

Have you thought about the Fc=mw^2R equation? That might help.

9. Jan 10, 2010

### jegues

I want you to solve for this equation yourself using your understanding of the relationship between linear and angular velocity.

I'd rather you understand how the equations came to be, rather than simply memorizing them.

Last edited: Jan 10, 2010
10. Jan 10, 2010

### jap129

I believe I figured it out, the answer is 120 N. It makes sense now. Thanks for the help. :)