# Homework Help: Force on an object in circular motion

1. Sep 30, 2007

### BuBbLeS01

1. The problem statement, all variables and given/known data
A hammer thrower whirls a 18.2 kg iron ball on the end of a 1.2 m string at 1.0 revolutions per second in a horizontal circle. Calculate the force in the string.

2. Relevant equations
F=m(v^2/r)

3. The attempt at a solution
I am not sure how to do this...
I set up a table of forces in the x and y direction...
x = T & centripetal force
y = W
I am not even sure I am starting off right.

2. Sep 30, 2007

### G01

What type of force is acting through the string? (i.e. What type of motion is it causing?)

3. Sep 30, 2007

### Proggle

Since the circle is horizontal, you know that F=m(v^2/r) will be equal to the tension in the string.

Can you obtain that value from the known information?

4. Sep 30, 2007

### BuBbLeS01

No I need to get the revolutions/sec in radians/sec

5. Sep 30, 2007

### BuBbLeS01

So I multiplied 1 rev/sec by 2pi to get the velocity...
F= 18.2 kg(2pi^2/1.2) = 598.76 N
That seems high?

6. Sep 30, 2007

### G01

Ok, you have the velocity in radians per second, so you have the angular velocity. The formula you are using assumes that the velocity is in meters per second. So, you'll either have to convert or use the version of the formula where the angular velocity is involved. Do you know this form of the centripetal force formula?

Last edited: Sep 30, 2007
7. Sep 30, 2007

### Proggle

Hmm, still missing something.

The velocity in your formula needs units of m/s, not radian/s. You're almost there, can you change it to that?

8. Sep 30, 2007

### BuBbLeS01

I think the formula is v= wr to get it into m/s so that would be 598.76rad/s * 1.2 m = 718.51 m/s

9. Sep 30, 2007

### G01

This is not correct.

Check you math when computing the angular frequency.

Remember:

$$\omega = 2 \pi f$$

Last edited: Sep 30, 2007
10. Sep 30, 2007

### BuBbLeS01

ok if I am doing this right I am getting a huge number of 7818780.67 N

11. Sep 30, 2007

### BuBbLeS01

I think I did something wrong maybe its supposed to be...
w=2pi * f = 6.28 rad/s
v=wr
6.28 rad/s * 1.2 m = 7.54 m/s

12. Sep 30, 2007

### G01

Yes this is correct. Sorry, I apparently didn't notice the error either.

13. Sep 30, 2007

### BuBbLeS01

thanks!

14. Sep 30, 2007

### G01

No problem!

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