# Homework Help: Circular Motion help

1. Feb 15, 2010

### black_hole

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

A 4.03 kg object is attached to a 1.23 m long string and swung in a vertical circle at a constant speed of 18.5 m/s. What is the tension force on the string when the object is at the top of the loop?

2. Relevant equations

Fnet = (mV^2)/r

3. The attempt at a solution

Ft = Fnet = (mv^2)/r = (4.03 * 18.5^2)/1.23 = 1121.356 N

2. Feb 15, 2010

### housemartin

I think you forgot force due the weight (gravity F=mg) of an object: at very top it should help to keep object in circular track.

3. Feb 15, 2010

### black_hole

So what is Fnet equal to? Ft-Fg?

4. Feb 15, 2010

### housemartin

pretty much i guess ;]

5. Feb 15, 2010

### jhae2.718

Edit: looks like I didn't pay close enough attention to the problem...sorry. See PhanthomJay's post.

Last edited: Feb 15, 2010
6. Feb 15, 2010

### PhanthomJay

Let's be careful, when the object is at the top of the loop, there are 2 forces acting on it: It's weight and the tension force in the string. The weight acts down on the object. Now in which direction does the tension force in the string act on the object????

7. Feb 16, 2010

### housemartin

so you suggest that at the bottom the tension would be less than at the top? And the fact that both weight and tension acts in same direction doesn't just mean that required centripetal acceleration is gain from both of these forces? So mac=T+mg and T = mv2/r-mg

Last edited: Feb 16, 2010
8. Feb 16, 2010

### PhanthomJay

No, why do you say that?
At the top of the circle, yes, that is correct. Now draw a free body diagram of the obect at the bottom of the circle. You should find that the tension in the string is greater at the bottom.

Note: The problem assumes the object moves at constant speed. Go with it.

9. Feb 16, 2010

### housemartin

hmm... sorry, seems i misread some things