Calculating Tension Force in Circular Motion: Homework Help

[black_hole]

Homework Statement

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?

Homework Equations

Fnet = (mV^2)/r

The Attempt at a Solution

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

 

[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.

 

[black_hole]

So what is Fnet equal to? Ft-Fg?

 

[housemartin]

pretty much i guess ;]

 

[jhae2.718]

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

 

[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?

 

[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 ma_c=T+mg and T = mv²/r-mg

 

[PhanthomJay]

so you suggest that at the bottom the tension would be less than at the top?

No, why do you say that?

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 ma_c=T+mg and T = mv²/r-mg

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.

 

[housemartin]

hmm... sorry, seems i misread some things