Circular Motion: Ball on String

[physicaled]

Homework Statement

A 1.5 kg ball is tied to a string that is .75 m long and whirled in a vertical circle.
A) Find the tension in the string at the top of the circle
B) Find the tension in the string at the bottom of the circle

Homework Equations

F_c=ma_c
a_c= v²/r

The Attempt at a Solution

A)
Fnet= Ft + Fg
Fnet = Ft + (1.5)(10)

B)
Fnet = Ft - Fg
Fnet = Ft - (1.5)(10)

As you can see, I did not get very far in my calculations as I am very confused where to go from here. Both equations would require velocity which is not given and I don't know how to find it.

 

[kuruman]

Do you know how fast the ball is moving?

 

[haruspex]

Do you know how fast the ball is moving?

... or any other piece of information?

 

[scottdave]

Perhaps the spin rate is 'just enough' to keep it going. That would happen when, at the top, it is just fast enough to overcome gravity acceleration. So what is Ft + Fg at the top of the circle?

 

[kuruman]

So what is Ft + Fg at the top of the circle?

Fg is mg and you know what that is. You are asked to find the tension and that depends on the speed. If the speed is just enough for the ball to go around the circle, what do you think the tension ought to be? I think you should revisit the problem and see if you forgot to post some needed information.

 

[haruspex]

Fg is mg and you know what that is. You are asked to find the tension and that depends on the speed. If the speed is just enough for the ball to go around the circle, what do you think the tension ought to be? I think you should revisit the problem and see if you forgot to post some needed information.

Scottdave is not the originator of the thread.

 

[kuruman]

Scottdave is not the originator of the thread.

Oops, I missed that. Thanks and sorry Scottdave for directing my reply to you instead of the OP.

 

[haruspex]

Oops, I missed that. Thanks and sorry Scottdave for directing my reply to you instead of the OP.

It's an easy mistake when posters don't use the reply or quote buttons, so it is not obvious whom they are addressing (hint @scottdave)

 

[physicaled]

Oops, I missed that. Thanks and sorry Scottdave for directing my reply to you instead of the OP.

Sorry for the late reply. Hate to say it but that was all the information that was given in the problem.

 

[kuruman]

If you have ever swung a mass at the end of a string, you would know that the faster you swing the mass around, the harder the string pulls on your hand. Tension depends on speed. If the speed is not known, the tension cannot be known. You might wish to ask for clarifications from whoever asked you to do this problem.