# Homework Help: Circular Motion/Centripetal Forces

1. Oct 28, 2008

### omgitsmonica

1. The problem statement, all variables and given/known data
A 2.1 kg mass is attached to the end of a .5m string. The system is whirled in a horizontal circular path. If the maximum tension that the string can withstand is 80N, what is the maximum speed in m/s of the mass if the string is not to break?

2. Relevant equations

Ac=v2/r

3. The attempt at a solution
I have no idea how to go about solving this. I don't know what mass has to do with anything...

2. Oct 28, 2008

### hage567

You have the right equation for a, just put it into Newton's second law.

3. Oct 28, 2008

### omgitsmonica

F=ma, so 80N=2.1a so 38.1=a

Ac=v2/r, so 38.1=v^2/.5

So v^2=19.05, so v = 4.36

Is that right?

4. Oct 28, 2008

### Epsillon

Yes

So the Fnet is the Ft and it is equal to mv^/r

But the key word in this questions is its horizantal. Do you know what would be diffirent if it wasnt horizantal?

5. Oct 28, 2008

### omgitsmonica

I imagine it would be the acceleration, because the direction of motion would be changed. Or does that not matter, like trajectories?

6. Oct 28, 2008

### Epsillon

If it is horizantal the only thing would be different is that gravity acting on the object that is spinning should also be factored in into the Fnet formula.

So at the top

Fc= Ft + Fg

Bottom

Fc= Ft-Fg

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook