# Homework Help: Uniform Circular Motion-Velocity

1. Sep 17, 2008

### Guruu

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

Here's the exact problem:
Young David who slew Goliath experimented with slings before tackling the giant. He found that he could revolve a sling of length 0.650 m at the rate of 5.00 rev/s. If he increased the length to 0.950 m, he could revolve the sling only 3.00 times per second.

What is the speed of the stone for each rate of rotation?

Then there's more but this is the only part I have trouble with

2. Relevant equations

T=(2piR)/V

3. The attempt at a solution

I set it up as V=(2(3.14)(.650m))/.2s
I get 20.4m/s, but the webassign thing I'm doing says that's wrong. I honestly don't know what I'm doing wrong, this should be a simple plug and chug kinda problem.

2. Sep 17, 2008

### Dschumanji

I personally don't see anything wrong with your work. Assignments that require you to input things online tend to be extremely picky on how you type the answer. Make sure you are following the directions carefully. If not, the webassign is just a piece of crap.

3. Sep 17, 2008

### Guruu

Yeah I just checked my e-mail and the professor had sent out a message saying he messed up the webassign. Well, at least my first physics forums post wasn't a waste of time!

4. Sep 17, 2008

### edziura

You can also use

V = r $$\dot{\theta}$$

where $$\dot{\theta}$$ = 10 $$\pi$$ radians / sec.

The answer comes out the same, so I don't know why webassign considers it wrong.

5. Sep 17, 2008

### Staff: Mentor

Looks OK to me.