# Homework Help: What equation would I use to solve this problem.

1. Oct 4, 2011

### george256

A 2.4 kg object attached to a horizontal string moves with constant speed in a circle of radius R on a frictionless horizontal surface. The kinetic energy of the object is 75 J and the tension in the string is 330 N. Find R.

2. Oct 4, 2011

### SammyS

Staff Emeritus
Welcome to the PH Forums!

F = ma, & a few others.

Don't just look for the "magic" equation. Think about the quantities you are given and what's asked, How are they related to each other or to related quantities.

3. Oct 4, 2011

### george256

well i keep thinking i have my mass i know gravity so i should multiply those to get mg then i feel like i should multiply mg with my 330N because that is my tensional force, and i have no clue what to do with the 75 j. Am I going in the right direction though?

4. Oct 4, 2011

### SammyS

Staff Emeritus
You don't need the weight, mg.

How is kinetic energy related to speed and mass?

What do you know about circular motion?

5. Oct 4, 2011

### george256

i can get my velocity from k but with that what do i do?

6. Oct 4, 2011

### george256

I figured it out thank you so much for the help. I had one of those duh moments when i realized. I will write down what I did so you can tell me if it was the correct and simplest way to go about it if you do not mind. first off i found my velocity using k=1//2mv^2 then i used my force of 330N in f=ma and got my a then i used the circular motion equation a=v^2/r and that was my answer. Does that seem like the correct way to do it?

7. Oct 4, 2011

### SammyS

Staff Emeritus
Looks good !