How Do You Calculate Radius in Uniform Circular Motion?

[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.

 

[SammyS]

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.

 

[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?

 

[SammyS]

You don't need the weight, mg.

How is kinetic energy related to speed and mass?

What do you know about circular motion?

 

[george256]

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

 

[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?

 

[SammyS]

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?

Looks good !