# Kinetic Energy/Circular Motion and figuring out point charges

1. Feb 3, 2007

### succubus

For the life of me I can't seem to get this problem.

An electrically neutral model airplane is flying in a horizontal circle in a 3m guideline, which is nearly parallel to the floor. The line breaks when the Kinetic energy is 50J. Now, reconsider the same situation, except now there is a point charge of +q and -q on either ends of the guideline. The line now breaks when the KE is 51.8 J. Find the magnitude of the charges.

I tried relating v2 = Fr/m, and equating that to v2=2(KE)/m so Fr/m = 2(deltaKE)/m and got a force of 1.2 N. I figure I have to find the force before I can find the point charge, but I can't seem to find the force. When I worked backward from the solution, the force turned out to be around 7x10(4) N. Which makes more sense. What am I doing wrong?

Thanks for any help

2. Feb 3, 2007

### Dick

I completely agree with your derivation of the force change being 1.2N. What is the 'solution'?

3. Feb 3, 2007

4. Feb 3, 2007

### Dick

I get that this charge corresponds to a force of about 1.2N at a distance of 3m. I think the problem is in the "working backwards" part.

5. Feb 3, 2007

### succubus

I figured it out. I was dividing by two for some reason, and then realized I had to square root the answer. Thanks for your help :)

6. Feb 1, 2009

### smprall

I actually am working on this exact problem and can't figure out what to do. Is there a possibility you could type out the steps of approaching this problem?? That would be great!
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