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Old Aug 31st 2010, 08:01 PM   #1
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Electrostatic/charges on earth & moon

Hi, first post.
I need help for this question: How much equal charge would be needed on the earth and on the moon for the electrostatic repulsion to balance the gravitational attraction?

I'm not sure where to start; I know that F = (kq^2)/r^2 and FG = (Gmemm)/r^2 so do I set them equal to each other and the r^2's cancel? After that, I have no idea what to do... Also if I do set them equal to each other, why? Please help! Thanks.
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Old Aug 31st 2010, 08:27 PM   #2
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Originally Posted by BadAtPhysics View Post
Hi, first post.
I need help for this question: How much equal charge would be needed on the earth and on the moon for the electrostatic repulsion to balance the gravitational attraction?

I'm not sure where to start; I know that F = (kq^2)/r^2 and FG = (Gmemm)/r^2 so do I set them equal to each other and the r^2's cancel? After that, I have no idea what to do... Also if I do set them equal to each other, why? Please help! Thanks.
Hi and welcome to the forum.
You're in the right direction. They ask for q, so once you equal both forces, solve for q.
Why do you equal the magnitude of both forces? Because they act in opposite direction so if they have an equal magnitude the resultant force is 0 N, which is what they want by their "balance the gravitational attraction".
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