Finding charge q when gravitational and electrostatic potential energy balance

[airkapp]

Imagine that two identical steel spheres, each having a mass of 1.0 kg, are placed a certain distance apart. They are given an equal charge q such that when the separation between the objects changes, the change in the gravitational potentail energy between the objects is exactly balanced by an opposite charge in the electrostaic potential energy between the objects.
What is q?

I used both formulas; V(r) = +k q1q2/r, and V(r)= -G m1m2/r
I then set the two equal to each other and solved for q.
Should this method yield the correct answer?
thanks

 

[airkapp]

Anyone know what my "r" will be in this case?

 

[vsage]

The best you can get with one equation and two unknowns is a proportionality between the two. Yes that's the way you'd go about solving that problem but q is going to be related to r in some way. Edit I can't believe I didn't see this but r isn't relevant since both forces are inversely proportional to distance. (Not that it makes a difference but I thought your question asked for forces originally).

 

[airkapp]

maybe I should of done the simple algebra first...duh..the r's cancel out
thanks
i ended up w sq.root of G/k

 

[e(ho0n3]

I don't understand your question. You say that "when the separation between the objects changes" (which I interpret as: when the distance between them changes, i.e. becomes larger or smaller) "the change in the gravitational potentail energy between the objects is exactly balanced by an opposite charge in the electrostaic potential energy between the objects." What opposite charge? Where is it coming from? It is clear that changing r will change the potential energy of the system. Are you asking for the value of q that will make the potential energy of the system is 0?