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## Homework Statement

Two equally charged 1g masses repel each other. The lower mass is held fixed.

a)What is the charge on each mass for the Coulomb force to balance the gravitational force of the Earth on the upper mass?

b)How many elecetrons does this rep?

## Homework Equations

F=Gm

_{1}m

_{2}/r

^{2}, where G=6.67x10

^{-11}

F=Kq

_{1}q

_{2}/r

^{2}, where K=9x10

^{9}

## The Attempt at a Solution

I know to find the charge on each mass we first use the gravitational equation and then use Coulombs law equation to find out the charge on each mass:

F=Gm

_{1}m

_{2}/r

^{2}

=(6.67x10

^{-11})(1)(1)/r

^{2}

I'm pretty sure this is right but I'm just confused what r is. Could it be 5.0x10

^{-11}which is the distance between an electron and proton?

b)This is simple. When you find out the coulomb charge, you use 1Coulomb=6.25x10

^{18}electron charge.