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How far (in cm) apart must be proton from group of 13 protons if the magnitude of the electrostatic force acting on either one due to the others is equal to the magnitude of the gravitational force on a proton at Earth's surface?

So, I know that the mass of a proton is 1.67*10^-27 kg and that the charge on the proton is 1.6*10^-19 C. I am using the formula: 1/(4piEo) times (q1q2 divided by r^2) equals mg. So I thought you could set it up like this: (1.6*10^-19 times (13)(1.6*10^-19) divided by (1.6*10^-27)(9.8). That multiply that quotient by r^2=9*10^9. I don't think I did it right, I'm confused at what to do with the 13. Thanks!!!

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# Homework Help: Electrostatic and gravitational forces!

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