# Eletric force and gravitational force question

1. Aug 7, 2008

### strawberrysk8

1. The problem statement, all variables and given/known data

How many extra electrons would we have to place on earth and moon so the electric repulsion between these bodies cancels their gravitational attraction? Assume that the numbers of extra electrons on the earth and on the moon are in the same propotion as the radial dimensions of these bodies (6.38:1.74).

2. Relevant equations

3. The attempt at a solution

2. Aug 7, 2008

### LowlyPion

How would you think to address the problem? Can you think of any relevant formulas?

3. Aug 7, 2008

### strawberrysk8

F=kqq/r^2

F=GmM/r^2

Constants: k,G,m,M,r

what is q1 and q2?

4. Aug 7, 2008

### LowlyPion

Don't forget the additional constraint from the OP that q_earth is 6.38 to q_moon's 1.74.

$$q__{earth}$$= 6.38/1.74 * $$q__{moon}$$

With all the parameters but charge determined then what do you suggest doing with the two equations?

5. Aug 7, 2008

### strawberrysk8

set them equal to each other

kqq/r^2 = GmM/r^2

kqq = GmM

k(6.38/1.74)q^2 = GmM

solve for q(moon)

then solve for q(earth)

what is the extra electons?

6. Aug 7, 2008

### strawberrysk8

oh!! i got it! thank you for the advice :)

7. Aug 7, 2008

### LowlyPion

Congrats then and good luck.