I am supposed to balance the coulomb repulsive and gravitational force in a way that both forces between the earth and the moon have the same amount. For that I can just use [tex]F_C = F_G[/tex] [tex]1/(4\pi\epsilon_0) * (Q_1*Q_2)/(r^2) = f (m_1 * m_2)/r^2[/tex] and then put Q_1*Q_2 on one side. Now I have to assign a charge to the earth and the moon, which is where I am at a loss. My thought was that the coloumb repulsive force of the moon has to be equal to the earth's gravitational force and vice versa, so that I can do something like [tex]F_C_E / F_C_M = F_G_M / F_G_E[/tex] and with that calculate Q_1 and Q_2. However, I can just calculate the gravitational force between earth and moon and not only the force that the earth exerts on the moon, which I would need for the approach used above. Would it be sufficient to relate the masses of earth and moon to each other and the ratio of both coulomb forces would be equal to that?