Find the electric charge required on the earth and moon to balance their gravitational attraction if the charge on the earth is 10 times that on the moon.

Using law of gravitations, the force between the two point masses is F = M₁ = mass of moon = 6.7 x 10²² Kg M₂ = mass of earth = 6 x 10²⁴ Kg r = distance between masses = 380 mm G = universal gravitational constant = 6.7 x 10^-11 Nm²/Kg² Let Q₁ and Q₂ be the charge on moon and earth respectively. Now the gravitational force must be balanced by the force of repulsion Q₁Q₂ = 4pε₀M₁M₂G but Q₂ = 10Q₁ = 10 x 4pω₀M₁M₂G ∴ Q₂ = 173 x 10¹² Q₁ = 17.3 x 10¹² C Q₁ = 17.3 TC.