1. The problem statement, all variables and given/known data One charged plastic ball is vertically above another in a test tube as shown. The balls are in equilibrium a distance "d" apart. If the mass of the upper ball is reduced by a factor of four, the distance between the balls in the test tube would become : 1) 4d 2) 8d 3) 2d 4) square root of 2d 2. Relevant equations ? 3. The attempt at a solution How are charge and mass related? That's what I can't understand.