physics 1條問題唔識~~~Coulomb's law

(a) When sphere B is above sphere A,
0.003g = k(Q1).(Q2)/1^2 --------------------------- (1)
where k is a constant, g is the acceleration due to gravity, Q1 and Q2 are the charges on spheres A and B respectively

When sphere A is above sphere B,
0.001g = k(Q1).(Q2)/h^2 --------------------------- (2)
where h is the height of sphere A above sphere B

(1)/(2): 0.003g/0.001g = h^2
i.e. h = square-root[3] m = 1.732 m

(b) From the diagrams, the charges on both spheres should be of the same sign. Hence, there is repulsive force between them. The electrostatic force acts on sphere B is thus downward.

At equilibrium, upward force = downward forces
R = weight of sphere B + eelctrostatic force on B
where R is the normal reaction force acting on sphere B by the bottom of the tube
hence, R = 0.003g + k(Q1).(Q2)/h^2
i.e. R = (0.003g + 0.001g) N = 0.004 x 9.81 N = 0.0392 N