Solving a problem in physics section 74 point 41 electr

41. A thin silk thread withstands maximum tension, the modulus of which is F = 18 mN. A small ball of mass m = 1.2 g with a charge q1 = 20 nC is suspended on a thread in the air. From below, along the suspension line, a ball is brought to it, the charge of which is q2 = –16 nC. At what distance r between the balls does the thread break?