100. A conductive ball weighing 120 g is suspended on a silk thread and has a charge of 1.2 nC. When a second charged ball was placed under it at a distance of 18 cm, the thread tension decreased by half. Determine the sign and charge modulus of the second ball. Acceleration of gravity should be equal to 9.8 m / s2.

Detailed solution. Decorated in Microsoft Word 2003. (Target decided to use formula editor)