# Problem P. 4140. (February 2009)

**P. 4140.** The centres of two small metal balls, whose radii are equal, are initially at a distance of *r*. If both balls are given different magnitude and opposite charges, and then they are released, they begin to move towards each other. They touch each other and then they move away from each other again. When the distance between their centres is 4*r*, then the force exerted between them is one-twentieth of the initial force exerted between them. The initial charge of one of the balls was 2^{.}10^{-7} C. Calculate the initial charge of the other ball.

(4 pont)

**Deadline expired on March 11, 2009.**

### Statistics:

141 students sent a solution. 4 points: 92 students. 3 points: 43 students. 2 points: 2 students. Unfair, not evaluated: 4 solutions.

Problems in Physics of KöMaL, February 2009