q and mass m

A small metal ball with positive charge  q and mass m is attached to a very light string, as shown in (Figure 1) . A larger metal ball with negative charge Acˆ’Q is securely held on a plastic rod to the ceiling. The distance between balls' centers is r.

Part A

Write an expression for the magnitude of the force T that the string exerts on the ball. Assume that the values of qand Q are positive.

Express your answer in terms of the variables q, m, Q, r, and appropriate constants.