Balls of mass m each connected by a thread move in a circle with a constant velocity v. The kinetic energy of each ball, equal to mv^2/2, does not change. If we switch to a reference frame in which the center of the thread moves in a straight line in the plane of rotation at a speed, the energy of each of the balls changes from zero to 4mv^2/2. What causes this energy change? Does the total kinetic energy change in the specified reference frame?