1. The problem statement, all variables and given/known data A small plate of mass m is suspended by a light string of length l .A monochromatic light beam starts falling on it and is completely absorbed. If the energy falling on the plate per unit time is W, then find the time period of oscillation? 2. Relevant equations 3. The attempt at a solution I feel the light falling on the disk exerts some force on the disk .If it is a constant force then the equilibrium position is shifted. But I am unable to relate the energy falling on the plate with the force it exerts. I would be grateful if someone could help me with the problem.