Problem 5.268

A small ideal mirror of mass \(m=10 \mathrm{mg}\) is suspended by a weightless thread of length \(l=10 \mathrm{~cm} .\) Find the angle through which the thread will be deflected when a short laser pulse with energy \(E=13 \mathrm{~J}\) is shot in the horizontal direction at right angles to the mirror. Where does the mirror get its kinetic energy?