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?