College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 17

A laser beam is used to levitate a metal disk against the force of Earth's gravity. (a) Derive an equation giving the required intensity of light, I , in terms of the mass m of the disk, the gravitational acceleration g , the speed of light c , and the cross-sectional area of the disk A. Assume the disk is perfectly reflecting and the beam is directed perpendicular to the disk. (b) If the disk has mass 5.00 g and radius 4.00 cm, find the necessary light intensity. (c) Give two reasons why using light pressure as propulsion near Earth's surface is impractical.