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. (Do not substitute numerical values; use variables only.)
I=
(b) If the disk has mass 3.89 g and radius 2.40 cm, find the necessary light intensity.
(c) Give two reasons why using light pressure as propulsion near Earth's surface is impractical.