일반물리학 2 — Electromagnetic Waves: Problem 63

63 Someone plans to float a small, totally absorbing sphere 0.200 m above an isotropic point source of light, so that the upward radiation force from the light matches the downward gravitational force on the sphere. The sphere’s density is 19.0 g/cm³, and its radius is 0.500 mm. (a) What power would be required of the light source? (b) Even if such a source were made, why would the support of the sphere be unstable?