The rate at which mass is being added to the bottle is and its velocity entering the bottle is . A bottle being filled with soft drink at a rate . The liquid enters the bottle with velocity . A light beam with power hits a plate which is oriented normally to the beam. Compute the force required to hold the plate in place if the plate completely absorbs the light, and the plate completely reflects the light.
Photons are massless, so the momentum of a photon with energy is .
Thus, the momentum per unit time hitting the plate is .