What is the thinnest film of a coating with n = 1.38 on glass (n=1.52) for which destructive interference of the red component (640 nm) of an incident white light beam in air can take place by reflection?

Two steel balls, each of mass m, are welded to a light rod of length l and negligible mass and are initially at rest on a smooth horizontal surface. a horizontal force of magnitude f is suddenly applied to the rod as shown. determine (a) the instantaneous acceleration a of the mass center g and (b) the corresponding rate at which the angular velocity of the assembly about g is changing with time.