1. The problem statement, all variables and given/known data FORCE on a sphere due to a beam of light a beam of light(thick enough to shower the whole sphere) is falling horizontally on a perfectly reflecting sphere of intensity I. what is the force exerted by this light beam on the sphere? 2. Relevant equationsp=U/c, area element=a^2 sin theta d theta d phi 3. The attempt at a solution while solving this, i get 4 sines , 1) for area element on sphere R^2 sin theta dtheta dphi 2)for momentum component of photon in radial direction 3)for dot product of intensity with area element to get energy incident on area element normally. 4) for net component of forces along horizontal direction. (vertical ones get cancelled out due to symmetry). but my book shows 3 sines and 1 cos.and the answer is thus coming different(pi R^2 I/c) can anyone show me where i am going wrong? thanks.