text: the momentum added to an object by light energy  (e) shining on it (eg solar sails) is e/c. why? my physics is only up to a-level, so i'm probably wrongly mixing newtonian physics with photons...  can someone explain a simple derivation?  for kinetic energy: e = 1/2 x m v^2  so i wondered why mv [momentum] does not equal 2e/c (kinetic energy equation) when it's applied to photons. p = mv = (e/c^2) * c = e/c