My textbook states without proof that p, the momentum transferred by a wave onto a perfect absorber (normal incidence), is U/ c, where U is the total energy delivered in some time interval. Is there a simple proof for this? Just something to help me rationalize. I tried equating kinetic energy to U. Then I set mv equal to p. But what I get is 2U/c, which is the p for a perfect reflector. Any help is greatly appreciated.