If you do some simple manipulations of the equations E=hf, E=1/2mv^2, P=mv, and p=h/<lambda>, one can derive that the momentum is h/<lambda> or that it is h/(2<lambda>). I know that photons do not always follow normal mechanics, but do they follow the usual rules for mechanics for collisions? I have heard of numerous situations in which photons have transfered their momentum to other particles or even larger masses, but it is unclear to me how exactly this works. It seems that one could build a device which would have an efficiency greater than 100 percent. In other words, it would not follow the law of conservation of energy. Why can this not be done? Is it because the photons transfer energy through various mechanisms, depending on energy?