Hi, a general question involving heat, light, kinetic energy, and momentum. A man has a 100W laser. He fires the laser at a black body which absorbs the photons of the laser and gains heat energy from it. In practice the conversion efficiency from the 100W laser power to the black body molecules should be high, 90% should be attainable. Heat is molecular kinetic energy, so the molecules in the black body are given momentum by the laser photons, at a rate of 90J/sec. Q : By conservation of momentum, if the photons are giving 90J/sec of KE to the BB molecules, why isn't the man experiencing a kick back from laser with a power of 100W, similar to the kickback from a 100W water-hosepipe ? My Answer : Guess : Since light waves exert EM forces perpendicular to their line of travel... a.) kickback and knock on momentum they exert is mostly perpendicular to their line of travel. b.) as a horizontal traveling wave oscillates it will exert a EM force up then down... so for large objects the energy given by one half of the EM wave is often taken back in the next half. Only very small objects, typically ions or electrons have so little mass they can be knocked clear by the 1st half of the wave with enough velocity to so as to escape the 2nd half of the wave that would return them to their original position and rest velocity. c.) any kickback is also thermalized instantly, just like the BB molecules, i.e. the laser device heats up a bit. Is this about right? At 1st glance I couldn't understand why since a 100W laser, or a light torch shouldn't have a 100W kick back.