Exactly! Te energy must go somewhere, and conventionally is converted into heat instead of mass.
--- "Stephen A. Lawrence" <[EMAIL PROTECTED]> wrote: > > > Merlyn wrote: > > Having loads of fun here > > > > <my email truncated the interesting and relevant > parts > > of the previuos messages, so no quote possible> > > > > Horace, your gedanken experiment involving the > dropped > > rock neglects the fact that light carries > momentum. > > In order for the rock to be turned into light > > ttraveling the opposite direction, a force must be > > applied to reverse its momentum. Equally, Einstein > at > > the top of the ladder must apply a force when he > > catches the light to stop it and turn it into a > > stationary rock. > > Actually it was my gedanken, or rather my quote of > Einstein's gedanken > experiment. But you're right, force is necessary to > change the momentum > of the rock/photon. > > But we can deal with the momentum issue. The rock > can exchange momentum > with the person who catches it _without_ exchanging > more than a > negligible amount of energy, and it's the total > energy we were concerned > with. Just make the planet on which the person who > catches it is > sitting sufficiently massive, so that the planet's > motion, and by > extension the motion of the person, is negligible. > > We see this effect all the time in real life. > Bounce a ball off a hard, > solid wall. The ball's momentum reverses, which > implies the wall gained > momentum equal to twice what the ball had to start > with, but if it's a > good hard rubber ball and the wall is good and > solid, the ball loses > almost none of its energy. The wall gains momentum > but (almost) no energy. > > A massive mirror, for another example, will flip the > momentum vector of > a beam of light very nicely while absorbing > essentially none of the energy. > > The reason is that "net impulse" -- transfer of > momentum -- depends only > on the duration of the applied force, while "work" > -- energy transfer -- > depends on the force and the distance the body it > acts on moves during > the application of the force. If the body is > massive and hence doesn't > move more than a miniscule amount during application > of the force, only > a negligible amount of energy will be transfered. > > Finally, if you throw a _sticky_ ball at a wall, and > it sticks but > doesn't bounce off, _and_ if the wall is good and > solid (and massive), > you find that the wall gains momentum equal to what > the ball had, _but_ > it still gains almost no kinetic energy. Instead, > the ball's kinetic > energy (almost) all turns into heat. > > Merlyn Magickal Engineer and Technical Metaphysicist __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
