Science Forums 2

[Erasmus00]

If you place a rock in a fast moving stream, the question of wether the rock does work is easiest analyzed by thinking in terms of energy. Originally, the water molecules are fast moving, so the kinetic energy of a water molecule is fairly high. After colliding with the rock, the water molecules are now moving much more slowly, so their energy has in fact been decreased. Where does this energy go? The rock heats up a bit(in other word, the rock molecules start vibrating), and that heat gets carried into the river bed its anchored in.
In general, when you have friction processes like this (rock in a stream, ball sliding to a stop) the energy that looks "lost" becomes heat.
Now, from a galilean relativistic point of you, this situation should by dynamically equivalent to a rock moving through a still stream. Now, the rock must do work, becasue the water molecules start speeding up, gaining energy. Where does that energy come from? Well, either there are no outside forces, and in fact the rock starts to slow down, or else the rock has another power source (you pushing it, for instance). So, which system is gaining energy and which is losing energy (hence which does work, rock or water) totally depends on your frame of reference. Just another of those tricky things about the whole energy/work concept.
-Will