|MadSci Network: Physics|
the question you are asking is actually quite a fundamental one, and with this I mean the equivalence of heat and mechanical work.
I would prefer another formulation: Heat is nothing different than mechanical work - except for one aspect, and this is the direction in which the motion of molecules occurs, whether they all move in one direction or all move in different directions.
I want to give you an example - imagine a car braking. Before that event you have all particles (atoms/molecules) of the car moving in one direction, that's quite obvious. What happens if you brake? On the one hand the car stands still - but what happend to the energy connected to the motion? If you touch the street or your tyres you can feel that they are quite hot - because the atoms move quite fast. But now they are moving in all directions at the same time, so the tyre as a whole stands still.
Do you see the difference? Before you had a concerted motion, all in one direction, thereafter all the particles are still moving quite quickly, but in different directions.
If that's not enough, just ask again or have a look at some physics-resources in the web (like studyweb.com or in the yahho-directory)
Try the links in the MadSci Library for more information on Physics.