Blog: Lifting a rock
By Kent W. Mayhew
Lifting a Rock: Reversible or not?
Imagine that you lift a rock into the air, thus you increase its potential energy. Is
this a reversible process.
In the process of lifting, you are doing the work and the rock gains potential. So if you consider
reversible to mean that you personally can gain that energy back that you as a system gave onto the rock, then the answer is no.
If
you consider that the rock's increased potential can now be converted into kinetic energy i.e. by letting go of the rock, then the
answer is still no because the rock has the extra energy and not you.
If you tie a rope onto the elevated rock, then yes
the rock can be used to move other objects including yourself. Again the answer remains it is not actually reversible because
you yourself have not completely regained the energy that you used to lift the rock in the first place.
It is not
that much different than expanding sysytem's lifting our atmosphere. Just as you transferred hence lost energy onto the
rock, so too does the expanding system lose energy into the atmosphere!
Sommerfield quote:"Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, so it doesn't bother you any more."
