# B Order of energy

1. Jun 21, 2017

### JLT

Between kinetic energy, potential energy, thermal energy - it seems like some forms of energy are preferred over others.
A rock on a hill wants to change its energy from PE → KE
and anything with KE wants to change KE into heat.
Is there are reason that some forms of energy are preferred over others?

2. Jun 21, 2017

### Staff: Mentor

I rock on a hill doesn't "want" anything. You are simply thinking of a metastable situation. When the rock falls down the hill, it will eventually end up with gravitational potential energy and no kinetic energy. So there is no preferred form of energy.

The case of thermal energy is different. Not all energy can be turned into useful work, and thermal energy is generally speaking such a case. This is not to say that thermal energy is "preferred," but you have to start taking entropy into account.

3. Jun 21, 2017

### JLT

A rock in air then - given the choice of the energy staying in the form mgh, or the energy changing into 0.5mv^2 the system naturally wants to change mgh into 0.5mv^2 → it likes KE better. If it did not like KE better than it would just remain floating in mid air - it would just keep the original PE.

Systems with KE tend towards heat - the energy likes to change itself into heat.

I think there are definitely some forms of energy that are preferred over other forms - why would some types of energy be preferred over others?

4. Jun 21, 2017

### Staff: Mentor

How can an inanimate system like or dislike something? Statistical mechanics explains why entropy must increase and hence why kinetic energy is more likely to turn into heat than the other way around; we don't need any notion of "wanting" or "natural tendency" for that.
They aren't. In your example of the rock that seems to want to exchange its potential energy for kinetic... Start it with a different initial velocity and will enter an elliptical orbit in which kinetic energy is transformed into potential energy and back again, more or less forever.

5. Jun 21, 2017

### Staff: Mentor

I think that is really the main answer. If we remove the anthropomorphism, we are left with simply that systems tend toward thermal energy due to entropy. All of the examples are the nuts and bolts of how that happens (friction, impacts, light absorption, etc).