# Mechanical motion VS thermal motion

1. Jul 20, 2010

### KFC

Hi there,
I wonder if there is anything call mechanical motion(movement)? How does it compare to so called thermal motion? I know in microscopic area, particles are moving randomly due to thermal effect. If we call this thermal (Brownian) motion, so what's the name of motion of macroscopic object moving governed by Newton's law?

By the way, as we know, even we cool the object to 0K, there is still something called zero-point energy. So will particles move with zero-point energy? If so, how do we call this kind of motion (thermal motion is no longer suitable in this case, right?)

2. Jul 20, 2010

The difference that comes to my mind is thermal motion is disordered or random, while mechanical motion is ordered or coherent.

Zero point energy, I believe, is not the energy left in a particle that is super cooled - it is the energy of 'empty' space, the vacuum.

3. Jul 20, 2010

### KFC

So you mean the zero-point energy doesn't even affect the motion of particles?

By the way, what about the variational motion among particles? Is it ordered or disordered? Why?

4. Jul 20, 2010

I believe that zero point energy comes from the random motion of quantum particles.

Based on my understanding, quantum particles will continue to move even at 0k. However it is not possible to reach that temperature.
You mean is the motion random? As far as I know, yes. Quantum fluctuations are not ordered, but they do follow certain statistical trends. For example, heat generally does not go from a cold body to a warmer one. (This would require the application of work.)

As to the why... I'd also love to know.

5. Jul 20, 2010

### Gerenuk

There is only one type of motion, namely the one governed by Newton's laws (or quantum mechanics if you want to be more precise).
"Thermal motion" is just molecules flying around like billard balls on the table. You might invent other terms like "circular motion", but fundamentally it's all the same.

0K does not mean the particle have no energy left. It only means all particles are in the lowest energy state. This lowest energy state might still have some energy left.