# Homework Help: Degrees of Freedom

1. Mar 4, 2013

### Cisneros778

1. The problem statement, all variables and given/known data

I want to know why the average kinetic energy FOR A SOLID is 3/2*RT.

2. Relevant equations

For every degree of freedom = 1/2*RT
Possible Degrees of freedom are:
Translation, rotation, vibration

3. The attempt at a solution

In a solid, I am certain there is vibrational energy because of the spring like forces from interacting particles in a lattice. This would only account for 1 degree of freedom so far.
I am against saying there is any translation or rotation because the particles are fixed in their respective positions since it is a solid.

So I am confused why the average kinetic energy is 3/2*RT when I see it as 1/2*RT.

(1 degree of freedom) x 1/2*RT = 1/2*RT

2. Mar 4, 2013

### Staff: Mentor

You can have vibrations in 3 dimensions.
Every vibration dimension corresponds to 2 degrees of freedom (potential and kinetic energy), so the result is 3RT, for an appropriate temperature range and in a crystal.

3. Mar 4, 2013

### Cisneros778

Thanks a lot!