# Thermal Dynamics, gases

1. Sep 18, 2009

### kpou

1. The problem statement, all variables and given/known data
Thermal Dynamics question, gases?
So I have this box with lengths 20cm on each side. There are 100 balls inside of it with diameter 5mm each. The density in the box is 7.8 g/cm3. The bottom of the box vibrates so the balls bounce around. The top of the box has a movable piston of mass 1kg. What is the root mean square velo of the steel balls if the top of the box is in dynamic equilibrium with the gas of steel balls? Ignore gravity for motion of the balls.

2. Relevant equations
Equations
pV=NkT,
p=m<v^2>N/V=m(2/3)(U/mN)(N/V)=(2/3)(U/V)
pV=2/3U
(p+a(n/v)^2)((V/n)-b)=RT
U=N<K>=1/2Nm<v^2>

3. The attempt at a solution

What I know is there is const V, const N
And what I am basically stuck on is how can I find pressure without temperature? Or vise versa? I have a feeling the answer might be lying in the statement of the top of the box being in dynamic equilibrium to the gas of steel balls.
Also, what does the mass of the movable piston have to do with this?
All the work I've been doing is likely just garble working with the knowns. I haven't found a formula with <v^2> that works with what I can see.

I feel like if I knew how dynamic equilibrium fit into this it would make this doable. And maybe what the 1kg piston has to do with it as well.

Not being able to find T or p is starting to get to me (sad face)

Last edited: Sep 18, 2009
2. Sep 18, 2009

### Redbelly98

Staff Emeritus
Looks like an interesting -- and challenging -- problem. Do you have a figure along with the problem statement?

Since there is a moveable piston, it seems to me that the volume is not fixed. Also, the weight of the 1 kg mass must be balanced by the "gas" pressure, whatever that is.

If you know the area of the 1 kg piston (is it the entire 20x20 cm^2 of the top of the box?), then you can figure out what the pressure is. Hint: the pressure pushes upward on the 1 kg. Acting down on the 1 kg are the force of gravity and the pressure of the atmosphere.

The density is another clue, since it relates the mass and volume of the gas.