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# thermodynamics question

Registered User
I wouldn't usually consider posting homework questions here, but physicsforums is down sooo

I'm completely stumped on this question. It seems very vague to me, and I don't even see how the cart being on frictionless rails is even relevant.

http://www.physics.gatech.edu/research/riedo/Teaching/Thermodynamics/3141_HW1.pdf

HalberdBlue on

## Posts

• Registered User
Pretty sure the frictionless rails do matter. Conservation of momentum implies the whole system's center-of-mass can't move, but they're enforcing a thermal gradient, which creates a density gradient, which moves the center-of-mass of the gas. Probably.

• Decorative Monocle Registered User regular
I'm not sure why the rails are there, but if I had to guess I'd say it's so that the tank can expand horizontally.

Normally I'd be pretty good at this sort of thing, seeing as I'm a mechanical engineering student, but Thermo was a year ago, it's after midnight, and I got out of class after 7 and am too tired to try and figure it out. My apologies.

• Registered User
Yeah, I'm pretty sure that the center of mass can move, since momentum is being added from outside of the tanker as opposed to inside. But since that's the bonus, I'm more concerned with the first part right now.

• Registered User
Edit: nevermind I'm pretty sure I got it, thanks anyways!

• Registered User
Yeah, I'm pretty sure that the center of mass can move, since momentum is being added from outside of the tanker as opposed to inside. But since that's the bonus, I'm more concerned with the first part right now.

I don't recall seeing anything that was adding momentum, just heat.

Whatever though.

• Registered User regular
Yeah, I'm pretty sure that the center of mass can move, since momentum is being added from outside of the tanker as opposed to inside. But since that's the bonus, I'm more concerned with the first part right now.

I don't recall seeing anything that was adding momentum, just heat.

Whatever though.

Increasing the temperature increases pressure, which adds momentum to the system. (and i'm assuming this pressure inside the tanker is greater than the atmospheric pressure outside of the tanker). Since both sides are held to different fixed temperatures, there will be a net momentum flux.

But actually, since this is a closed system, I am probably completely wrong.

parabol
• Registered User
Demerdar wrote: »
Yeah, I'm pretty sure that the center of mass can move, since momentum is being added from outside of the tanker as opposed to inside. But since that's the bonus, I'm more concerned with the first part right now.

I don't recall seeing anything that was adding momentum, just heat.

Whatever though.

Increasing the temperature increases pressure, which adds momentum to the system. (and i'm assuming this pressure inside the tanker is greater than the atmospheric pressure outside of the tanker). Since both sides are held to different fixed temperatures, there will be a net momentum flux.

But actually, since this is a closed system, I am probably completely wrong.

Looking at the diagram, yes, you are wrong, because there's no way for the pressure to move the system of the tanker and gas (which is what I was referring to). Yeah the gas is moving faster but we describe that with the temperature.

I'm pretty well convinced that you end up with something like a linear density distribution of gas mass-biased to the cold side, which would put the CM of the gas 1/3 tank length away from the cold side of the tank. The tank would move so that the CM of the tanker+gas stays in the same spot.