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Diagram

So the test question asked what would happen to the gas if this container were tilted on its side. The piston shown here is 10 kg and moves frictionlessly. I said that the pressure inside the container would decrease and and the piston would move out; however the answer key said that there would be no change in pressure or in position of the piston.

My reasoning was that lets say the gas inside the container exerts an arbitrary force of 1000N upwards because of the pressure. Then lets say the 10kg exerts 100N downward, and the atmosphere exerts an arbitrary 900N downward. The system is obviously in equilibrium. However, when turned on the side, the force of gravity no longer acts right or left, so the 1000N against the 900N of atmospheric pressure will accelerate the piston right until the pressures equalize.

Personally, I thought my answer was the intuitive one. Am I right?

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Yes you are right.

The 10kg piston acts as a force over the area of the piston, increasing the pressure and decreasing the volume of the gas inside.

When the set up is tilted, the force no longer acts on the gas, but sideways, so the pressure equalizes.

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