My hypothesis : A gas in a container, when expands exerts a force on the piston(freely moving and frictionless piston)[F1] and the piston would thus exert an equal and opposite force on the gas[F2] this force will travel all the way to the other end of the container and this will push the ground whose normal force will nullify the force. Thus the piston moves up. If the container-piston apparatus was kept horizontally instead of being kept vertically then the same would have been observed but the role of normal force would be played by friction between the container and the ground. This thus leads to the conclusion that the piston cannot be moved when forces like these are absent. Therefore, a gas in a container cannot move the piston up in the outer space.

Is this hypothesis correct? If no, then why not? Please explain with an example.

  • $\begingroup$ What if the piston is frictionless? $\endgroup$ May 12, 2021 at 13:01

1 Answer 1


Without an external force, the centre of mass of the gas/piston/container system cannot accelerate - if we work in a reference frame in which the COM is initially at rest, then it will stay at rest.

However, this does not prevent the piston moving in one direction and the rest of the container moving in the opposite direction, which allows the gas in between to expand.

  • $\begingroup$ Wouldn't the forces however be nullified by the container's internal tension in its walls? $\endgroup$ May 12, 2021 at 13:15
  • $\begingroup$ @LumbiniATambat I am not sure what that means. If the friction between the piston and the cylinder walls is low enough then the gas will expand and the piston will move. If the friction is too great then the piston will not move. $\endgroup$
    – gandalf61
    May 12, 2021 at 14:01
  • $\begingroup$ I got it, so it’s like the gas kicking on the container’s bottom and this pushing the position up, right? $\endgroup$ May 15, 2021 at 13:51

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