Is expansion of gases a violation of inertia law?

The first Newton's law (also called inertia law) says that, when viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a net force.

Let us suppose that we have a box with a wall splitting it in two halves. The left half contains a rest gas (no wind inside), while the right half is empty. Suddenly, we remove the wall. Because of the fact that gases expand to fill their containers, we know that the gas of the left half will move to the right side in order to fill the whole box:
But there is no net force acting on the gas, so why has it moved to the right half of the box? Do gases violate the inertia law?

Are you able to give an explanation? Accept the challenge!


  1. 'No wind inside' means that the gas is macroscopically at rest, but microscopically is made of molecules or atoms moving at different velocities, although there are as many molecules moving to the right as molecules moving to the left. Gas pressure is due to the impacts, on the wall, of those molecules. But if we remove the wall, those molecules will continue to move at a constant velocity, so they will fill the whole box while inertia law is not violated.

    1. Very good!
      This is a very useful cognitive conflict in order to motivate students to learn the kinetic theory of gases in freshman year (9th grade)