Consider the arrangement in upper figure. When a particle of mass m that is travelling with a component of velocity νx parallel to the x-axis collides with the wall on the right and is reflected, its linear momentum (the product of its mass and its velocity) changes from mνx before the collision to −mνx after the collision (when it is travelling in the opposite direction). The x-component of momentum therefore changes by -2mνx on each collision (the y- and z-components are unchanged).