For instance, the particles in a gas are constantly moving, and thus occupy a different position at each moment of time; their momenta are also constantly changing as they collide with each other or with the container walls.
气体中的粒子不断移动，因此在每个时刻占据不同的位置；当它们相互碰撞或与容器壁碰撞时，它们的动量也在不断变化。