Work Step by Step
Hitting the rug causes it (dust in it) to move forward.Both have inertia. The metal frame exerts a force on the rug in the opposite direction, overcoming the inertia of the rug and preventing it to move further forward.No sufficient force acts on the dust to counter its inertia, so it continues moving. With the baseball, having been acted upon by the thrower's hand gains inertia. After leaving the hand, no counterforce acts to overcome its inertia, so it keeps on moving.