風扇同架車

Scenario (1)
A force is acting on the fan that pushes it (and the cart) to move forward.

Scenario (2)
A force is acting on the fan as in scenario (1).
Another force is acting on the paper card, which is equal in magnitude and opposite in direction to the force on the fan. Therefore, the cart does not move because it is subject to the action of two equal (in magnitude) but opposite (in direction) forces.