Here is a summary of the results to be expected for the investigations above:
Cart doesn't move.
Cart goes forward.
Cart goes forward.
Cart goes backward.
Initially the cart doesn’t move, but when the file folder or paper is in place, the cart moves backward.
The behavior of the cart is a classic example of Newton’s Third Law: For every action, there is an equal and opposite reaction.
Investigation 1: The fan pushes the air forward, and the air pushes the fan backward. (A crucial thing to keep in mind is that action and reaction forces, often called an action-reaction pair, do not act on the same object.) If this was all that was happening, the cart would move backward: The fan is being pushed backward, and since it’s attached to the cart, the cart is also pushed backward. But the sail is set in place, so there is a second action-reaction pair, with the air pushing forward on the sail, and the sail pushing backward on the air. In this situation, there ends up being two forces exerted on the cart: the air pushing backward on the fan, and the air pushing forward on the sail. These two forces balance each other, and the cart doesn't move.
Try to identify the action-reaction pairs in Investigations 2, 3, 4 and 5, and use them to predict why the cart behaves as it does. Compare your expectations and results with the information offered in the discussions below.
Investigation 2: With the air direction reversed (either by physically reversing the fan or by reversing the electrical connections to the fan), the fan pushes the air backward and the air pushes the fan forward. Since the fan is attached to the cart, the cart moves forward.
Investigation 3: The fan pushes the air forward and the air pushes the fan backward. But because the fan is held in your hand and is not attached to the cart, there is no effect on the cart for this action-reaction pair. The air pushes the sail forward and the sail pushes the air backward. Since only the sail is attached to the cart, the cart moves forward.
Investigation 4: The fan pushes the air forward, and the air pushes the fan backward. Since the fan is attached to the cart, the cart is pushed backward.
Investigation 5: Initially, the cart is in the same configuration as in Investigation 1, so it doesn’t move. However, when a barrier is inserted between the fan and the sail (without touching either one), then the first action-reaction pair is still the same as in Investigation 1: The fan pushes the air forward, and the air pushes the fan backward. In the second action-reaction pair, the air pushes the file folder forward, and the file folder pushes the air backward. But the only force in the two reaction pairs that acts on the cart itself is the force on the fan, which is connected to the cart, so the cart moves backward.