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If a sailboat is stranded because there is no wind, is it possible to set up a fan on deck and blow wind into the sail to make the boat move? This is a classic physics problem that you can explore by using simple materials to build a low-friction cart with a removable motor and sail. The Fan Cart provides an elegant demonstration of action-reaction pairs described in Newton’s Third Law, and can also be used to demonstrate other aspects of force and motion.
Investigation 1: Attach the sail to the cart, and then attach the fan so it blows air toward the sail. Turn on the fan by attaching the battery clip leads to the motor, and observe what happens. (Note: If the air is blowing the wrong way, reverse the motor direction by switching the alligator clip connections.)
Investigation 2: Leave the sail in place, but remove the fan assembly and turn it around (or leave the fan assembly in place and reverse the electrical connections to the motor) so the fan blows air away from the sail. Turn on the fan, and observe what happens.
Investigation 3: Remove the fan assembly, and hold it in your hand while it blows air toward the sail. Observe what happens.
Investigation 4: Replace the fan assembly so that it blows air toward the sail, but then remove the whole sail assembly. Turn on the fan, and observe what happens.
Investigation 5: Return to the original situation, with the fan and sail both attached to the cart, and the fan blowing air toward the sail. Now hold a file folder or stiff piece of paper between the fan and the sail (but not touching either one) and observe what happens.
Here is a summary of the results to be expected for the investigations above:
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.
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Attribution: Exploratorium Teacher Institute