The Fan Cart

The classic physics problem, the action-reaction pairs in Newton’s Third Law can be explored from one of the projects I have made at The Exploratorium Summer Institute Teacher Training Program.

Let us ask a question to ourselves: “If a sailboat is stuck 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?” The answer to this question can be solved by constructing a “Fan Cart” using simple materials, e.g. a cart, a motor, 4 CDs, a few drinking straws, a fan, a sail, straight round sticks, Velcro fasteners, a pair of small batteries and a battery case.

Make the fan cart look like the one in the pictures or you can design your own. 

Now notice the following observations:

1. Attach the sail and then attach the fan to the cart with Velcro so that it will blow air towards the sail when it is running. Turn on the fan, and observe what happens.

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 that the fan will blow air away from the sail when it is running. Turn on the fan, and observe what happens.

3. Remove the fan assembly, and hold it in your hand while it blows air towards the sail. Observe what happens.

4. Replace the fan assembly so that it will blow air towards the sail when it is running, but then remove the whole sail assembly. Turn on the fan, and observe what happens.

5. Return to the original assembly, with the fan and sail both attached to the cart, and the fan blowing air towards the sail. Now insert a stiff piece of paper between the fan and the sail, and observe what happens.

What's going on?

Here is a summary of the first result from the situations above:

1. Cart doesn't move.

The behavior of the cart is a classic example of Newton's Third Law: For every action, there is an equal and opposite reaction.

In case 1, the fan pushes the air forward, and the air pushes the fan backward. A crucial thing to keep in mind is that the 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 backwards; the fan would be pushed backward, and since it's attached to the cart, the cart would be pushed backwards also.

Try to identify the action-reaction pairs in cases 2, 3, 4 and 5 and use them to predict why the cart behaves as it does.