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Sail Fans

I got the idea for this experiment while driving around town. We were driving across the Bridge of Lions in St. Augustine, and I pointed out a sailboat that had a large fan-like propeller sticking up behind the sail. The propeller is attached to a generator, using the wind to recharge the batteries. We joked about it being a fan to provide wind if the breeze died down, which lead to a discussion about what would happen if you tried that. That lead to this experiment.

To find out what would happen if you did mount a big fan to blow on the sails, you will need:

  • string
  • tape
  • a paper cup
  • a large balloon
  • three drinking straws
  • two chairs
  • a clothespin or chip clip

Tie one end of the string to the back of a chair. Thread the string through two of the drinking straws and then tie the other end to the back of the other chair. Move the chairs apart until the string is tight. The straws should now slide easily back and forth on the string.

Since most people don't have miniature electric fans sitting around the house, we will use a balloon instead. Blow up the balloon. Twist the mouth of the balloon several times and clamp it with the clothespin. That should keep it inflated. Tape the balloon to one of the straws, so that the mouth of the balloon points towards the other straw on the string. Test it by releasing the clip. The air should rush out of the balloon, pushing the ballon along the string in the opposite direction.

OK that makes sense. As the air from the balloon is pushed one way, the balloon is pushed in the opposite direction with the same amount of force. That is how a jet engine works. It is also how a propeller plane works. The propeller pushes the air backwards, and the plane is pushed forwards.

Now we need a sail. The easiest sail I could think of was a paper cup. Tape the paper cup to the other straw, so that the opening of the cup is facing towards the mouth of the balloon. Blow into the cup and it should slide along the string. Some of the energy from the moving air is transferred to our "sail," moving our "boat" along the string.

Once you have both parts working, inflate the balloon again. Move the cup so that the mouth of the balloon is inside the cup. Release the balloon. What happens? The balloon zips one way and the cup zips the other.

The last step is to connect the balloon and the cup to each other. Just as the fan and the sail would be attached to the same boat, the balloon and the cup need to be connected. Take the third drinking straw. Tape one end of it to the straw the balloon is attached to. Move the balloon until the mouth is near the opening of the cup. Then tape the other end of the third straw to the cup.

Before you inflate the balloon and test it, make a guess about what you think will happen. Compare it to our imaginary sail boat with the big fan on the back. Would our boat go forward? Would it sit still? Would it go backwards?

Think about the forces involved. The balloon is being pushed backwards with a force equal to the force that is pushing the air forwards. Part of the energy of the air is transferred to the cup, but only part of it. Some of the energy is lost to air that is deflected out of the cup. Since we have more force pushing the balloon (fan) backwards than is pushing the cup (sail) forwards, our boat should move backwards!

OK, lets test our hypothesis and see whether it is correct. Inflate the balloon. Move the "boat" to the center of the string. Then release it. Which way did it go? Was our hypothesis correct? Does it make sense now that you can't use a fan behind the sail, unless of course, you want to go backwards.

Have a wonder filled week.