Category : Science Projects And Inventions
Cut notches along the edge of a pencil. Stick a pin through the centre of the cardboard and attach it to the pencil's eraser as in the top illustration. The hole in the "propeller"' must be a bit larger than the pin, in order to cut down on friction. In your left hand, hold the end of the pencil. With your right hand, rub the second pencil back and forth across the notches as in the second illustration. If the tip of your first finger slides along the right side of the notches, the cardboard "propeller" will rotate rapidly towards the left. If the pencil being rubbed is moved a trifle forward so that the tip of your thumb now slides along the left side of the notches, the "propeller" will stop and start turning towards the right! The movement of the right hand is undetectable. This allows you to command the propeller to rotate first one way and then the other, without revealing how you do it.
HOW DOES IT WORK?
The propeller spins because of the horizontal – or the back and forth – vibrations in the notched pencil as you run the second pencil over it. These horizontal vibrations have a specific oscillation pattern that creates – vertical or up and down – vibrations in the pin. The pin’s vertical vibrations take the form of circular or elliptical motion, and this motion causes the propeller to spin.
When you hold a finger against the left side of the notched pencil you force the horizontal vibrations to travel in one direction and create vertical vibrations in the same direction. This causes the propeller to spin left. When you hold a finger against the right of the notched pencil, the vibrations travel in the opposite direction and causes the propeller to spin right.
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