Kids, the “seesaw” candle is a fire science trick that teaches how combustion and Newton's Third Law of Motion work. A candle, balanced between a pair of glasses, seesaws up and down on its own as it burns. The most important thing you’ll need to try this trick is an adult partner. Have them get a long, thin candle, a needle, 2 glasses of the same height, aluminum foil, and watch them do this trick.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
First, protect the experiment area with a sheet of aluminum foil. Then, expose the wick at both ends of the candle. If it has some wick pressed onto the bottom of the wax, loosen it. If there isn't any wick then use a dull knife to cut away some wax. Push the needle all the way through the candle about halfway down its length, so half of the needle sticks out either side of the candle. If the candle wax is soft, the needle may push through easily, but if the wax is hard or thick, then grasp the needle with pliers or tweezers, heat it in a flame and push it through the candle. Use the needle as an axle to balance the candle between a pair of glasses. It is okay if one end of the candle is heavier than the other. Your adult partner will then light both ends of the candle. The candle will rock up and down, like a seesaw.
The candle moves in response to forces acting on it, trying to reach equilibrium. The combustion reaction turns the candle wax into carbon dioxide gas and water vapor, making the burning end of a candle lighter. If one side of the candle burns more quickly than the other, the lighter side of the candle moves up. The lower side of the candle is angled such that the flame melts the wax, causing it to drip down. This lessens the mass at that end of the candle, plus the force from the dripping wax actually pushes the end of the candle up! This is Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction.
Tips and Safety
This is a fire project, so have an adult perform all of the steps and stay away from curtains, pets, towels... you get the idea. Lighter candles respond more dramatically to changes in mass than heavier ones. You can watch a video of the project.
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Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
November 2012
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References:
Anne Marie Helmenstine at: http://chemistry.about.com/od/funfireprojects/a/Seesaw-Candle-Fire-Magic-Trick.htm
