by STEM Scouts
Watch our experiment on the STEM Scouts Facebook page.
The rocking candle experiment 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, rocks or seesaws up and down on its own. The motion continues as long as the candle continues to burn. If one side of the candle starts out heavier than the other, the motion of the candle will act to equalize the mass on either side of the pivot point.
It’s a simple trick but eye-catching and interesting!
- Long candle
- 2 glasses or jars that are the same height
Long, thin candles work best for this trick. You can even use a pair of candles that are connected to each other.
- The first step is to expose wick at both ends of the candle so take a look at the bottom of the candle. If it has some wick pressed onto the bottom of the wax, loosen it so that you will be able to light it. On the other hand, if there isn’t any wick at the bottom of the candle, use a knife to cut away enough of the candle to expose wick. You don’t need a particularly sharp knife. In fact, it’s better to use a dull knife so that you don’t accidentally cut the wick.
- Push the needle through the candle about halfway down its length. You don’t have to be exact, but if you aren’t very good at gauging halfway points, then use a ruler to measure your candle, divide that number by two and push the needle through the candle at that point. If the candle wax is soft, you may be able to push the needle through the candle with minimal effort, but if the wax is hard or your candle is thick, then grasp the needle with pliers or tweezers, heat it in a flame and push it through the candle. The hot needle should pass through the wax fairly easily. The trick still works if you accidentally bend the needle.
- Balance the needle and candle between a pair of glasses. It is okay if one end of the candle is heavier than the other.
How Does It Work?
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 not only lessens the mass at that end of the candle, but 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 use adult supervision and avoid trying this trick near curtains, pets, gasoline… you get the picture.
- Lighter candles respond more dramatically to changes in mass than heavier ones. That is, lighter candles will give you a better range of motion than heavier candles. If you use a big candle, you won’t see much motion at all.