Key concepts
Physics
Force
Gravity
Mass
Lever
Introduction
When the days grow shorter and it gets dark early in the evening many people enjoy candlelight. Candles are also a great tool for doing science—so why not combine both and add a little light to your science? In this activity you will use candles to investigate the balancing forces of a seesaw.
Background
Seesaws are classic playground equipment for children. They are not only fun to play on but also provide an excellent opportunity to explore a type of simple machine designed to lift objects much heavier than your strength would normally allow. A seesaw is a specific type of lever; it consists of a long beam attached to a pivot called the fulcrum. As soon as you put weight on one end by sitting on one side of the beam it drops to the ground. This is because the force of gravity is acting on the weight of your body, pulling it and the beam down. How much weight is pulling down on the beam depends on your body mass. The heavier you are, the larger the gravitational force. To balance the beam again, you need a counteracting force on the other side. One possibility is to place a second person at the same weight on the other side of the beam. Once the same force is pulling down on each side of the beam the seesaw is balanced.
You might know from experience, however, it is not only weight that matters but also where people are seated along the beam. Two people with the same body mass sitting on opposite sides of the beam can still change the balance and make the seesaw rotate by moving farther from or closer to the beam’s center. The beam’s rotation is caused by the turning force, also called torque, which takes into account the force pulling down on the beam and location of the applied force.
According to the law of the lever, the seesaw will stop rotating once the turning forces are equal on both sides and cancel each other out. This law also explains why you are able to lift very heavy objects with a lever. When the object on one side is too heavy for you to lift you just have to move farther away from the beam’s center until your turning force is greater than the one on the other side.
Who would have thought such a simple activity would entail such complex physics? Don’t worry if this sounds complicated; it will become more obvious once you try this project—using a seesaw made of candles!
Materials
- Two identical birthday candles
- Strong tape
- Needle that is longer than the candle's diameter
- Aluminum foil
- Knife
- Two glasses of the same height
- Lighter
- Adult helper
- Safe location and surface for using candles
Preparation
- Tape the birthday candles together at their ends so both wicks are facing opposite directions.
- Put a large piece of aluminum foil on your work area to protect it from any wax spills.
- Set the two glasses next to each other in the middle of the aluminum foil. The gap between the glasses should be small enough to place the needle across it.
- Have an adult push the needle all the way through the side of the candle exactly where the ends of both candles meet. This should be exactly in the middle between both wicks. If it is too difficult to push the needle through the candle, try to heat the needle in a flame before you push it through the wax.
Procedure
- Place the candle in the gap between the glasses so the parts of the needle that are sticking out on each side of the candle rest on the rim of each glass. What do you notice once the candle is placed between the glasses? Can you see the similarities between your experimental setup and a seesaw? Where is the fulcrum of your candle seesaw?
- If your candle seesaw is unbalanced, change the location of the needle in the candle. Once the needle is placed exactly in the middle of the two candles, the seesaw should be balanced. Why does the needle have to be exactly in the middle of the candle to balance the seesaw?
- Make sure your surface is covered with aluminum foil along the entire length of the candle.
- Once the candle is balanced and doesn’t drop down on either side, ask your adult helper to carefully light both candles. Don’t light both candles at the same time—wait for a couple of seconds before you light the second one. Do you think it would make a difference if both candles were lighted simultaneously?
- Watch how both candles burn and observe the movement of your candle seesaw. What happens with the seesaw after a while? Does it stay balanced or does it start moving? If you see movement, can you explain why the candle seesaw is moving?
- Once the candles burn down by about one fourth, blow out both candles and cut the top (approximately one centimeter, or about one-half inch) of one of the candles. How do you think this will affect the seesaw balance?
- Place the candle between the glasses again, so the parts of the needle that are sticking out on each side of the candle rest on the rim of each glass. Is the seesaw still balanced as before? Why or why not?
- Ask your adult helper to light both candles the same way as before.
- Watch the candles burn and observe what happens. Does the seesaw move again? How are your results different from the previous observations?
- Make sure to blow out both candles before they burn completely.
- Extra: Repeat the activity, and this time don’t place the needle in the middle of the candle beam. One side should be heavier, similar to when you cut the top of one candle. This time, however, only light the candle on the longer side. Then watch the candle burn. What happens to the candle seesaw this time? Does it start moving? Can you explain your observations?
Observations and results
Did you notice that what you were building resembled a seesaw? Both candles taped together formed a long beam that was attached to the needle, which acted as the pivot (or fulcrum). The candle beam was able to rotate freely from one side to the other just like a real seesaw. Because you don’t put any extra weight on the candle beam as you would on a playground seesaw, the only force pulling down on the beam is the weight of the candle itself. To balance the seesaw it is important that both forces pulling down on each side of the beam are exactly equal. This is only true if the needle is placed exactly in the middle of the candle beam. If one side is slightly longer, this would also make it heavier and it would drop down, as you might have observed. If the needle is placed in the middle, however, the gravitational forces pulling down on each side should cancel each other out and it should stay balanced.
This changes once you light the candles. When the candle is burning a chemical reaction occurs that converts the candle wax to a gas. You probably also noticed the solid wax turned into a liquid and dripped onto the aluminum foil. The wax lost through burning and dripping makes the candle shorter and therefore lighter. As this side of the candle beam becomes lighter it moves upward whereas the other, heavier side drops down. The rotation is reversed once the other candle loses wax and becomes lighter again. The key to this seesaw movement is that both candles are not burning the same amount of wax at the same time. This is the reason why you have to light them one after the other. If both candles start burning at exactly the same time and lose the same amount of wax simultaneously, the candle seesaw would stay balanced.
If you cut part of the candle on one side, the longer, now heavier side of the beam would drop down. When you lit the candles you probably noticed the candle seesaw didn’t move at all, because the longer side will always be heavier while both candles are burning. If you did the extra test, however, and only lit up the candle on the heavier side, you would have noticed that as soon as the candle burnt down enough to make it shorter and lighter, the seesaw dropped down on the other side.
Cleanup
Make sure both candles are fully extinguished and dispose of them and the aluminum foil. If you do not want to reuse the needle, make sure to dispose of it in a sharps container.
More to explore
The Simple Physics of Seesaws, from MythBusters
Heavy Lifting with a Lever, from Scientific American
The Lever, from the Math and Science Activity Center
Balancing the Load: The See-Saw as a Simple Machine, from Science Buddies
Science Activity for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
