Introduction

Do you love playing on a seesaw? Why is it that depending on where you sit on the beam, and the weight of the person on the other side, you either fly up into the air or fall down to the ground? And why is it so difficult to perfectly balance the seesaw? It can all be explained with physics! In this activity, you will investigate the balancing forces of a seesaw—with a seesaw made of candles!

Time: 20-30 minutes

Key Concepts: Physics, gravity, force, lever, mass

 

 

Materials

  • Two identical birthday candles
  • Strong tape
  • Needle that is longer than the candle’s diameter
  • Aluminum foil
  • Knife
  • Two glasses the same height
  • Lighter or matches
  • Adult helper
Prep Work
  1. Tape the birthday candles together at their ends, so that both wicks are facing opposite directions.
  2. Put a large piece of aluminum foil on your work area to protect it from any wax spills.
  3. 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.
  4. Take the needle and push it 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
  1. Place the candle in the gap between the glasses so that the parts of the needle that are sticking out on each side of the candle rest on the rim of each glass. Question: Can you see the similarities between your experimental setup and a seesaw?
  2. 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. Question: Why does the needle have to be exactly in the middle of the candle to balance the seesaw?
  3. Make sure that your surface is covered with aluminum foil along the entire length of the candle.
  4. Once the candle is balanced and doesn’t drop down on either side, ask your adult helper to carefully light up both candles. Don’t light both candles at the same time—wait for a couple of seconds before you light the second one.
  5. Watch how both candles burn and observe the movement of your candle seesaw. Question: What happens? If you see movement, can you explain why the candle seesaw is moving?
  6. Once the candles burn down by about one fourth, blow out both candles and cut the top (approximately 1 cm or about ½ inch) of one of the candles.
  7. Place the candle between the glasses again, so that the parts of the needle that are sticking out on each side of the candle rest on the rim of each glass. Question: Is the seesaw still balanced as before? Why or why not?
  8. Again, ask your adult helper to light both candles the same way as before.
  9. Watch the candles burn and observe what happens. Question: How are your observations different this time?
  10. Make sure to blow out both candles before they burn completely.

 

 

What Happened?

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. As 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 the same. 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. However, if the needle is placed in the middle, 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 that 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 upwards, while 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 losing 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 at the same time, 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 dropdown. When you lit the candles, you probably noticed that the candle seesaw didn’t move at all, as the longer side will always be heavier while both candles are burning. However, if you would have 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.

Credits
Svenja Lohner, PhD, Science Buddies