Circus-Trick Science: How to Balance Anything
IntroductionHave you ever wondered why it is harder to keep your balance with a heavy backpack on? Or why it is difficult to make a toddler's sippy cup tip over? Maybe you are the kind of person who wonders about circus balancing acts and would like to learn how to ride a bike on a rope. Or perhaps you want to know how to make your toy car less prone to toppling over when racing through a sharp curve. In this science activity, you will get to investigate balance using marshmallows, skewers, and toothpicks. Sticky, yummy balancing fun!
- Large marshmallows
- Bamboo skewers
- Paper and a pen or pencil
- On a skewer, place one marshmallow in the middle and one on each end. Place the tip of your finger under the middle marshmallow and try to balance the structure on your finger.Can you balance it? Is it easy or difficult?
- If you cannot balance it, move the middle marshmallow a bit to one side or the other until you find just the right spot that enables you to balance the structure with your finger. We will call this spot the original balance point.
- Poke a toothpick into each end marshmallow. Add one marshmallow to the end of each toothpick and connect the two new marshmallows with a skewer so you get a rectangular shape (with a marshmallow on each corner and one marshmallow on the center of one skewer).
- Place your finger under the original balance point on your new rectangular structure (with most of the structure hanging below this point) and try to balance it on your finger. Can you balance it? Is it easier or more difficult than balancing the simpler structure? If you cannot balance this structure, move the middle marshmallow a bit to one side or the other until you can balance the structure.
- Draw the structure on a piece of paper and then try to balance the structure by putting your finger under the original balance point again. Is the structure completely horizontal, or is it tilted to one side? Rotate the paper your drawing is on so that it has the same angle as your actual structure. On your drawing, mark where your finger is with an "X" and draw a vertical line down from the X.
- Now balance the structure by putting your finger under one of the toothpicks. Is the structure hanging down completely vertically, or is it tilted? Rotate the paper your drawing is on so that it has the same angle as your actual structure. Make an "X" where your finger is and draw a vertical line down from the X.
- Now balance the structure by putting your finger under one of the two corner marshmallows that are farthest away from the middle marshmallow. Is the structure hanging down completely vertically, or is it tilted? Rotate the paper your drawing is on so that it has the same angle as your actual structure. Make an "X" where your finger is and draw a vertical line down from the X.
- On your drawing, the area where the three lines intersect is the center of mass of your structure.Where is the center of mass of your structure? Where is it in relationship to the middle marshmallow?
- Reorient your structure so that the middle marshmallow is on top. On the middle marshmallow, use toothpicks to attach two more marshmallows, clustered near the middle marshmallow. Place your finger under the original balance point and try to balance it on your finger. Can you balance it? Is it easier or more difficult than balancing the previous structure?
- Repeat the drawing steps you did before to determine where the three lines intersect and figure out where the center of the mass is now.Based on your new drawing, where is the center of mass of your new structure? Where is it in relationship to the middle marshmallow? If it has changed, how has it changed? Why do you think this is?
What Happened?You should have felt like the 5-marshamallow rectangular structure you made was the easiest to balance. In that structure the center of mass was just below the middle marshmallow. When you added two more marshmallows above, the center of mass shifted up and the structure became harder to balance.
An object will be stably balanced if you can make a (virtual) vertical line pass from its center of mass passes through the balance point. In this activity, the balance point was the spot you marked with an "X," where you finger was, and the vertical line is the same as the one you drew. (This vertical line is also known as the plumb line.) By finding where the lines intersected, you could figure out where the structure's center of mass was. You should have found that the first rectangular structure had a center of mass below the middle marshmallow. When you added two marshmallows on top of the middle marshmallow, the structure's center of mass should have shifted to be higher up, within the middle marshmallow. Because an object should be more stably balanced if its center of mass is under the balance point, but unstable if its center of mass is above the balance point, the first structure you made (with just three marshmallows) should have been less stable than the rectangular structures you made, and likewise the original rectangular structure should have been more stable than the one with two marshmallows added.
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For Further Exploration
- Try building some other structures using marshmallows, toothpicks, and skewers. Where is the center of mass for your other structures? Can you alter it by adding marshmallows to specific places?
- Find some non-symmetrical objects, such as a toy truck, and see how their distribution of mass affects how stable they are. If you park a toy truck on a hill, how will its distribution of mass influence its balance?
- As you did at the beginning of this activity, take a skewer and place one marshmallow in the middle and one on each end. Use toothpicks to stack marshmallows on top of the middle marshmallow and try to balance the structure on your finger each time you add another marshmallow. How many marshmallows can you add until you can no longer balance the structure? Make other structures and similarly try stacking marshmallows until it is no longer stable. Can other structures hold more stacked marshmallows, indicating that the basic structure may be more stable?