Science Fair: Finding a Balance

Much of nature is based on balance. When you walk, you maintain balance so you don't fall down. In the wild, animal populations are forced to stay in balance with the supply of food. If the food supply dwindles, the animal population does the same. Your parents spend time each month making sure their checkbooks are balanced, so there is enough money coming in to equal the money they have to pay for things like bills and food. In physics, finding a balance point is often as simple as finding the middle of an object. For example, if you were to place a broomstick on the table, you could easily measure from either end to find its middle. That, in all likelihood, would be its balance point, something you could test by holding a finger underneath it at that point to see if it balances. But what if the object you are trying to balance is irregular in shape, such as a baseball bat, or a golf club? Could you still find its middle, or center of gravity, simply by measuring? This experiment will answer that question for you and might give you some ideas for how to extend this idea even further.

CENTER OF GRAVITY: The balance point of an object, regardless of its size or shape.

Question: How do you find an object's center of gravity?

Experiment Overview

You will begin with a yardstick (a meter stick is fine, too), and will perform some simple tests to verify that the center of gravity of the stick is indeed in its exact middle. Then you will be adding weight to various locations along the yardstick and will repeat the experiment to find the new center of gravity. Once you have mastered the technique with a yardstick, you will be challenged to try it on irregularly shaped objects you might have lying around your house.

Science Concept

Finding the center of gravity of an object can be very helpful. For example, if you were to swing a baseball bat, you could tell if there was too much weight in the end, and if there was, you could hold the bat farther from the bottom to make up for it. Also, if you had to carry something large and heavy by yourself, you would have to find the center of gravity and make sure it was supported. Otherwise, as soon as you started to carry it, it would twist and fall to the ground.

When working with a plain yardstick, as you move your hands closer to the center, one hand will be holding up more of the yardstick's weight than the other. The second hand will therefore be able to slide toward the center of the stick. However, before long, that second hand will now be supporting more of the yardstick's weight, and this will allow the first hand to begin sliding toward the center. Eventually, you will find that both hands will slide to the exact center of the yardstick and it will be balanced at that point. Adding weights or changing to an irregular shape will not change this process.

Materials

  • Yardstick

  • Molding clay

  • Various irregularly shaped objects, preferably long and fairly thin (A baseball bat would be a good example.)

Procedure

1. Hold the yardstick in front of you, resting on your two index fingers. Your index fingers should be placed near the ends of the yardstick.

2. With your eyes closed, slowly move your two index fingers toward the center of the yardstick. They will take turns sliding, one stopping while the other slides, and then the first beginning to slide. They should meet in the center of the yardstick.

3. Try this experiment again, placing your fingers at different locations to begin with. Convince yourself that they will always meet in the exact middle of the yardstick.

4. Now form a palm-sized ball of clay and place it somewhere on the yardstick other than the middle.

5. Repeat step 2 until your fingers meet. This is the new center of gravity of the yardstick. You can confirm this by the fact that the stick balances when your fingers are placed there.

6. Repeat steps 4 and 5 with various amounts of clay, placing them at various locations on the yardstick. Each time, your fingers should end up at the center of gravity of your yardstick.

Questions for the Scientist

1. After completing this experiment, give a definition in your own words for the phrase “center of gravity.”

2. After placing a ball of clay on the yardstick, try to predict the center of gravity that it will produce. How close was your prediction?

3. Why do you suppose your fingers do not slide at the same time, but instead take turns sliding toward the center of the stick?

4. In what sorts of situations, other than those listed here, would it be helpful to know an object's center of gravity?

5. In addition to weight, what other factors might affect the amount of friction your fingers experience?

6. Can you think of an object for which this process would not work?

Conclusion

Knowing the center of gravity of anything that is manufactured, from a baseball bat to a golf club, makes it more reliable, and safer and easier to use. Engineers work very hard to make sure that products they design and build are balanced, easy to operate, and safe for those who buy them. For example, if a car's center of gravity is located too high above the road, it could easily tip over when turning a corner. See if you can look for other ways to find the center of gravity of objects you encounter every day. You may be surprised at how easy it is to do once you know what you are looking for.

Did You Know?

All cars can roll over, but the taller a car is the more likely it is to be involved in a roll-over accident. Forty percent of roll-over accidents in which someone dies involve excessive speeding.

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