Knight Foundation Summer Institute

Emily Dorean, Haverford College


Physical science is an important starting point for all sciences; it covers motion, forces, energy, matter, heat, light, sound, and atoms. All other sciences are based on physics at an empirical level. In this lab, students will be investigating an important physical concept, balance, that effects their lives everyday. I have chosen to use sports and physical activities as the basis on which the students can then build their understanding of balance. In this lab, students will be able to understand some of the physics that explains their performance in physical activity. They may also be able to learn some tactics for enhancing their own performance!

Balance is one of the processes our body takes care of without our even realizing it. Yet it is amazing to think how much a part of our daily lives it is! We need balance to sit, to stand, to walk, to run, and to work. Our bodies have a complicated and delicate system for helping us to maintain this balance. This skill can be improved through practice. For example, athletes learn to practice this within their sport in order to increase their performance level. What sports can you think of that require balance? The students may come up with the following: shooting foul shots in basketball, swinging a baseball bat, tumbling in gymnastics, ice skating etc.

So how does someone learn to understand how balance effects their performance. Well, the key to balance is the center of mass. This is what determines how you balance. By shifting your body weight, or your mass, around, you are also shifting the center of that mass, and your body has to compensate quickly. The mechanism our bodies have for dealing with these changes is described below.

"When you spin around and make yourself dizzy, you are confusing your body's system for keeping itself balanced. This balancing system involves the inner ear and the brain. Look at this picture. It shows the outer ear on the left and the many different parts of the inner ear on the right. Notice the three tubes called the semi-circular canals of the inner ear. At the ends of these tubes are bulb-like structures that each contain a little sack called an ampulla. Just below the three ampullas is another larger bulb-like structure, which contains another sack called the utricle.

The ampullas and utricle help your body keep its balance by sending nerve messages to the brain about the position and movement of your head. Here's how it works: The ampullas and utricle are filled with a thick fluid. They also have tiny fine hairs attached to their inner walls that stick into the fluid. The hears are attached to nerves that go to the brain. When you tilt or rotate your head in a certain direction, the fluid pushes against the hairs and causes nerve signals to go to the brain. The brain recognizes the signals as meaning that the head has moved in a particular way and is in a particular position. You use this information to adjust the movements of your body to maintain your balance." (taken from Wonder Science magazine, April 1996, v. 10 no.7)

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The first part of this lesson is a couple of demonstrations to emphasize the relationship between the center of mass and balance. Then the students will work in pairs to attempt to define for themselves what "center of mass" means and how to locate it.


  1. to discover through experimentation what "center of mass" means
  2. to appreciate how science can explain events we encounter in our daily lives
  3. to develop a basic understanding of how the body reacts to changes in balance


center of mass

balancing point


Demonstration 1: The Chair Lift

  1. Sit in a chair with your back against the back of the chair and your hands on the seat beneath you.
  2. Try to stand up from the chair without leaning forward at all. Is it possible? What do you think you will need to do in order to stand up?
  3. Try to stand up again, bending a little forward at the waist, and keeping your back straight. Keep leaning forward until you are able to stand up. How far forward did you have to bend in order to stand up? What does this have to do with the center of mass? What did you find helped you to stand up without bending as much?

It may be a good idea to jot down some notes on the board to answer these questions. They will be useful later for the students to come back as they try to understand exactly what the center of mass is.

Demonstration 2: The Foot Lift

  1. Stand sideways along a wall with your foot, hip, shoulder, and arm all touching the wall.
  2. Keeping all these parts on the wall, try to pick up your outside foot.
  3. Can you do it? Why or why not? Do you know any other ways to test the importance of the center of gravity? Where do you think your center of mass is? Is this different when you are standing? When you are sitting?


This can be presented as a game, in which the partners compare the proximity of their choice for the center of mass with the actual center of mass. Separate the students into pairs, give each pair an index card and 8 paper clips.

  1. Each student should take a turn marking on your index card where you predict the center of mass is. To test their hypothesis try to balance the index card on the eraser end of a pencil or on the tip of the index finger. If the card balances on the spot you marked then the guess for the center of mass is correct! Who came the closest to guessing the correct point? If you weren't right the first time, keep trying until the card does balance.
  2. Now add a paper clip to one corner of the index card.
  3. Each student should take a turn marking on the index card where the center of mass is now.
  4. Test to see if the predictions are correct. Who is closest? If you aren't right the first time, make another guess.
  5. Next, add another paper clip to the index card in a different spot. (See diagram below.) Can you predict where the pencil needs to be placed in order to balance the card?

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  1. Continue this procedure, placing the paper clips in different places, until you have experimented with all eight paper clips in several positions.


This project is meant to help the students understand a basic concept. Therefore, an assessment could include having the students write out their own definitions for "center of mass" and "balance". Essentially, for an object to balance, its center of mass must be over the support point.

Have the students compile a descriptive list of the importance of balance in our daily lives -- not just in sports but in other areas as well. This could include writing a short essay on what it would be like if we had no balance.

Return to the demonstrations presented at the beginning of class. The students should be able to explain what is happening in each demonstration now that they understand the concept of center of mass.


One interesting activity is to have the students design an experiment so that they could test where the center of mass is for other objects. For example, if an object is hung from a string, the object will rotate until its center of mass is directly underneath the string. Try it for a simple object whose center of mass can easily be predicted. First, attach the string directly above the center of gravity. Next attach the string off to the side from the center of mass and watch the object rotate.

Another really fun activity is "Balancing Bernie". Again the idea is to try to balance the figure. Give the students two paper clips and the attached figure to cut out of strong paper. Prompt them to figure out how to balance Bernie on his head. Have them place Bernie's head on their forefinger and try to balance him. Does it work? How can you add the paper clips to make the center of mass right where your finger is? Basically what is happening here is that when Bernie does not have paper clips in his hand, his center of mass is above your index finger (i.e. in his neck area). But when you add the paper clips, his center of mass is lowered. It is now on the other side of his head; therefore, he can balance on your finger.

An interesting application of the center of mass concept can be found in the sport of track. Ask the students if they have ever watched how a runner begins a sprinting event such as the 100 meter dash. Perhaps the students will realize that the smart runners crouch down on their hands and feet and lean forward so that their center of gravity is well beyond their feet. This extreme position makes the runners feel as if they will fall. However, they practice their start enough times to be able to time it perfectly so that just when they think they cannot balance any longer, the gun goes off, and the runner quickly brings their feet forward. (Remember, the only way you can balance is if your center of gravity is above your base. In the activity above, the center of mass was the pencil or your index finger.) In this case, the support base is the runner's feet. The fact that the runners have to bring the feet forward quickly to keep from falling enables them to push off with greater force and therefore gather speed more quickly in the race.

Philadelphia Science Content Standards:


This experiment satisfies Benchmark 1 for grades 5-8: "design, modify, and conduct an investigation through testing, revising, and occasionally discarding ideas, all of which lead to a better understanding of how things work."


This experiment satisfies Benchmark 7 for grades 5-8: "describe how all organisms maintain a relatively stable internal environment while living in a constantly changing external environment."


This experiment satisfies Benchmark 5 for grades 5-8: "investigate how interactions among the senses, nerves, and brain make learning possible."


This lesson would work well in a physics unit. It would also fit into a unit on the nervous system of the human body. Or it could simply be a fun activity to show students how science does fit into their everyday lives.

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