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Balancing Act: Finding Your Center of Gravity

Difficulty
Time Required Very Short (≤ 1 day)
Prerequisites None
Material Availability Readily available
Cost Very Low (under $20)
Safety Minor injury possible

Abstract

Like to have the balance of a tightrope walker? Try the more close–to–the–ground balancing test in this easy experiment to learn a few trade secrets of the high wire experts. In this project, you'll find your center of gravity and explore the physics of balance at the same time. No net required for this balancing act!

Objective

The goal of this project is to explore how changing your center of gravity affects how well you can balance.

Credits

Darlene E. Jenkins, Ph.D.

Sources

The idea for this project came from this DragonflyTV podcast:

Cite This Page

MLA Style

Science Buddies Staff. "Balancing Act: Finding Your Center of Gravity" Science Buddies. Science Buddies, 9 Oct. 2014. Web. 20 Dec. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Sports_p017.shtml>

APA Style

Science Buddies Staff. (2014, October 9). Balancing Act: Finding Your Center of Gravity. Retrieved December 20, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Sports_p017.shtml

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Last edit date: 2014-10-09

Introduction

The project video below focuses on three talented students at a circus arts school. The girls performed one of the most difficult circus acts, walking the tightrope. They knew from experience that it was easier to balance on the tightrope when they held a balancing pole in front of them instead of just using their arms stretched out to the sides. But they wondered if the length of the pole would make any difference in their ability to successfully walk the wobbly rope.

Watch DragonflyTV circus stunts video

Click here to watch a video of this investigation, produced by DragonflyTV and presented by pbskidsgo.org

The girls decided to develop an experiment to answer their question. Each of them attempted to walk across a low tightrope using poles of three different lengths, and they counted the number of times they wobbled or fell with each pole. They found their balance improved as the pole length got longer. They figured that holding a long balancing pole was like having "super-long arms" to help keep them upright on the rope.

In this project, you can do a similar experiment without having to worry about falling from a circus tightrope. In fact, your balancing challenge need only go as high as a roadside curb in these experiments. You'll stand on a curb with your heels hanging off of the edge, and ask an assistant to record the number of seconds you maintain your balance. If you find that's too easy a test, you can try to balance while slowly raising your heels up and down or stand on one foot to increase the difficulty.

This curbside balancing act will serve as a simple but useful test to see how changing arm position or holding a pole affects how long you can stay on the curb. Using a short pole or a long pole will help you measure the effect of pole size on your balance. You may not have the fancy costumes or the equipment of a circus performer, but in this project you'll be able to collect interesting data about balance just the same. Read on to see how to organize your experiments and get started on this fun, light-footed project.

As a first step, do some background research on the science that explains the physics of balance. We've provided a list of useful search terms and basic questions in the next section to get you started. One of the major concepts you will need to understand is the idea of center of gravity or center of mass. This is an imaginary point about which all weight (mass) is evenly distributed in an object or in our bodies. It's a little easier to think of the center of mass of a perfectly round object like a ball, because its center of mass is located at the centermost point of the ball. Our bodies are not evenly symmetrical in all directions, but for most people when they are standing, the body's center of gravity is midway between the stomach and back, about two inches below the belly button.

Athletes use the idea of center of gravity to improve their performance in all kinds of sports, dance, and martial arts. Generally, a lower center of gravity in the body means an increase in stability. That's why football players bend down, take a wide stance and shift their weight forward when they block and tackle. This position lowers the center of gravity in their bodies, makes their base of support broader, and makes it harder to push them over. The same idea applies to cars and buildings where engineers tailor designs to keep the center of gravity low to make safer, more stable vehicles and structures.

Whenever our center of gravity lies directly over the base of support (for example, our feet when standing), we remain perfectly balanced and steady. But every time we move, our center of gravity shifts in response to the change in our shape and the new distribution of mass. This means while standing almost any change in position comes with a risk of falling if our center of gravity shifts too far beyond the base support of our feet. In order to remain upright while doing something as simple as walking, our body must continuously compensate to the changing center of gravity by slightly adjusting our arms, head, and shoulders forward or backward to keep the center of gravity always directly above our moving feet. So there's some complicated mechanics going on even when one takes an easy stroll down the street. Imagine the rapid adjustments the body must automatically make when you try something as challenging as traversing a high wire.

For a high wire performer, the body's natural center of gravity must be kept directly over the wire in order to stay balanced. That's difficult because the base of support (usually just one foot on a thin wire) is so narrow and the wire is constantly moving. Such a limited base of support means just a little too much lean to one side or the other can cause a serious wobble and dramatically increase the risk of falling off the wire, hopefully into a safety net. Anything performers can do to lower their centers of gravity will make their walk on the wire easier. That's where the balancing poles come in. In your curbside experiments, see if you can figure out just how the position, weight, and length of a pole changes your center of gravity and increases or decreases your stability when trying to balance.

You also should notice how using the poles influences the speed of your wobbles on the curb. Physicists explain that bodies rotating around a fixed point, like a tightrope walker falling off (around) a fixed high wire, follow similar laws of torque and angular velocity. Angular velocity is basically how fast an object spins around a pivotal point. Torque is the amount of force that causes an object to rotate about that point. In a general sense, the speed of a spinning object varies in direct proportion to the torque applied to it and in inverse proportion to its length or distance from the pivot point. The more torque, the greater the spin. The longer the distance, the more time it takes the object to complete the entire circle around the pivot point, and the slower the spin.

For a tightrope walker, a foot on the rope represents the pivotal point. When she is not carrying a balancing pole, her head or the fingertips of her outstretched arms mark the maximum distance from the pivot point. Adding long poles to a tightrope walker essentially extends the distance from the pivot point out to the limits of the ends of the poles. So the circus school students in the video were not far off when they said they thought their balancing poles acted like "super-long arms." Check out how well the poles you carry in your experiments serve as long arm extensions and if they slow the speed of your wobble when you try to balance on the curb.

You'll find a few recommended activities on finding your center of gravity in the Bibliography section. Do at least two of the activities before you start your experiments. They will quickly demonstrate how your center of gravity changes as you move or an object's center of mass shifts when you change its shape or weight.

Good luck, have fun, and watch that wobble!

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

  • Mass
  • Gravity
  • Center of gravity (or center of mass)
  • Rotational motion
  • Torque
  • Angular velocity
  • Physics of tightrope balancing

Questions

  • What is the difference between mass and weight?
  • What is the center of gravity (center of mass) of a spherical object? of an irregularly shaped object?
  • How does center of gravity relate to balance?
  • Where is the approximate center of gravity of a human when standing? when sitting?
  • How do the long poles used by a tightrope walker make balancing easier?
  • What is torque? How does torque influence a tightrope walker?
  • How does a tightrope walker's center of gravity change with the position or an increase in length of a balancing pole?

Bibliography

As part of your research and preparation, do at least two of the following activities to become better acquainted with the concept of center of gravity:

Here are some additional websites you might want to check out as you start your research:

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Materials and Equipment

To do this experiment you will need the following materials and equipment:

  • Stop watch or timer that measures seconds
  • Sidewalk curb with at least 3 meters (approx. 9 feet) clearance on either side
  • An assistant to take times and spot you, if needed
  • Two poles of the same material
    • One pole should be at least 2 meters (approx. 6 feet) longer than the other.
    • You can use wood poles, PVC pipe, or small diameter plumbing pipe.
  • Notebook
  • Pen or pencil

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Experimental Procedure

  1. Gather your materials and arrange with your assistant the day and time of your experiment.
  2. The day of your experiments, first practice balancing without poles while standing on the edge of the curb facing the sidewalk. Your heels should not touch the curb.
  3. If you find this too easy, balance while slowly raising your heels up and down, or balance on one foot for your trials. Decide which type of balance test you want to do in your experiments.
  4. Prepare a data table similar to the example shown below.
  5. Perform the three experiments listed in the table. The first experiment involves balancing while placing your arms in three different positions. The second and third experiments involve balancing with a short or long pole held close to your body at three different levels. Do five balancing trials for each position in all experiments.
  6. Your assistant should record how long, in seconds, you stay on the curb for each trial.
  7. Also note how stable you feel while balancing in each position. Record a "wobble rating" for each position (i.e. slow = steady, medium = less steady, fast = unsteady).
Balancing Experiments Data Table
Name:   Date:
Pole Material:   Location:
Experiment No.
& Positions
Balance Time (sec) Wobble Rating
(Slow, Med, Fast)
Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Total
Exp 1. Arms Only              
A. Arms down at sides              
B. Arms out to sides, shoulder level              
C. Arms above head              
Experiment No.
& Positions
Balance Time (sec) Wobble Rating
(Slow, Med, Fast)
Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Total
Exp 2. Short Pole Only              
A. Pole at waist              
B. Pole at shoulder level              
C. Pole above head              
Experiment No.
& Positions
Balance Time (sec) Wobble Rating
(Slow, Med, Fast)
Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Total
Exp 3. Long Pole Only              
A. Pole at waist              
B. Pole at shoulder level              
C. Pole above head              

Analyzing Your Data

  1. Total the seconds for all trials. Calculate an average balance time for each position.
  2. Use the three average times within each experiment to calculate a "total average time" for Experiments 1, 2, and 3.
  3. Prepare a bar chart showing the average and "total average" times for Experiments 1, 2, and 3. Group the the data by experiment number so you can more easily compare the results between the three experiments.
  4. What positions were best and worst for balancing? Were they the same in all three experiments?
  5. Which experiment showed the longest average times? Were you surprised by your results?
  6. What did you notice about the speed of wobbles when using no poles, a short pole, or a long pole?
  7. Construct stick figure diagrams to represent your positions in each experiment. Indicate where you think the center of gravity is located on each stick figure. Hint: Start with the center of gravity at the belly button for Experiment 1A. Then show on other stick figures how the center of gravity moves up or down when the arms or poles are added in the next positions or experiments.
  8. What do you notice about the placement of the center of gravity and the number of seconds you could balance in each position?
  9. For help with data analysis and setting up tables, see Data Analysis & Graphs.
  10. For a guide on how to summarize your results and write conclusions based on your data, see Conclusions.

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Variations

  • More data. Have your assistant or a few friends try the same experiments. Are the results similiar? If not, try to explain why.
  • Heavy lifting. Repeat the experiments using two soup cans as weights. Hold them in your hands or duct tape them to the ends of the poles. (You can also use wrapping flexible ankle or wrist weights if you have them.) How does the additional weight affect your results? Adjust your placement of the center of gravity in the stick figures you drew to show how it shifts when weights are involved in each position.
  • Change your view point. Repeat the experiments with your head turned to the side or with your gaze upward. Be sure to have your assistant close by in case you need a spotter for some of the balancing tests. Compare these results with your results from the original three experiments. How do visual cues help us in balance? Do some research into how the eyes and ears are important for balance.
  • Stay on the beam. Repeat the experiments using a wooden 2x4 beam nailed into the grass to keep it from wobbling. Record the number of times you can travel up and back. Are your results on balance while traveling along the balance beam similar to the results you got when you balanced while standing on a curb? For an experiment that describes how to make this type of balance beam and includes various tests to try, see
    "Balancing Act,"http://www.darylscience.com/Demos/TightRope.html.
  • Baby steps. Research how a baby learns to walk. Investigate the importance of the ratio of the baby's head size to body size during the first couple of years and how the ratio influences center of gravity and the ability to walk.
  • Do the math. For students interested in mathematical descriptions of balance, look up the equations for torque, angular velocity, and angular acceleration. Calculate the changes in angular acceleration when a tightrope walker of your height wobbles or falls without a pole and then with poles of various lengths.

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