What would happen if you played on a teeter totter under water?
Learning Objectives
After completing this lesson, the student will be able to identify the center of gravity and center of buoyancy of a body.
Standards
- NGSS HS-ETS1-3
- CCSS.Math.Practice.MP1
- CCSS.Math.Practice.MP2
- CCSS.Math.Practice.MP4
Supplies
- Balloon
- Helium
- Putty
Units Used
- Mass: kilogram (kg)
- Length: meter (m)
- Time: second (s)
- Force: Newton (N) (1 N=1 kg m/s2)
Terminology
A couple key terms you’ll use with balloons and ships are:
- Center of Gravity: The center of gravity is the point about which all weights are evenly spread, so both the magnitude of the weight and the location of the weight matter. On a see-saw for example, a heavier rider close to the center can be balanced by a lighter rider further from the center. For your ellipsoidal hull, where is its center of gravity?
- Center of Buoyancy: The center of buoyancy is the center of mass of the displaced fluid. For an aircraft carrier floating in water, the center of buoyancy is the centroid of the displaced water. In your case, instead of a ship displacing water, you have a helium filled hull displacing air. Where is the centroid of the air displaced by your ellipsoidal hull?
To visualize these terms with a tuna blimp, check out this video:
Experiment
See this for yourself with an experiment. Inflate a balloon with helium. Put a piece of sticky putty on the side of your balloon. What happens? What changed?
Your balloon is in equilibrium whenever the weight acting down at the center of gravity, and buoyant force acting up from the center of buoyancy are aligned. Your balloon with putty stuck to it has stable and unstable equilibria. A stable equilibrium will return to that position if perturbed (given a nudge). An unstable equilibrium will transition to the stable equilibrium if perturbed. Can you find stable and unstable equilibria for your balloon? Draw the stable and unstable equilibria you’ve found.
Next Steps
Continue this teeter totter logic by thinking about how and where forces balance on an airfoil with the next lesson that discusses centers on an airfoil.
Last updated: November 23, 2022.
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