Focus question

Why do some floating objects tip over when a load is added, while others do not?

Essential materials

• lemon
• glass or container of water large enough for the lemon to float freely
• coins of several different sizes
• large and small paper clips
• safety goggles

Main ideas and background information

Note: If your students do not have much prior experience with floating and stability, or if you might find it helpful to review your own understanding, we recommend that you click here for an introductory stability activity. When you return, you will have a much better context for understanding the information that follows.

• See the Digging Deeper: Floating and Stability section for a review of the main ideas and background information.
• A lemon sinks into water until it displaces a volume of water equal to the weight of the lemon. The center of gravity of the lemon is above the center of buoyancy.
• If you place a coin on top of the floating lemon, it may remain there for a moment. However, chances are that you placed the coin slightly off center. This will cause the lemon to tip in that direction. However, when the rounded lemon tips, the shape of the displaced water does not change. The center of buoyancy does not change its location. This means that the lemon will continue to tip until the coin falls into the water.

Procedural tips

• Wait to place an object on the lemon until it is stable in the water, showing no signs of motion.
• Place a coin as gently as possible on the top of the floating lemon without touching it. Bring the fingers as close to the lemon as possible before releasing the coin.
• Observe closely what the lemon does, and change how you place the next coin. For example, if you observed that the lemon rotated quickly to the right and dumped the coin into the water, then place your next coin a little more to the left.

Discussion

1. What happened when you placed the coin on the lemon? What different ways did you try placing it to get it to stay on top?

2. Did the size or weight of the coin make any difference? Why or why not? (You might be able to balance a very lightweight coin on the lemon.)

3. Do you think that there is any way to win this game? Explain.

4. Anyone who is familiar with small boats or canoes knows that it is never a good idea to stand up in the boat. How is this connected to what you have just observed with the lemon and the coin?

Assessment

Are students able to:

• explain why some floating objects tip over when a load is added, while others do not? (If the tipping of the floating object does not change the shape of the volume of the displaced water, the object keeps tipping; no restoring force is produced. However, if the tipping of the floating object changes the shape of the volume of displaced water, it can produce an opposing buoyant force that keeps the object from tipping further.)

Extensions and further investigations

• Try this experiment with other small objects placed on top of the lemon. You might be able to balance a lightweight object such as a paper clip on the lemon.

• Repeat the experiment using a small rubber or plastic ball instead of a lemon.

• Weight one side of the lemon with a thumbtack and repeat the experiment. The thumbtack acts like the keel of a boat.

• Find out what happens if you use other fruit or vegetables such as oranges, apples, potatoes, or plums.

• Vehicle rollovers are serious safety concern, particularly for vans, sport utility vehicles, and light trucks. Research the importance of center of gravity in vehicle design. What recommendations would you make that might improve the safety of these vehicles?

• Find the center of gravity of various objects—and your own center of gravity! (See Resource Links)

Career connections

• Design engineer for boats, kayaks, surfboards, etc.
• Loadmaster for ships
• Designer of lifesaving flotation devices
• Designer for ocean rescue systems
• Crews for sailboats

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Standard B - Physical Science: B1, B2

Grades 9-12
Standard B - Physical Science: B2, B4

Britain: This activity correlates with the English National Curriculum standard Sc1, Science Enquiry, and the following additional standards:

Sc 1 BoS: 1a,d,e
Sc1 Sc3: 1a Sc4: 2b,c BoS: 1a,d,e,f
Sc1 Useful Background BoS: 1a,d,e,f

Glossary/vocabulary

center of gravity
center of buoyancy
buoyant force

Resource links

Information about vehicle rollovers and safety concerns:

Consumer News

U.S. Starts Real-World Rollover Tests
from The Center for Auto Safety

Formula Predicts Rollover Risk
from The Center for Auto Safety

Finding the center of gravity:

Center of Gravity: How to balance a checkbook using the physics method
from Exploratorium

How to find your center of gravity

Where is your center of gravity?
from Cislunar Aerospace, Inc.