Focus question

Why do some floating objects remain in an upright position when tipped?

Essential materials

• a small block of wood
• a few moderately heavy coins
• a clear dish for water (We made one from a two-liter drink bottle.)
• a centimeter ruler
• a pencil or other slender rod
• tape
• water

Main ideas and background information

See the Digging Deeper: Floating and Stability section for a review of the main ideas and background information.

Procedural tips

• If you modify a clear two-liter bottle for this activity, use filament tape, duct tape, or masking tape on the sharp edge around the top. Cellophane tape is not as safe for this purpose.
• When the students are measuring the water level, make sure that they align their eyes with the top of the water before reading the ruler. If they look at the ruler from above or below the level of the water, their measurement will not be as accurate.
• In steps 2, 3, and 4, it is not necessary to press down very far on the end of the wood block. A small displacement is usually all that is necessary to tell whether a situation is stable or not.
• If you use a lightweight coin in step 3, the water level might not change noticeably. We suggest that you use a coin with enough weight to change the water level by a measurable amount.

Discussion

1. What happened when you placed the wood block in the water? Why do you think that happened? (The block floated and the water level rose. The block was able to displace water equal to its weight without having to use its entire volume, so the block floated. That is, the wood was less dense than water. The water level went up because the weight of the block pressed down on the water surface.)

2. What happened when you pressed down momentarily on one end of the floating wood block? Was it stable? (The wood block tipped downward when one end was pressed down. The buoyant force could be felt pushing upward. When the pencil was taken away, the block came back to its original orientation. This means that the block is stable.)

3. What happened when you placed the coin in the middle of the floating wood block? Why? (The water level went up a small distance, because the block and coin were pushing on it more than the block by itself. The block with the coin sank lower into the water because of the added weight of the coin. The block with the coin had to displace more water in order to create a large enough buoyant force to keep it afloat.)

4. What happened when you pressed down momentarily on one end of the floating wood block with the coin in the middle? Was it stable? (As without the coin, the wood block tipped downward when it was pressed on one end. When it was released, the block with coin returned to its original orientation. It was stable.)

5. What happened when you placed the coin near the end of the floating wood block? Why? (The end of the block with the coin sank deeper into the water and then stopped. The buoyant force moved over to push upward against the off-center weight, leaving the block in a tilted position in the water. The water level did not change, because the weight of the floating block and coin had not changed.)

6. What happened when you pressed down momentarily on the floating wood block with the coin near the end? Was it stable? (When the end with the coin was pressed down, the block tipped downward more. When it was released, the block returned to its original tilted orientation. It was stable.)

7. What do you think would happen if you moved the coin closer to the end of the block? (At some point, the block might tip enough for the coin to slide off.)

8. Why did the block with the coins taped to the corner tip over in the water? (The center of gravity was outside of the center of buoyancy. The weight of the coins overcame the buoyant force, causing the block to tip over.)

9. Thinking about what you have learned in this activity, how would you go about loading boxes of different sizes and weights onto a boat or ship? Explain. (For maximum stability, the weight of the cargo must be distributed as evenly and as low in the boat as possible.)

Assessment

Are students able to:

• explain why some floating objects remain in an upright position when tipped? (The tipping of the floating object changes the shape of the volume of displaced water. This can change the position of the buoyant force so that it restores the object to an upright position. Sometimes this is called a restoring force.)

Extensions and further investigations

• Try this experiment with other objects of different weights placed on top of the wood block.

• Repeat the experiment using a block of another material besides wood.

• Try the experiment with the wood block floating on an edge or on its end.

• Find out what happens if you use blocks of different lengths, widths, or thickness.

• Try the experiment with floating objects of shapes other than rectangular (e.g., a wood cylinder).

• Boat rollovers are a serious safety concern. Research the importance of the center of gravity and the center of buoyancy in boat design. What recommendations would you make that might improve their safety?

• People who paddle kayaks sometimes like for their boats to tip over easily. Can you find out why this is so? How might some kayaks be designed so that they are unstable?

• Try different sizes and weights of coins or other objects. How does the size or weight relate to the amount of water displaced?

• 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
restoring force
stable
unstable

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.