Floating and Stability

Have you ever wondered why some floating objects return to an upright position after they have been tipped? For example, a sailboat tips over a little when the wind blows, but it returns to an upright position when the wind dies down. Why does it do that?

Here are a few activities to explore this situation.

Tools and Materials

Small block of wood
A few moderately heavy coins
Clear 2-L (0.5-gal) or larger plastic beverage bottle, dish, or basin
Centimeter ruler
Pencil or other slender rod
Cellophane tape
Water

What to Do

Record level We set up our equipment as shown above. We carefully cut off the top of a clear 2-L plastic drink bottle and taped a centimeter ruler to it. Note that we also taped over the sharp edge around the top for safety. If you use something that does not need to be cut, such as a clear dish or basin, you do not need to tape the top. When we added the water, we included a tiny drop of food coloring for better visibility in the photographs.

1. Record the initial level of the water.

2. Place the small block of wood in the water. Record the level of the water. Press down slightly on one end of the block of wood with the pencil and release. Describe what happens.

3. Place a moderately heavy coin as close to the center point of the block as you can. Record the water level. Press down slightly on one end of the block of wood with the pencil and release. Describe what happens.

4. Now let’s mix things up a bit. Move the coin about halfway toward one end of the block, but keep it centered between the edges. This causes the block to tilt in the water. Record the water level once more. Press down slightly on the low end of the block of wood with the pencil and release. Describe what happens.

5. Place another coin on top of the first one. Record the water level. Press down slightly on the low end of the block with the pencil and release. Describe what happens.

6. Tape two or more coins to a corner of the block.

What now? What do you predict will happen when you place this in the water? Try it! Record the water level. Describe what happens.

Course:

Science

Result/Solution(s)

Here are our results for the floating and stability activities. Your actual measurements will probably be different; however, the changes will probably be similar.

Step	Description	Water level (cm)	Press and release results
1	Water alone	10.5	--
2	Water and block	10.6	Returned to original position
3	Water, block, one coin on center	10.7	Returned to original position
4	Water, block, one coin off center	10.7	Returned to original position
5	Water, block, two coins off center	10.8	Tipped over and dumped one coin
6	Water, block, two coins taped to corner	10.8	Tipped over and floated with coins beneath

The water level we measured shows how much water we put into the container at first.

When we tested the water and the block, the water level went up because the weight of the floating wooden block pushed down on the water. When one end was pushed down and released, the block returned to its original position. The block was stable in the water.

Next, we added a coin to the center of the block. This caused the water level to go up a bit more because the combined weight of the block and the coin pushed down on the water more than the weight of the block by itself. We also noticed that the block and coin went down into the water a bit more than for just the block. When one end was pushed down and released, the block returned to its original position. The block was stable in the water.

When we moved the coin to one side of the block, the water level stayed the same, even though the block tilted. This was because we did not add any more weight to the block. All we did was move the coin to another location on the block. When the low end was pushed down and released, the block returned to its original position. The block was stable in the water.

Next, we added another coin to one side of the block. The water level went up slightly because of the weight of the second coin. When the low end of the block was pushed down and released, one of the coins slipped into the water and the block returned to its original position with the remaining coin still in place. The block with two coins was not very stable in the water; however, once the top coin fell off, the floating block with the remaining coin looked stable.

Note: Your results may vary depending on the size of your block, the type of wood from which it is made, the weight of your coins, and where your coins are placed on the floating block.

Finally, we taped two coins to one corner of the block. The wood block tipped over immediately and came to rest floating with the coins beneath.

In this configuration, the coins are like the keel of a sailboat. The extra weight low in the water creates a very stable situation.

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