Buoyancy Science Project

Buoyancy is the concept in physics used to explain the forces at work that enable an object to float, basically. These forces are, in essence, the fact that an object seems to lose weight in water. If the volume of water that object displaces is great enough, the object will float. This explains why boats are broad and flat, not thin and deep.

Density also plays a role in whether or not an object will float. Try the following experiment.

Obtain two 2” cubes of wood. One should be made of pine and one made of oak.

Fill two identical jars with water.

Drop the wood in each jar.

Observe the results.

What can you conclude?

Try dropping a piece of clay that is a two inch cube into the same jar of water, after you take the wood out. Now, what have you learned?

There was a Chinese prodigy named Cao Chong according to Chinese folklore. He was reputed to have an adult’s intelligence by the age of six. Some of us never do reach that point…. Here’s a problem for which Cao Chung devised a solution. His father was a prominent man in the court of the king. As such when given a gift that included an elephant, back around 200 A.D., he wanted to weigh this elephant. Having such an intelligent child for a son, he enlisted the son’s help. The son did indeed figure out a way to measure the weight of the elephant, using the principle of buoyancy. Do you know how he did it?

Not being prodigies, we’ll need a little help. This incident is recorded in an ancient book, entitled Sanguo zhi. Cao Chong put the elephant in the boat, which is a feat in itself, and noticed how deeply the boat sunk into the water. He then put things into a similar boat until it sunk to the same degree as the one holding the elephant. Next, he weighed the articles that he had put in the 2nd boat to see how much the first boat weighed. This would be the same amount as the elephant, since that second boat displaced as much water as the boat did that held the elephant.

You can try this in the bathtub with a hamster if you want.

Step 1. Weigh an empty one pound coffee can.

Step 2. Place the hamster in the one pound coffee can and place the can in a tub of water.

Step 3. See how far the can sinks into the water. Mark a line on the can with a permanent marker.

Step 4. Take the can out of the water.

Step 5. Scoop the hamster out and place it securely back in its cage.

Step 6. Wash the can out.

Step 7. Place the same coffee can into the water.

Step 8. Fill with rocks until the can sinks to the permanent marker line on can.

Step 9. Weigh the can with the rocks in it.

Step 10. Subtract the weight of the empty can. This is how much the hamster weighs.

P.S. Can you figure out how many hamsters it would take to make up the weight of an elephant, without killing either of them?