MadSci Network: Edible/Inedible Experiments Archive

Bubbling, Burbling, Buoyancy Business

Area of Science: Physics
Meant for at least Grade 4-6 (age 8-10).
This experiment is inedible.
An adult should be present.

What is buoyancy? It's the force of water pushing up on something. Sometimes it's enough to make things float, sometimes not. How does it work?


  1. bucket
  2. water source to fill up bucket (back yard hose will be fine)
  3. air tight jar, not small, but small enough to fit in bucket a bunch of rocks, different sizes, but small enough to fit in the jar at the same time
  4. swimsuit (optional: if it's summer, you might have more fun with this if you are in one)

1. Don't drink the water - the bucket and rocks will make it dirty.
2. Don't get your mom or dad wet without asking first!

How to do the experiment:
1. Go outside and fill up the bucket with water.
2. Put the rocks, one at a time, in the water. (We put them in one at a time because we are polite, and also because it is important to keep all the water inside the bucket.) Do they float?
3. Take the empty jar, close the lid tightly and put it in the water. Does it float? If it doesn't, get another jar... Empty jars are supposed to float.
4. Take the rocks out of the water. Put the smallest one in the jar. Close the lid and put the jar back in the bucket. Does it still float?
5. One rock at a time, add more of the rocks to the jar. Each time you add one more rock, put the lid back on and see if the jar still floats.

If you can fill the jar with enough rocks, you will eventually be able to make it sink. Why?

Why do rocks float when they are in the jar, but sink if they are not?

Everything in or on water pushes some water aside, even if just a little bit. This is called displacement.

The smallest rock sank, didn't it? When it did, it had to displace some water to make room. The water level went up a little when this happened.

If we could weigh the little rock and the amount of displaced water, we would see that the rock weighs more than the water, since it sank right to the bottom of the bucket!

Next, lets say we weighed our jar with the smallest rock inside and the amount of water IT displaced. Our jar with the little rock floated, didn't it? On our scale, it would weigh THE SAME as the water it displaced. If we kept weighing our jar with more rocks, and the water it displaced each time, we would see that the jar displaces more and more water. The heavier the jar gets, the more water it displaces, but as long as the two weights are the same, the jar will still float. At some point, the jar begins to be heavier than the water it displaces, and this is when it finally begins to sink.

When you made the jar heavy enough sink, were you able to see the water level in the bucket rise?

Useful References:
Displacement was first discovered by a great Mathematician named Archimedes who lived in Ancient Greece. Read more about him at:

Archimedes Home Page

Further comments:
Want more to think about? So far we have just talked about weight, and not much about shape or material. Experiment with different materials and shapes to see more about buoyancy and displacement!

Experiment submitted on Sat Jul 19 10:54:18 1997 by:
Name: Mrs. Gail Watson
Institution: John F. Pattie Elementary School
Position: Computer Technologist

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