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We are going to use a sprinkling can of the type used in gardens, and we will call it an "overflow container" so we can feel like real scientists. It needs to be filled with
water so that the water barely runs out of the spout, and then we must weigh it.

Next, we will weigh a smaller container, which we will place beneath the spout of the overflow container so that it can catch any water that may pour out from the spout.

We will tie a string to a piece of wood, or something else that floats, tie the other end of the string to a spring scale, and then write down the weight of the float. The fun
part is lowering the float onto the top of the water in the overflow container.

Now, aren't you glad we put the small container under the spout? Otherwise we'd have a mess!

Check out the weight of the float now that it is floating. It doesn't weigh anything! Did it lose weight? That's just silly. Floats can't go on a diet!

I guess that if the buoyancy of the water is holding it up, the scale doesn't have to hold it up, so the scale shows that the float does not have weight. Wowie! That's neat!

The next thing to do is to weigh the small container, and subtract the weight we noted when it was empty. That gives us the weight of the water that poured out when we put the float onto the water in the overflow container. Does that water weigh the same as the float?

It has to. Real scientists have taught us that the buoyant force of water is the same as the weight of the water that is displaced by a floating object.

We can try this experiment with several different things that float. Except aircraft carriers.

We should try different kinds of wood such as balsa and mahogany, or teak. If we were really lucky, we could get a small piece of lignum vitae. That kind of wood is so dense that it does not float! We can try a cork and a ping-pong ball.

We could even try this experiment with a different liquid, such as salt water, or cooking oil.


This writer just thought about something. This stuff about "the buoyant force being equal to the weight of the displaced water" is a rule.

We need to find out what the rule is about things that don't float. If we put a small rock in there, it would scoot right down to the bottom. It would also displace water, because the rock takes up space. Since we filled the overflow container to the very brim, there is not enough room in there for anything else unless some of the water spills out. The rock weighs a lot more than the water it displaced.

So what is the rule?


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