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DISPLACEMENT OF WATER

We can have some more fun with water. We'll weigh a large dishpan, and write down its weight, then place a one quart can in it. Actually, a coffee can will do very nicely. Now, we need to fill the can to the brim with water. All the way to the top. Let's wipe the outer surface of the can, and the inside of the dishpan to be sure they are dry.

This block of wood also must be weighed, and the weight written on our lists. We are going to see a really neat trick. Very gently, we put the block of wood onto the water. OOPS! Oh, slop, we might need a mop! Why did some of the water leave just because we put the wood onto it?

It seems as though the wood floats, but a little bit of it is down in the water. I mean to say, the bottom of the wood is not just sitting on the top of the water.

When you put a boat into the lake, a few inches of the boat sinks into the water, doesn't it? We say it floats, because it doesn't just ZOOM down to the bottom of the lake, and we can get into it, and it still stays up, but more of it goes down into the water. We would say that the boat "draws" a few inches of water.

If the water in the lake was all the way up to the top of the edges of the lake, and we put a boat into it, some of the water would slop out onto the ground around the lake. Why is that?

It is because an object put into water displaces some of the water. There is a limit to the amount of space that can be taken, and there is a limit to the amount of weight that can go down into the water. When that limit is passed, the extra water has to go somewhere.

Now, let's take the wood out of the can, and take the can out of the dishpan. We need to weigh the dishpan to see how much water is in it. Since we already know how much the empty dishpan weighed, all we have to do is subtract its weight when empty, from the weight of the dishpan with water in it.

Is the weight of the water the same as the weight of anything else we have weighed today? The wood, maybe? Now, let's try this with some other objects that float.

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