Density

Pop quiz - Sinking and floating depend on what?

Weight Size Shape Temperature Density Magic fairies

OK, the title of the page is density, so I sure hope you got that one correct.

Take a look at these two little boats. Both boats are solid. They're the same size and shape, but let's see what happens when we put them in the water. Click on each boat to find out.


One boat remains floating, but one sinks. Through this experiment, we can say that sinking or floating doesn't depend on size and shape. But what about weight?

Second experiment - We take two sheets of aluminum foil. Each weighs exactly the same and each is the same size.The first we shape into a bowl and the second we wad up into a ball. Now, let's gently place them on the water. Click on each piece of foil to see what happens.


The bowl-shaped foil floats but the ball sinks. Why?

You may be tempted to say it's the shape, but from experiment one, we learned that shape alone isn't an influence. It's not weight, because both pieces weigh the exact same. What could it be?

If you said DENSITY, you're on target. Density is how tightly packed the material is. For example, a block of wood has a lot of holes and spaces in it. Most are small enough not to be noticeable, but they're there. Wood isn't that dense.

Metal has very few spaces. The molecules are close together and tightly packed. Because of this, we say that metal is denser than wood.

So why did the bowl-shaped foil float? By bending the metal, you've increased the surface and decreased how tightly packed the foil is. By wadding the foil up, you've made a DENSE little ball. It's denser than the water, and so it sinks.

To find more information on why a big metal cruise-ship floats and a small little rock sinks, read about isostasy. Either that, or go back to the earth's layers.
Or you could return to the first or third terms.