Floating and Sinking

You have seen lots of things floating in your life, but it is worth taking a moment to make sure that you understand (on a very basic level) what is happening.

Let's use the example of a boat floating in water. If a boat is lowered into water it will push aside (displace) some water.

The boat will continue moving down into the water until that mass of the water it has displaced is equal to the mass of the entire boat. At that point, it will stop sinking and remain where it is.

Let's try to make some sense out of that.

You can think of this working like a see-saw. on one side of the see-saw we have the boat and everything that is inside it. On the other side we have a container that holds the displaced water.

We'll start with the boat just beginning to push down on its side of the see-saw.

As the boat sinks into the water, the displaced water is added to the other side of the see-saw.

The boat will sink, and water will be added to the other side of the see-saw until the weight on both sides is the same.

So, as the weight of the boat increases (the boat is larger or more cargo is loaded into it) the boat will sink deeper into the water and more displaced water will be added to the other side to balance that weight. This means that as long as the boat and everything it is holding weighs less than an equivalent volume of water, it will float.

On the other hand, if we fill the boat with something that is as dense (or denser) than water, the displaced water will never weigh enough to balance the boat and it will keep sinking.

So, we can say that something sinks until it floats. That is, it sinks until the water it displaces weighs as much as it does. Something that weighs more than the water it displaces will sink all the way down to the bottom.