Why does a penny sink in water but a boat doesn't?

Answer 1

A boat floats because it gets to displace its weight in water before it has a chance to submerge. If you designed a penny to do that, it would float too. The penny never gets to displace its weight in water; it sinks, and displaces only its volume. Here's what I mean by "displacing volume, and displacing weight". Take a two-inch cube of aluminum, and a two-inch cube of lead. The lead cube will weigh a lot more than the aluminum one, but when you put them in water, they will sink, and "push aside" exactly the same amount of water; a two-inch cube of water, in fact. Now take two identical model boats, and put one cube into each one. Assume the boats are large enough so the boats don't sink! Using a large cylindar, you will see that the boat with aluminum will make the water level go up a certain amount. The amount of water "pushed up" will weigh the same as the (boat plus the aluminum). Do the same with the other, and more water will be "pushed up". For the second boat, the amount of water "pushed up", or displaced, is equal to the (boat plus the lead).