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Why do Boats Float?

JEFF DECURTINS: So we’re about to have the AP Boat Races and one of the things the students are required to do, each group, is calculate whether or not their boat will float. It’s kind of amazing, people have an intuitive sense of what will float, so if they throw a block of wood in there it will float, but if they throw a penny in the water it will sink to the bottom so they have sense of what things may float but why they float may be a different story. And it’s not readily apparent to people why things will float.

 

Before the race begins

The usual excuse for floating is that when you put things in water it pushes things aside and the amount of force pushing it up to make it float is equal to the weight of the water that gets pushed aside but it’s not clear why that is — why that may be the case. The real answer, or the technical answer, is that if you get deeper, the pressure of the water pushing in on you gets greater. If you’ve ever dived into a swimming pool, like 10 feet deep or something, you can feel the pressure on your ears. So as you go deeper in water, the water pressure gets bigger and bigger, so when you put something in the water… the water pushes in on the side of the boat but it pushes the same amount on the left and right.

But if you look at the bottom of the boat, the only thing pushing down on the boat is the weight of the boat itself but underneath the boat the water is pushing up on the boat and it’s pushing up more because the bottom of the boat is deeper in the water than the sides of the boat. So the deeper you go, the harder water pushes and on the bottom of the boat the water is pushing up. That difference between the top of the water and the bottom of the boat, that extra water pressure, is what keeps the thing afloat. And the more something weighs, the deeper it sinks into the water — and as it gets deeper the water pushes back on it more. And that’s the whole thing, it’s basically adding the water pushing up and your weight pushing down. And you keep sinking until the water is pushing up the same amount you weigh down.

 

The “Hamster Wheel” boat

So if you look at ice cubes you drop them in water, they almost float but a little bit will be sticking out because the weight of the ice is less than the equivalent space of water. The water underneath is pushing up the ice cube, the same amount of weight as the ice cube. And that’s the way anything goes. It’s really strange — for example, in World War 2 they built ships out of concrete, which seems bizarre… but once you put that in water it starts going deep and the water presses back, as much as it takes to hold the ship up. So as long as the ship isn’t solid concrete, which is heavier than water, but a mix of concrete and air, the ship is lighter than the equivalent amount of water until it goes deep enough for the water to push back.

So if you look at ice cubes you drop them in water, they almost float but a little bit will be sticking out because the weight of the ice is less than the equivalent space of water. The water underneath is pushing up the ice cube, the same amount of weight as the ice cube. And that’s the way anything goes. It’s really strange — for example, in World War 2 they built ships out of concrete, which seems bizarre… but once you put that in water it starts going deep and the water presses back, as much as it takes to hold the ship up. So as long as the ship isn’t solid concrete, which is heavier than water, but a mix of concrete and air, the ship is lighter than the equivalent amount of water until it goes deep enough for the water to push back.

The infamous “Banana Boat” – constructed entirely from banana boxes

An example of how heavy water is, if you took a cubic meter of water that amount would weigh 2200 pounds… I don’t think we appreciate how heavy water is. That’s really the secret, the floating.

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