Giant Metal Bubbles

by Deb on May 28, 2012

Boats

It’s a bit of a mystery how boats float if you think of them as big lumps of metal or wood. But change the focus and think of them as giant metal bubbles and it’s suddenly clearer.

Buoyancy

When you have a column of water, the pressure at the bottom is higher than the pressure up the top. Imagine there were 100 people standing on each other’s shoulders – it’s a lot harder being at the bottom than being on top. Because of this stronger pressure at the bottom, there is a general upward force.

pressure

Modified from Peter Southwood

Floating comes down to a battle between two things – gravity pulling down and buoyancy pushing up. Gravity and buoyancy both work on mass, which is the amount of matter in an object. If the buoyancy pushing up is greater than the gravity pulling down, the object will float.

Density

Density is a way of measuring how much mass is packed into a given size. If lots is packed in, like a box filled with books, it will be heavy. If things are only loosely packed in, like a box of jumpers, it will be light. Boats aren’t just made out of the shell – they include the air inside them as well and the density averages out, making them lighter than the water.

density

Same size but different density, the denser one (right) is heavier.

Floating

Going back to our column of water, an object in it will replace the same mass of water. But if it is our boat which is mostly air, the low density means it is much bigger than the same mass of water, so it only replaces a little bit and will float. As you put more and more into the boat, you are increasing its mass and increasing the amount of water it is pushing out of the column. This makes it float deeper and deeper. Eventually it won’t be able to push away enough water and down it goes.

boats

The light blue shows how much water would have been there, but has been replaced by the red boat.

We will be doing this 🙂 I just wanted to get the explanations out of the way first.

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