Science Puzzle
Why Do Drink Cans Have Domed Bottoms?
Turn over a can of fizzy drink and you'll find the bottom isn't flat. It curves up into the can like an upside-down bowl, which means the can holds a little less drink than it could. Cans are made in their billions, and every bit of wasted aluminium costs money.
So why do can makers give them a domed bottom?
The Answer
Because the drink inside is under pressure, and a dome can hold that pressure with far less metal than a flat base could.
Fizzy drinks are full of dissolved carbon dioxide, which keeps the inside of a sealed can at several times the pressure of the air outside, more on a hot day. That pressure pushes outwards on every part of the can. A flat sheet of thin aluminium would simply bulge out under it, turning the bottom into a rounded lump, and the can would wobble instead of standing up.
A curved shape behaves differently. Push on a dome and the load is spread round through the curve, squeezing the metal along its surface instead of bending it, just as an arch carries the weight of a bridge. So the dome resists the pressure even though the metal is very thin; the walls of a can are only about a tenth of a millimetre thick. Making the base flat but strong enough would need much more aluminium in every one of those billions of cans.
The pressure actually helps the walls too. Squeeze a sealed can and it feels rock hard; squeeze an empty, open one and it crumples easily. The gas inside stretches the thin walls tight, a bit like air in a tyre.
The principle: Curved shells resist pressure. A dome or arch spreads a load round through its curve, so a thin curved sheet can hold back a force that would bend a flat sheet of the same metal.