Try this: Uncrushable cans

This activity looks at a bit of engineering and how strong ordinary aluminium cans are.

You will need:

• Four empty soft drink cans. Make sure they are in good condition, with no dents in them.
• A flat-bottomed thing that can survive you standing on it. An upside-down desk can work well.
• A table, pole or tree you can use to help yourself balance. It should be on level ground.
• A second person

What to do:

1. Put the cans upright on the ground, as far apart as the corners of the desk. Do it close to the table, pole or tree.
2. Gently put the desk on top of the cans. You may need the second person to help you.
3. Using the table, pole or tree to help you balance, carefully step up onto the middle of the desk.
   • While getting up on the desk, its important not to push to the side at all, or the cans may tip over
   • Have the second person help hold the table steady.
4. You should find the cans easily support your weight.

What's happening:

The walls of a soft drink can are only a fraction of a millimetre wide and it's easy to squash a can with your hands. How can it be strong enough to support your weight?

It turns out that standing on the table applies force to the can differently to squashing it. When you squash a can, you push on a point on the side, which causes the side to bend at that point. When you stood on the cans, the force was being applied to the top and bottom of the can. When a force is applied this way, it is evenly spread out all around the side, rather than being concentrated on one point. Not only that, but the force is a compression force, which tries to squash the metal instead of bending it. The result is that the hollow can is able to withstand force being applied from the ends almost as well as if it were a solid lump of metal.

Engineers and architects use this idea when designing buildings or machines. For example, by using hollow columns in buildings, the column is still strong, the columns are lighter, so it's easier to construct, and it saves money on materials.

If one of your cans collapsed as you stood on it, it probably had a small dent in it that you hadn't noticed. When the force from above is applied to a can with a dent in it, the dent makes a weak spot that will bend, which starts the can crumpling. This is why columns in buildings still need to have reasonably thick walls; because they need to withstand having things bump into them.

There are lots of ways that an objects shape, and not just the material it is made of , contributes to its strength. A nice example from nature is the eggshell. You can easily squash or crack an eggshell by pushing on its side, but if you try to push in on the ends of an egg, it is much harder to crack (if you want to try this, do it outside). Engineers and architects need to understand how an objects shape determine its strength, so they can make buildings and machines which are strong and tough enough to do their job, but still light and cheap enough to build.