The Strength of Shapes — Part III

Impress your friends with this neat trick!

Present them with a potato and a straw and ask them to push the straw through it!

Let them have a few attempts — they’ll probably give up having not managed it — then dazzle them by using your knowledge of science to get the straw through!!

You will need:

• A grown-up (to assist you while you learn)
• A thick glove
• One medium-sized potato
• A drinking straw (we used paper straws)

You may also need:

• Several attempts to perfect your technique!

Then, later:

• An audience (for when you’ve mastered it!)

How to do it:

1. Hold the straw in the hand you use to write with.
2. Place your thumb on top of the straw and wrap your fingers around its middle, as shown in the photo below:

3. Hold the potato in your other hand, as shown in the photo below:

4. Line up the tip of the straw with the point on the potato where you’ll push the straw through

5. Raise the straw and…

SAFETY FIRST!!

Keep your fingers (and palm) away from the area of the potato that you’re going to push the straw through!

Also, wear a glove on the hand that’s holding the potato

6. …STRIKE!!! sharply downwards on the potato; keep your thumb on top of the straw and carry on pushing…

7. …It will go through!! Don’t believe us? See the video:

8. Once through the potato, show your friends and enjoy your success! YOU knew it would work, because of your knowledge of science (and adherence to safety principles)!!

9. Explain to them why it works (see below).

The science behind it:

There’s quite a lot going on here!!

Strength of Cylinders — as we demonstrated in an earlier challenge*, cylinders (in this case our straw) are one of the strongest geometrical shapes, regardless of the material they’re made from (in this case paper). They are immensely strong, particularly in situations like this when a force is applied across their length (in this case a compressive force).

Forces — because the tip of the straw has a small surface area, and by moving it quickly, we’re able to deliver a lot of force through it. This force is big enough to move (displace) the potato in front of the straw, allowing it to go through.

*The Strength of Shapes — Click on Link

Where next?

Once you’ve got the hang of it, you could try other fruit and vegetables — but get permission first! Also, you might like to make your own straw out of a piece of paper and see whether it works as well. But don’t forget to wear a glove!!