A good visual demonstration to question our intuition regarding forces.
- straws (need to be straight – cut off the bendy bits if there are any)
- large baking potatoes
- Challenge members of your audience to see who can insert a straw the furthest into a potato. To increase the excitement you may want to line up a row of volunteers, each with a straw and potato, and run the demo as a race.
- Give them a while to try the challenge – they will almost certainly twist the straws slowly into the flesh of the potato. When their straws are bent and not going in any further, show them how to do it:
- Hold the potato between thumb and fingers (note: do NOT have your hand behind the potato!).
- Grasp the straw firmly about 2/3 of the way up (so you have plenty of straw to go into the potato).
- Use a sharp thrusting movement to force the straw through. Be confident – it really will go!
How Does it Work?
There are two principles contributing to this trick:
- The sharp thrusting movement delivers a much larger instantaneous force than the slow gradual twisting motion, thereby allowing the straw to get further.
- The end of the straw has a very small surface area, so the force you impart is concentrated very strongly. If you tried to do the trick with a pencil roughly the same size as the
straw you would find it much more difficult – you would need to displace a much greater area of potato, which would require a much greater force.
Tips for Success
The straws need to be straight, with no defects. Don't try to re-use the same straw. Have confidence that the straw will go through – you don't have to be particularly strong to make this trick work, but you do have to be aggressive. Practice this one in advance so that you gain confidence and believe it will work!
If you still have difficulty getting the straw very far into the potato, try changing your grip – some people find that holding the straw with their thumb over the other end works best, while others like to pinch the straw part way down.
This demonstration works best when run as a competition between different members of your audience, especially for teenagers. Regardless of their age, many members of your audience won't believe this trick, even after they've witnessed it with their own eyes! It would be worthwhile having a good supply of straws and potatoes available so that everyone can have a try for themselves.
Did You Know?
This effect (the concentration of force into a small area) can be seen quite frequently in everyday life, from vinyl floors damaged by stiletto heels to its inverse, extra wide tyres on tractors and wheel barrows.