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Balancing Forks
Feel the Force / Party Tricks
Set up the forks-in-cork assembly and challenge the audience to balance it on the edge of a glass. When they maintain it cannot be done, produce the cocktail stick and commence the trick.
NB: This should only be attempted by an adult as it involves flames, glass and potentionally flying forks!
Age range 11-14 with adult supervision
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- One cork
- One cocktail stick
- 2 identical metal forks
- One glass
- Matches
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- Push the two forks symmetrically and firmly into the side of the cork so that the handles of the forks form about a 90-degree angle.
- Push the cocktail stick carefully into the end cork; take care the cocktail stick does not break at the tip when you push it into the cork.
- Now put the cocktail stick on your finger, and try to balance the above assembly on your finger. If the angle of the forks is suitable, you should find a point on the cocktail stick that you can balance the forks. Mark this point.
- Now try to balance the assembly on the rim of the glass. It should balance at about the marked point on the cocktail stick – even though this seems highly unlikely, being such a top-heavy construction … So, just to clarify, you’ve got part of the cocktail stick inside the rim of the glass, and everything else balancing on the outside.
- Now for the even crazier bit. Light the cocktail stick at the end that’s in the glass and watch it burn away. The burning will stop once it hits the glass rim, but the fork assembly will continue to be balanced at that point.
- Accept the applause from your audience!
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The ‘centre of gravity’ of any object is the point about which you can balance the object as if all the masses were concentrated or gathered at this point. In other words, the net torque of all the masses of the object about this point is zero, regardless the shape of the object. The centre of gravity does not have to be on the object, it can be in the open space. For instance, the centre of gravity of this fork assembly is in between the forks in the empty space.
How do you make it stable? When you try to balance an object, if the point of support, the pivot point, is not at the centre of gravity then the object will rotate either clockwise or anti-clockwise depending on which side has more torque. However, if the pivot point is on the same vertical line as the centre of gravity, then the object, no matter what shape, is going to balance. It will be stable if the centre of gravity lies below the pivot point. The pivot point is where the cocktail stick rests on the rim of the glass. The actual centre of gravity must lie in the empty space between the two forks and below the pivot point to achieve stability. Consequently we can easily burn away the cocktail stick that extends into the glass because it is playing no part in the balancing act.
As the heat of the flame is absorbed by the glass, the temperature drops below the wood's ignition temperature and the burning of the toothpick stops exactly at the fork or the glass rim.
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The Planet Science Newsletter.
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This should be performed by an adult due to the hazards involved with glass (possible breakage and cutting injury), forks (sharp points) and flame (possible burns).
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