Two forks, a cork, and a matchstick balance on the lip of a glass.
What it shows:
Balancing two forks in an unlikely configuration is a lesson in finding the center of mass and stable equilibrium. Nothing too deep here, but it's fun.
How it works:
Two forks, a cork, and a matchstick are assembled as shown in the photograph above.
The assembly balances on the edge of a glass. The center of mass must lie below the glass-matchstick pivot point. Since it contributes very little to the location of the center of mass, if you light the match it will burn to the lip of the glass and go out, leaving the fork assembly balancing in a most precarious manner!
Setting it up:
The only tricky part is adjusting the angle between the forks and the matchstick so that it balances properly without sliding off the glass. A video camera with a short focal length lens should be set up on the bench so that the audience can get a closer look of the pivot point. A blowtorch is usually supplied for comedic effect.
The configuration should be stable enough to show that a small nudge on the cork will cause the system to oscillate but not fall. Before lighting the match it's a good idea to wet the top of the match above the pivot point.