A Balancing Act

The counterintuitive demonstration in the video is a beautiful example of the concept of center of gravity. It sure looks like the fork and spoon should immediately teeter off of the glass and crash onto the table. And it’s even more dramatic when half of the matchstick burns away but the contraption still balances on the edge of the glass. But it’s all physics.

If the system is in a state of static equilibrium (meaning no motion) then according to Newton’s Second Law, the net force and the net torque acting on the system must both add up to zero. In order for the forces to cancel, the downward force of gravity must be balanced by the upward force of the glass acting on the matchstick. So far, so good. But for the torques to cancel, the center of gravity of the object must lie directly over the rim of the glass. The center of gravity of an object is the point at which the gravity force seems to act. It’s like the statistical average of all the gravitational forces acting on each small piece of the object.

Now if you look carefully you can see that a substantial part of the mass of the spoon and fork actually lies behind the balance point. The illusion is that there is much more mass on the outside but it isn’t true. There is in fact sufficient mass in the handles to place the center of gravity right over the rim. When you burn away half the matchstick it still balances. Why? 1) Because the part of the matchstick resting on the rim is still intact, and 2) Because the mass of the matchstick is so small, that although you shift the center of gravity very slightly to the outside when you burn part of it away, the c.g. still lies over the rim of the glass.

Do this one at parties and you'll really impress your friends!