The physics of throwing pottery

The art of pottery requires a lot of patience, focus and a high tolerance for getting messy. Having a grasp on some basic physics concepts can also be a big help.

The wheel spins as a result of centripetal acceleration. The diameter of the wheel and the velocity of the wheel (controlled by the potter) determine the wheel's centripetal acceleration (a=v^2/r). The lump of clay is subject to centrifugal force (the tendency for an object to fly outwards on a circular path) because of the wheel's centripetal, or center seeking force. Thusm the clay's natural tenency is to fly off the wheel (not uncommon).The objective of the potter is to fight this force, manipulating the clay upward instead of outward. If we increase the velocity of the wheel, its centripetal force increases and requires the person to put more force on the clay with their hands in order to keep it from flinging off the wheel. The wheel spins faster around the outer edge than it does in the center. This is why one of the most important steps is to center the clay, making it less vulnerabel to the centrifugal force.

Another force working against the clay is friction. The force of friction is dependent on the surface of the clay and the potter's hands ("mu" or the coefficient of friction) and the ammount of force placed on the clay by the person (N). F_f=muN

When working with clay, a potter always as a bucket of water on hand to keep the clay wet especially during the initial centering process. The coeficent of friction is higher between dry clay and skin than it is between wet clay on skin. Because we must apply a high force on the clay (N) in order to shape it, we must counteract this force with a low coeffiecient of friction by keeping the clay wet and slippery.

The key to successfully throwing a pot is to keep these forces in ballance. Apply too much external force and the clay will collapse. Allow friction and centrifugal force to take over too much and the piece will fly off the wheel.