When you bite down hard, you put force on the object between your teeth and on the joint. In terms of physics, the jaw is the lever and the TMJ is the fulcrum. Actually, more force is applied (per square foot) to the joint surface than to whatever is between your teeth. To accommodate such forces and to prevent too much wear and tear, the cartilage between the mandible and skull normally provides a smooth surface, over which the joint can freely slide with minimal friction. Therefore, the forces of chewing can be distributed over a wider surface in the joint space and minimize the risk of injury. In addition, several muscles contribute to opening and closing the jaw and aid in the function of the TMJ.