The gear effect arises when a ball collides with an object and the object recoils with a component of its acceleration in a direction parallel to the two contacting surfaces. Two experiments are described that show how the surfaces of the ball and the object can engage without slip similar to two gears. The effect is used in golf to correct for off-center impacts and it plays a minor role in baseball.
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Baseball bats, golf clubs, and tennis racquets all differ in this respect. For an off-axis impact, can act through the center of mass of a bat [as in Fig. 7(b)], but it does not normally act through the center of mass of a club or racquet. For a tennis racquet, the center of mass lies in the plane of the impact surface, and the center of mass of a bat or a club lies behind the plane of the impact surface.
A ball impacting on a soft surface such as rubber would cause the surface to accelerate locally in the contact region due to the finite tangential stiffness of the surface. Acceleration of the surface will also result in a gear effect bounce if the ball grips the surface, in the sense that the static friction force on the ball arises from local acceleration of the surface, possibly as a result of elastic deformation in both the ball and the contact surface.
