An experiment is described where a billiard and a rubber ball were filmed when they were released from rest on an inclined plane. Simultaneous measurements of the linear and angular velocities were used to determine if sliding was involved and to estimate the offset distance in the normal reaction force. The results show that the normal reaction force does not necessarily act through the center of the ball, altering the expected linear and rotation speeds of the ball down the incline.

Motion of a ball or a cylinder down an inclined plane has been described by many authors in physics teaching journals, in terms of either its linear or its angular acceleration down the incline.1–6 Measurements of both quantities are needed to obtain a detailed understanding of the effects involved, but this is rarely attempted. At low inclination angles, a ball rolls down the incline without slipping, and at...

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