51. (III) An Atwood’s machine consists of two masses,  and  which are connected by a massless inelastic cord that passes over a pulley, Fig. 10–57. If the pulley has radius R and moment of inertia I about its axle, determine the acceleration of the masses  and  and compare to the situation in which the moment of inertia of the pulley is ignored. [Hint: The tensions  and  are not equal. We discussed the Atwood machine in Example 4–13, assuming  for the pulley.]