49. No external torques act on the system consisting of the train and wheel, so the total angular momentum of the system (which is initially zero) remains zero. Let I = MR2be the rotational inertia of the wheel. Its final angular momentum is Lf→==−IMRωω##kk,2where#kis upin Fig. 11-47 and that last step (with the minus sign) is done in recognition that the wheel’s clockwise rotation implies a negative value for ω. The linear speed of a point on the track is Rand the speed of the train (going counterclockwise in Fig. 11-47
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