In frozen Minnesota the winter sports carnival includes some unusual events. Since it is dangerous to run on ice, each runner runs on a heavy (240 kg) and long (40 m) wooden plank, which itself rests on the smooth and horizontal ice. One of the competitors is a 60 kg woman who runs the length of the plank in 4.4 seconds, quite an impressive time. Her performance is viewed by a crowd huddled on the ice. The performance that they see is less impressive. With what speed do they see her moving?

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  1. consider the log.
    the log has to be moving at 60/240 *40/4.4 meters/sec=10/4.5

    Consider relativity:
    speed runner relative to observer=sro
    speed runner relative to log=srlog
    speed log relative to observer=slo
    sro=40/4.4 + (-10/4.4)=30/4.4 m/s

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