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In times gone by men moved very heavy stones by using logs as rollers. Assuming all the logs have the same radius and that the logs roll and do not slide explain why the stone moves in the direction the men push it twice as fast as the logs. (Try a similar experiment yourself, for example with some pencils and a heavy book, and you will find you leave the 'logs' behind at the back and have to move them into position in front of the 'stone'.)