Guest Posted November 27, 2009 Report Share Posted November 27, 2009 A disc of 1 unit diameter is in contact and coplanar with a disc of 5 units diameter. If the smaller disc is rotated around the larger disc, how many rotations does it have to make before the same two points are in contact again? Quote Link to comment Share on other sites More sharing options...
0 superprismatic Posted November 27, 2009 Report Share Posted November 27, 2009 6 (6/5 of a rotation for each of the 5 times the original touching point on the small disc contacts the large disc on its trip once around the large disc) Quote Link to comment Share on other sites More sharing options...
0 Guest Posted November 27, 2009 Report Share Posted November 27, 2009 The small disc makes 6 rotations. It must rotate 5 times to cover the difference in the circumfrences of the two disks, but because the large disc is not turning, there is one additional rotation needed to travel fully around the large disc. I was excited to be the first to post a reply... but then had to register. By the time that was finished, someone else had posted their solution (and I believe they are correct). Such is life. Rob E. They don't come any newbier than this. Quote Link to comment Share on other sites More sharing options...
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A disc of 1 unit diameter is in contact and coplanar with a disc of 5 units diameter. If the smaller disc is rotated around the larger disc, how many rotations does it have to make before the same two points are in contact again?
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