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Machine Problem

  1. Oct 1, 2008 #1
    1. A machine has a steel shuttle that is moving along a fixed unlubricated steel rail pulled by an elastic shuttle cord, which is at an angle

    The shuttle has a mass of 800g and is released when the elastic cord has 20N tension at an angle of 45 degrees.

    I know how to solve the problem, but I do not know why friction does not play a role here.
  2. jcsd
  3. Oct 1, 2008 #2


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    Because you are in an introductory class and it makes the maths too hard.

    It's the strange world of classroom physics, where springs have no mass, air has no resistance and inclined planes go on for ever!
  4. Oct 1, 2008 #3
    What strikes me though is the term "fixed unlubricated," which makes it seem as if the problem is trying to hint that there is friction.
  5. Oct 1, 2008 #4


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    It would normally mean you are considering a rolling rather than sliding wheel - this makes a difference because of angualr momentum.
    Often the questiosn don't make sense because they were originally longer and had a second part that needed the extra info - but for a new edition, or a lower level course they chop off part 2.

    If you ever think - "is it me or is this a stupid question in the textbook", consider how little money you get for writing and checking textbook questions!
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