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A question, answer please

  1. Aug 14, 2004 #1
    1. Suppose that a boy is pulling(Force: P) a train of 4 toy cars(The boy and toy cars are linked with a string) at a constant velocity.
    The mass of each car is m kg. If friction is negligible, what is the tension of the string?
    Is it equal to force P? That's my guess.....
    Last edited: Aug 14, 2004
  2. jcsd
  3. Aug 14, 2004 #2
    no man.....apply the free-body diagrams....and see for yourself...the tension varies in evry wire...
  4. Aug 15, 2004 #3
    The tension force should be zero because you are neglecting friction, and the cars are moving at constant velocity.
  5. Aug 15, 2004 #4

    Doc Al

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    Staff: Mentor

    If the boy pulls with a force P and there is no friction, how is it moving at constant velocity? (Unless P = 0, of course. :confused: )
  6. Aug 15, 2004 #5
    With constant velocity, we can safely change "should be" to "is". The boy only needs to pull to get it started from zero mouvement and then to keep it going if there is friction. That's why wheels were invented. And that's why I'm confident Summer will be back next year.
  7. Aug 17, 2004 #6


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    It was a riddle yeah! P=0 :biggrin:
  8. Aug 17, 2004 #7
    Wouldn't there always be tension in the string? I mean, it cannot just float in midair, can it? If we neglect gravity completely, as zero friction would seem to suggest, then there would be no tension in the string, but otherwise there would have to be some tension in the string, otherwise it wouldn't be real! I am thinking specifically of a hanging cable and how (if I remember correctly) it hangs in the shape of a parabola under the influence of gravity and no other forces. Hmmm, so how do you make the string have no tension? Maybe you would just place it on a horizontal, flat, frictionless surface too! :uhh:
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