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Homework Help: Simple vector train problem drawing

  1. Oct 7, 2011 #1
    A ball is hanging from a long string that is tied to the ceiling of a train car traveling eastward on horizontal tracks. An observer inside the train car sees the ball hang motionless.

    Draw a clearly labeled free-body diagram for the ball if the train has a uniform velocity.

    i drew two vectors, but it says i got it wrong. both of my vectors are equal in lenth and are 180 degrees apart.

    here's my answer image: http://i53.tinypic.com/21et2za.jpg
  2. jcsd
  3. Oct 7, 2011 #2
    Maybe my intro mechanics are a bit rusty, but I can't see why that would be incorrect. If the system is in equilibrium (Fnet = 0) that means that the train is moving at a constant velocity (i.e., anet = 0). Perhaps since there is a friction force between the tracks and the train, and (by virtue of Fnet being 0) the train exerts a forward force to offset it, they want you to add vectors Ftrain = [itex]\mu[/itex]Fgi and Fk = [itex]\mu[/itex]Fg(-i)

    where i is the unit vector in the direction of the train's velocity vector.

    That's the best I can come up with.
  4. Oct 7, 2011 #3


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    Looks good, maybe they want 'mg' instead of 'w' for the weight vector?? I don't know why it doesn't accept it, though.
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