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Friction at constant velocity

  1. Feb 14, 2012 #1
    1. The problem statement, all variables and given/known data
    A 10 N force is horizontally applied to a cart from the left. The cart moves to the right with a constant velocity. What is the magnitude and direction of the frictional force? (In terms of mass)

    2. Relevant equations
    Fnet=ma
    fk=ukN

    3. The attempt at a solution
    I know Newton's second law shouldn't apply since the acceleration equals zero so there is no net force. But I'm confused as to how I can get all the forces to cancel out if the applied force needs to be greater than the frictional force in order for the cart to move. So the frictional force can't be 10 N. Is there a force I'm missing or is there something wrong with my reasoning?
     
  2. jcsd
  3. Feb 14, 2012 #2

    PeterO

    User Avatar
    Homework Helper

    your problem lies in the statement in red, above.

    You do, and have, used Newton's second law when you concluded "... so there is no net force".

    getting all the forces to cancel out is easy ....

    Weight down
    Normal Reaction Force up
    Applied Force to the right [applied from the left]
    Friction Force to the left.

    They cancel in pairs.
     
  4. Feb 15, 2012 #3
    Oh okay, I was getting static and kinetic friction mixed up. Thanks
     
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