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Is my solution good? - kinetic friction

  1. Sep 18, 2005 #1
    in first situation block is pulled with force F .

    in second situation we have the same block with the same force F but under angel 60 degrees.

    it is possible that in both situation motion is uniform. ?? (we have friction)

    so imo it isn't possible because The block is in the both situation in motion, so we have kinetic friction and it's depends on a gravitacion force, so in both situation should be the same.

    My thought is correct or no? somebody could give me an answer ?
     
  2. jcsd
  3. Sep 18, 2005 #2

    Doc Al

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    Staff: Mentor

    Careful: The kinetic friction depends on the normal force, not just the weight of the block. When the force F is applied at an angle, the normal force changes.
     
  4. Sep 18, 2005 #3
    Thx now i correctmy solution
    i have somthing like this:

    F-uQ=0
    and
    F/2 - u(Q-Fcos60)=0

    my result is:
    u=3/cos60
    so the solution depends on the u (coefficient of kinetic friction)

    Now am J correct ?
     
  5. Sep 18, 2005 #4

    Doc Al

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    Staff: Mentor

    Not sure what you are doing here, or what you are trying to solve for.

    In situation 1, where the applied force is horizontal:
    - the normal force equals the weight of the object [itex]N = mg[/itex]
    - the net horizontal force is [itex]F - \mu N[/itex]

    In situation 2, where the applied force is at an angle with respect to the horizontal:
    - the normal force equals [itex]N = mg - F\sin \theta[/itex]
    - the net horizontal force is [itex]F \cos \theta - \mu N[/itex]

    (I'm moving this to the homework help section.)
     
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