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Homework Help: The Vector Nature of Newton's Second Law

  1. Jan 11, 2009 #1
    1. The problem statement, all variables and given/known data
    A crate of mass 50.0 kg is pulled across a level concrete floor by a force of 300.0 N acting 30.0 degrees above the horizontal. The crate moves at a constant velocity of 0.962 m/s. What is the force of friction acting on the crate?

    2. Relevant equations
    F = ma
    a = f/m
    Fnet = F1 + F2

    3. The attempt at a solution
    I'm not sure how the mass and velocity comes into play in this question. Because can't I just find all the sides of the triangle to find the force of friction?
  2. jcsd
  3. Jan 11, 2009 #2


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    Science Advisor

    [tex]\vec{F}= m\vec{a}[/tex]
    and the acceleration vector is 0. There are FOUR forces acting on the crate but only two of them are relevant to this question:
    (1) 300N at 30 degrees above the horizontal. What are its horizontal and vertical components?
    (2) Friction force which is purely horizontal. That's what you want to find.
    The two forces that are not directly relevant are
    (3) The weight of the crate which is purely vertical.
    (4) The force of the floor on the crate which is purely vertical.

    The last two are not relevant because you are asked only for a horizontal force.
    Because of that, the mass and velocity do NOT play any part in this problem.
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