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Homework Help: Velocity of a mass being pulled across a level surface

  1. Oct 1, 2007 #1
    1. The problem statement, all variables and given/known data

    A man pulls a crate of mass 67.0 kg across a level floor. He pulls with a force of 172.0 N at an angle of 28.0° above the horizontal. When the crate is moving, the frictional force between the floor and the crate has a magnitude of 124.0 N.
    If the crate starts from rest, how fast will it be moving after the man has pulled it a distance of 2.70 m?

    2. Relevant equations

    W = Fsin(angle)
    W = KE
    KE = 1/2 m v^2

    3. The attempt at a solution

    I started by finding the net force so that I could use the work equation:

    Fx = 172 cos(28) - 124 = 27.87 N
    Fy = 172 sin(28) (Fn and W cancel) = 80.75 N
    Fnet = sqrt(Fx^2 + fy^2) = 85.42 N

    Work Equation

    85.42(2.70)cos(28) = 203.6 J

    KE Equation

    203.6 J = .5 (67 kg) v^2

    v= 2.47 m/s
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 2, 2007 #2
    Careful, your frictional force acts in a opposing direction of your motion.. which is also angled.

    And why does your [tex]F_n[/tex] and [tex]W[/tex] cancel? Weight comes straight down and your normal force is perpendicular to the surface.

    Draw a free body diagram - it'll give you much better insight
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