1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook