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A 20.0kg sled is being pulled across a horizontal surface at a constant velocity

  1. Oct 7, 2012 #1
    A 20.0kg sled is being pulled across a horizontal surface at a constant velocity....

    Hi. This is a problem on my physics homework that I can't seem to get the (correct) answer to. I would love to be told what I'm doing wrong. (:


    This is a problem and what I did:

    A 20.0kg sled is being pulled across a horizontal surface at a constant velocity. The pulling force has a magnitude of 80.0 N and is directed at an angle of 30° above the horizontal. Determine the coefficient of kinetic friction.

    m(sled) = 20kg
    F(gravity) = 20kg * 9.8 m/s2 = 196 N

    F(pull) = 80 N
    F(pull,X) = 80sin30° = 40 N
    F(pull,Y) = 80cos30° = 69.28 N

    F(normal) = 196 N - 69.28 N = 126.72 N

    Kinetic Friction = μ * F(normal)

    μ = 40 N / 126.72 N = .3157

    I looked in the back in the book, and apparently the correct answer is .444. What am I doing wrong?

    Thanks!
     
    Last edited: Oct 7, 2012
  2. jcsd
  3. Oct 7, 2012 #2
    Re: A 20.0kg sled is being pulled across a horizontal surface at a constant velocity.

    Nevermind, I figured out what I did incorrectly.
    Lol, I mixed the vertical and horizontal components of the F(pull) by accident.
    ....

    m(sled) = 20kg
    F(gravity) = 20kg * 9.8 m/s2 = 196 N

    F(pull) = 80 N
    F(pull,Y) = 80sin30° = 40 N
    F(pull,X) = 80cos30° = 69.28 N

    F(normal) = 196 N - 40 N = 156 N

    Kinetic Friction = μ * F(normal)

    μ = 69.28 N / 156 N = .4441
     
    Last edited: Oct 7, 2012
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