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Homework Help: Work and Zero Work

  1. Mar 18, 2012 #1
    Hi, I am taking a highschool physics course and I just need a little help grasping the concept of zero work and whether or not it applies to this situation... The question reads:

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

    A child on a sled (having a combined mass of 47.0kg) is pulled by a force directed along a rope that make a 45° angle with the horizontal axis. The force exerted on the rope is 100.0N. The force of friction acting on the sled is 30.0 N. If the child is pulled a distance of 10.0 m along a level field, determine the total work done on the child and on the sled.

    So I worked this out using W = F(cos θ)• ∆d
    I calculated the work from the pull and then the work from friction and added the values to get 407 J which I'm pretty confident is right..

    2. The attempt at a solution

    Where I need help is in deciding whether or not this value is the work done on the child and the sled..
    is this the work done on the sled. And the work done on the child is zero.

    I'm at this dilemma because my text says that 'whenever a force is exerted perpendicular to the direction of displacement, it does not contribute to forward motion.'
    So I'm thinking that the child is just kind of along for the ride and the sled is experiencing work. I think this because the child is only experiencing ForceNormal and ForceGravity.. but then again isn't the child experiencing some kind of ForceFriction from the sled and does the fact that the child's weight is definitely affecting the answer something to consider?

    Please help!
    Last edited: Mar 18, 2012
  2. jcsd
  3. Mar 18, 2012 #2
    Hi chubbyorphan, as you have clearly used the combined mass of both the sled AND the child, the work is been done on the two of them, not just the sled.
  4. Mar 18, 2012 #3


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