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Relative Work - not sure what it means-

  1. Nov 16, 2011 #1

    Two people push in opposite directions on a block that sits atop a frictionless
    surface (The soles of their shoes are glued to the frictionless surface). If the
    block, originally at rest at point P, moves to the right without rotating and ends
    up at rest at point Q, describe qualitatively how much work is done on the
    block by person 1 relative to that done by person 2?


    2. Relevant equations

    W = F*dx

    3. The attempt at a solution

    I could not make up my mind with the question, what it asks. I said from the direction of the motion we know F1>F2 and thus W1>W2. And also W1 is positive whereas W2 is negative. But what else should i say? Shall i say total work is W1-W2 or something like that ?
  2. jcsd
  3. Nov 16, 2011 #2


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    Homework Helper

    A strange question! I'm wondering why the motion stops. Does person 2 start pushing harder, to overcome F1 and decelerate the block? Maybe it hits his glued-down shoes.
    Anyway, the total work is ultimately zero when the kinetic energy disappears. Before that, all is as you said.
  4. Nov 16, 2011 #3
    Can you please express your opinion about the last sentence of the question? What does it imply by saying "by person 1 relative to that by person 2?" I mean it does not ask about total work, but I feel uncomfortable that I think I am not answering properly if I say things only in post 1.
  5. Nov 16, 2011 #4


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    Homework Helper

    Everything you say is "relative", since you have no way to calculate any absolute numbers.
    I'm just thinking of some qualifiers, like F1 > F1 at some point because the box moves to the the right. One does some work on the box and on two. Two absorbs all that work eventually, some gradually, the last of it when the box hits his foot and loses its kinetic energy.