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Homework Help: Confusion with work and constant velocity

  1. Apr 6, 2009 #1
    1. The problem statement, all variables and given/known data
    So I'm confused about work and constant velocity. In a sailboat example, if the boat is going at constant velocity then there is no acceleration, therefore no net Force and no work.

    However, let's say there is an elevator being pulled upward by a pulley at constant velocity. This would mean the elevator is not accelerating so no net Force, and then no work.
    But I'm confused about PE. Since the elevator is being pulled upward, wouldn't PE increase and isn't change in PE = W? So work would increase with increase in PE?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 6, 2009 #2


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    Homework Helper

    Not exactly. For the ship there is work against friction in pushing the boat through the water, likely even can be somewhat determined from the weight of the water that is churned by the propeller. There is no acceleration, so no work goes into kinetic energy of the mass of the ship.

    For your elevator there is no work again going into kinetic energy, because there is no change in speed. But there is a change in potential energy. For instance if you cut the rope and the elevator fell back to the starting point there would be that much kinetic energy carried by the elevator as it passed that point going down.
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