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Homework Help: Potential Energy during Constant Velocity Motion

  1. Jan 20, 2013 #1
    1. The problem statement, all variables and given/known data

    Could you please clarify the confusion I have about potential energy? Consider an object being raised from the Earth's surface upwards at a constant velocity. Is the potential energy increasing while it is moving upwards at a constant velocity?

    2. Relevant equations

    3. The attempt at a solution

    This is my reasoning:

    While the object is being raised upward at a constant velocity, the net force on that object is 0. Therefore, the net work on that object is also 0. As a result, the kinetic energy of the object is constant while it is moving up at a constant velocity.
    Since the kinetic energy is not changing, the potential energy is not changing while the object is moving up. When the object STOPS moving, and is held at a constant height, then the net work done on the object during that interval when it is slowing down would be negative, which would decrease the kinetic energy until it reaches 0. This would be accompanied by an increase in potential energy. However, WHILE it is moving upward at a constant velocity, the potential energy is not changing.

    When I look online, it is said that the potential energy is increasing while the object is moving upwards, even though it is moving upwards at a constant velocity (no change in Kinetic energy). My reasoning is, because of the Conservation of Mechanical Energy, if there is no change in Kinetic energy, then there is no change in potential energy if we are dealing with conservative forces, which we are in this situation. Then why does the potential energy increase while it is being raised?

    Last edited: Jan 20, 2013
  2. jcsd
  3. Jan 20, 2013 #2


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    Homework Helper

    In the first place, in order to raise something up, you are working against gravity. So you are inputting energy into a system. If you apply conservation of energy, then you will have

    KE1+PE1+ E = KE2+ PE2

    E would be the energy you are inputting into the object to get it to whatever height.
  4. Jan 20, 2013 #3

    Doc Al

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    Staff: Mentor

    Just to add to what rock.freak667 explained:

    All true. But realize that by talking about the net force and net work you are explicitly including the work done by gravity. There are two ways to deal with gravity: Either treat it as another force, just as you did here, or treat it by using a potential energy term. It's one or the other (your choice) but not both. Otherwise you are counting gravity twice.
    That's not true. Gravitational PE increases as the object moves up.
    Either talk in terms of net force and net work including gravity or in terms of gravitational PE and the work done by all forces other than gravity. Since you chose here to mention PE, you would not include the work done by gravity in your analysis.
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