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Total work done if it's zero?

  1. Oct 21, 2013 #1
    1. The problem statement, all variables and given/known data
    An object is moving along some path. the total work done on the object during this process is zero.

    2. Relevant equations
    no equation

    3. The attempt at a solution
    A the speed of an object has decreased.
    B the speed of the object may have changed.
    C the object must be moving in a circle
    D the velocity of the object must not have changed
    E the speed of the object has not changed
    F the velocity of the object may have changed

    I chose answer D and E. From what I understand so far work is zero when the Force is perpendicular to the displacement.

    would speed and velocity changed, when the work of an object is zero?
  2. jcsd
  3. Oct 21, 2013 #2


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    Homework Helper

    If you do no work on an object then it's kinetic energy can't change. Why do you think the velocity vector can't change if the force is perpendicular to the displacement?
  4. Oct 22, 2013 #3


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    Science Advisor

    Note that "kinetic energy" depends upon speed, not velocity. Now, what is the difference between "speed" and "velocity"?
  5. Oct 22, 2013 #4
    It there is no work being done on the object, the force acting on the object (if it exists) must be perpendicular to the velocity vector. With the force is perpendicular to the velocity vector, the direction of the velocity vector must be changing, but, with no work being done, the magnitude of the velocity (speed) must be constant.

  6. Oct 24, 2013 #5
    the difference between speed and velocity is that "velocity" is speed with direction, and "speed" is just direction.
  7. Oct 26, 2013 #6

    Doc Al

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    Staff: Mentor

    It's certainly true that if the force is always perpendicular to the displacement then the work done will be zero. But this problem only states that the total work done on the particle is zero.

    What does the Work-Energy theorem tell you?
    Last edited: Oct 26, 2013
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