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Work & energy

  1. Sep 30, 2005 #1
    I just thought of work & energy as follows

    A force F when moves through a distance x we say that work is done. In this we say that the component of force acting along the direction of displacement does the work. Why we say that? What the other component does?

    potential energy
    we say that potential energy of a system is its potential to do work. When we apply a force to a mass and the mass moves thro a distance x the the stored potential energy is F x X

    so i say that the mass has the potential to do a work of F x X.

    I imagine a situation where a box on a table is moved by a distance X horizontally. The box is not having any potential energy to do work. Why is this?
  2. jcsd
  3. Sep 30, 2005 #2

    Doc Al

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    Staff: Mentor

    Whatever it's doing, it's not transferring mechanical energy to an object.
    Not all forces have an associated potential energy. For example: Gravity does; friction does not.

    The situation is not fully specified. Are you exerting a force F in the direction of its displacement? If you are exerting a force F (and that's the only horizontal force) then work you do goes into increasing the kinetic energy, not the potential energy.
  4. Sep 30, 2005 #3


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    In order that the direction of displacement not be the direction the force is pointing, there must be some other force acting (if you push horizontally, and the object moves up an inclined plane, the plane is pushing back perpendicular to itself and is counter-acting the component of force in that direction.). The only net force is in the direction of motion.
  5. Oct 1, 2005 #4
    the box i referred comes to a stop because of friction. so the kinetic energy change is zero. so can i say that the force which had pushed the box doesn't do any work?
  6. Oct 1, 2005 #5

    Doc Al

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    Staff: Mentor

    I'm not sure I understand the situation. If the box is coming to a stop, how can the kinetic energy change be zero? If the only force on the box is friction, then it does negative work on the box, bringing it to rest.
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