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B Generalized version of work-energy theorem

  1. Oct 13, 2016 #1
    I know that for rigid bodies only the work-energy theorem states that the net work done on the body equals the change in kinetic energy of the body since a rigid body has no internal degrees of freedom and hence no other forms of energy such as potential energy. Is there a most generalized form of work energy theorem that is valid for rigid as well as non rigid bodies and for conservative as well as non-conservative force?
     
  2. jcsd
  3. Oct 13, 2016 #2

    Doc Al

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    This might not be what you're looking for, but it applies to rigid or non-rigid bodies, since it deals with center of mass quantities:
    $$F_{net}\Delta x_{cm}=\Delta (\frac{1}{2}m v_{cm}^2)$$
     
  4. Oct 13, 2016 #3
    No
    We can apply work energy theorem for a rigid body or a point particle substituting the body(center of mass) in former case(applying work energy theorem for rigid body) we may have terms like rational kinetic energy,...
    But in the later one(applying work energy theorem for centre of mass of the rigid body) we may have terms for work done due to those forces which did zero work on the rigid body(due to no movement of point of contact)

    Willy nilly we get the same result

    It all matters on what you choose your system to be
    What might be potential energy in one case can be work done by external force in another case
     
    Last edited: Oct 13, 2016
  5. Oct 13, 2016 #4

    Doc Al

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    Yes! (The theorem is a consequence of Newton's 2nd law.)

    One can certainly apply conservation of energy, but I thought the question was asking for something different.

    Exactly! Those terms have the appearance of work, but do not reflect actual work done (in the first law of thermo sense). Nonetheless, the theorem is valid.
     
  6. Oct 14, 2016 #5
    Hey
    What happened??
     
  7. Oct 14, 2016 #6
    @Doc Al, I think that your equation will work out for rigid and non-rigid objects but why is there no account of change in other forms of energy(potential energy, heat energy) in the equation? Another question:Does Fnet include the non conservative forces as well or only the conservative forces(is Fnet the resultant of the net conservative force and the net non-conservative force)?
     
  8. Oct 14, 2016 #7

    Doc Al

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    Don't confuse this equation with the much more general conservation of energy.

    Fnet includes all forces acting on the body, whether conservative or not.
     
  9. Oct 15, 2016 #8
    Could you please elaborate.
     
  10. Oct 15, 2016 #9
    Video 1 (work energy theorem-18:00 to the end)

    Video 2 (work energy theorem-full video)


    He' a great prof. (simply awesome)
    BUT YOU HAVE TO UNDERSTAND INDIAN ACCENT !!
     
  11. Oct 15, 2016 #10

    Doc Al

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    Sure. The equation I gave is derived from Newton's 2nd law, using the properties of the center of mass. The left hand side looks like a work term, but is not. (It's sometimes called center of mass work or pseudowork.) The work terms used in conservation of energy equations require the forces to be combined with the displacement of their point of application, not the displacement of the center of mass. And conservation equations can have all sorts of internal energy quantities.
     
  12. Oct 15, 2016 #11
    So these equalities are valid for point particles, rigid bodies and non-rigid bodies, Right?
    ΔK.E. = Wnet = Fnet . dcentre of mass = (Fnet conservative + Fnet non-conservative + Fnet external) . dcentre of mass = Wnet conservative + Wnet non-conservative + Wnet external
    And the potential energy is included in Wnet conservative. Correct?But is there any proof that Wnet conservative=-ΔP.E.?
    Also i could not understand one thing: do conservative forces change the K.E. and non-conservative forces change the P.E. or the opposite or do both the types of forces can change both the types of energy and what type of energy can external forces change.
     
    Last edited: Oct 15, 2016
  13. Oct 16, 2016 #12

    Doc Al

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    The center of mass equation that I gave is valid for all of these.
    Careful about setting the center of mass "work" term equal to the total change in energy. Don't confuse that equation with the more general conservation of energy.
    All forces contribute to the change in the KE of the center of mass.

    Here's what's tricky, and where the center of mass equation is useful. Imagine jumping into the air. Your KE obviously has changed, despite no external work being done on you. The center of mass equation still holds.
     
  14. Oct 16, 2016 #13
    @Doc Al , I know that the work done by conservative forces is independent of the path taken, i.e., the work done by conservative forces in moving an object from an initial point to a particular final point is the same for different paths taken. Thus the potential energy of the object at the particular fixed point is same for different paths taken to reach the particular final point from the initial point and so it is wise and useful to use the concept of potential energy in case of the work done by conservative forces.
    In case of non-conservative forces some difficulty arises because non-conservatives forces are path dependent. The work done by non-conservative forces is dependent on the path taken,i.e., the work done by non-conservative forces in moving an object from an initial point to a particular final point is different for different paths taken. Thus the potential energy of the object at the particular final point is different for different paths taken to reach the particular final point from the initial point and so it is not wise and useful to use of the concept of potential energy in case of the work done by non-conservative forces. Also mechanical energy is conserved in case of conservative forces while in case of non-conservative forces mechanical energy is not conserved. So far correct.Right?

    Now i recently understood the reason why W net conservative = -ΔP.E.(which had been troubling me for long). The work done by conservative forces have been defined like this, so that the law of conservation of mechanical energy is complied to when no non-conservatives are acting on the body(In which case the law of conservation of mechanical is not complied to).Correct?

    Also isn't it correct that both conservative forces and non-conservatives increase the potential energy of the body(although the concept of potential energy is not used for non-conservative forces) but we let the conservative forces account for all the changes in potential energy?

    So in conclusion, All the types of forces(conservative, non-conservative and external forces) increase both the kinetic and potential energy of the body but we let the only the conservative force account for all the change in the potential energy.Right?
     
  15. Oct 16, 2016 #14

    Doc Al

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    Potential energy is only defined for conservative forces.

    True.

    Not really. A change in potential energy can only involve conservative forces. If you think otherwise, perhaps you can give an example of what you mean.
     
    Last edited: Oct 16, 2016
  16. Oct 16, 2016 #15
    For example:When a non-conservative force moves an object from one point to another point there is indeed an energy change of the body and do you think of this:
    And what do you think of this:The work done by conservative forces have been defined like this, so that the law of conservation of mechanical energy is complied to when no non-conservatives are acting on the body(In which case the law of conservation of mechanical is not complied to)
     
  17. Oct 16, 2016 #16

    Doc Al

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    Right. But if there is any change in PE it is due to the action of conservative forces. (By definition, essentially.)

    Sounds OK to me.
     
  18. Oct 17, 2016 #17
    @Doc Al,If non-conservative forces do not change potential energy then why is it equal to ΔM.E.(ΔK.E.+ΔP.E.)?
     
  19. Oct 17, 2016 #18
    @Doc Al ,In addition to the above question can you also tell that whether the work done by forces on the point of application equal to the work done on the center of mass of a body(Can we substitute the work done by the forces on the point of application by the center of mass)?
     
    Last edited: Oct 17, 2016
  20. Oct 17, 2016 #19

    Doc Al

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    Perhaps we're getting lost in semantics. Can a non-conservative force change the the PE of a system? Sure. But there must be conservative forces acting--else there would be no PE at all.
     
  21. Oct 17, 2016 #20

    Doc Al

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    No, definitely not. The work done by forces on the point of application is the "real" work. A force can produce center of mass "work" yet do zero work against the point of contact. (Consider the example of jumping that I gave before.) And you can also have forces that do real work, yet no center of mass "work".
     
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