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Static friction does no work (energy is conserved)

  1. Mar 24, 2014 #1
    I have read that static friction does no work and energy of the system is conserved if only this type of friction exists. Is this only an experimental fact, or can it be proved from basic principles? If it can, how?
     
  2. jcsd
  3. Mar 24, 2014 #2

    maajdl

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    http://en.wikipedia.org/wiki/Friction#Static_friction :

    "Static friction is friction between two or more solid objects that are not moving relative to each other. For example, static friction can prevent an object from sliding down a sloped surface. "

    Where have you read that static friction does no work?

    Assume that by a static friction contact you are able to set a mass into motion.
    You must have transferred some energy to the mass, isn't it?
    Is there a fundamental difference between static friction and a contact force, excpet that one is tengential and the other normal to the contact surfaces?
     
  4. Mar 24, 2014 #3

    Tipler, "Physics for Scientists and Engineers", 5th edition, page 292: "Because the friction is static, it does no work, and there is no dissipation of mechanical energy".

    The problem in question is ball rolling down an incline without slipping. The friction between the ball and the incline is said to be static.
     
  5. Mar 24, 2014 #4

    A.T.

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    ... in the rest frame of the surfaces in contact.
     
  6. Mar 24, 2014 #5
  7. Mar 24, 2014 #6
    I am in agreement with both maajdl and A.T.. The work done by static friction is dependent on the frame of reference of the observer. In fact, work in general is dependent on the frame of reference of the observer.

    Chet
     
  8. Mar 24, 2014 #7
    Tipler said that? That's very disappointing 'cause it ain't always true. But sometimes it is, depending on the specific physical situation.
     
  9. Mar 25, 2014 #8

    maajdl

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    I think there is maybe a confusion here between "work" and "dissipation of energy".
    In static friction, no energy can be lost or dissipated.
    But work can be done.
     
  10. Mar 25, 2014 #9
    This is about definitions. Static friction is usually regarded as the force being reversibly/elastically applied to one surface by another. Generally, when you apply a force to a fairly rigid object in contact with another surface, some of that force will be transferred to the interface and cause various changes in the second surface. This can include compression, erosion, deflection, flow, and the creation of various surface defects (all on a microscopic level). In the ideal case (which is no doubt what you are being taught) the atomic nature of the surfaces is NOT considered, hence no work is done. (ie. ideal solids). In the real world, if you touch a glass window, you leave a little bit of you on the glass, and a little bit of glass on you. These effects are ignored, except by the writers of CSI (and by tribologists, surface physicists & chemists, forensic scientists, etc.)
     
  11. Mar 25, 2014 #10

    A.T.

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    Yes, or it is a (mis)interpretation of the "static" qualifier. In general "static friction" means there is no velocity difference between the contact surfaces, not that the contact is at rest.
     
  12. Mar 25, 2014 #11

    A.T.

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    Even in the ideal case work is done in any reference frame where the contact patch moves parallel to the force. Work is frame dependent.
     
  13. Mar 25, 2014 #12

    sophiecentaur

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    Static friction can be though of in just the same as chains and sprockets or gear wheels and teeth. Work can be said to be 'done on' the chain by the driving crank and the same work is 'done on' the output sprocket. There is a lot of angst, expressed in this thread and many others, concerning who or what 'does the work'. Is it really worth worrying about as long as you do the right Force and Displacement calculations. They will tell you the energy transferred.
     
  14. Mar 25, 2014 #13
    I'm not sure how well defined this is.

    For example: Stack a rigid body on another rigid body on a frictionless surface and apply a gentle force to the bottom body. Work is done on the upper body but by which force? It seems arbitrary to me.

    Alternatively: If we drop a rigid body onto a moving but unpropelled rigid body, then to argue that static friction does work on the upper body would require a non-physical infinite acceleration. If there is no normal force in the direction of the acceleration then there must be a period of dynamic friction.

    The reality is that there is no such thing as a rigid body, though. Conceptually in the idealised case, I think you can have it either way, but I don't think you can construct a case where you are forced to conclude that static friction must do work unless you wish to consider it an external force.
     
    Last edited: Mar 25, 2014
  15. Mar 25, 2014 #14

    A.T.

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  16. Mar 25, 2014 #15

    Nugatory

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    well, seeing as how there is only one force acting on the upper body, there's not a lot of room for arbitrariness there.

    The arbitrary choice was made when we considered this as two bodies with a force acting between them, instead of as one larger body. If we had glued the two bodies together, we wouldn't have any trouble with treating the two bodies as one; and if we instead chose to treat them as two bodies, we'd have no difficulty seeing how the adhesive force of the glue is what's accelerating the upper body. We can think of static friction as just a rather weak glue.
     
  17. Mar 25, 2014 #16

    A.T.

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    This and the choice of the reference frame of course. Once you decide what the bodies are and choose the reference frame, the rest follows.
     
  18. Mar 25, 2014 #17

    Dale

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    Nice example!
     
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