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Pulleys question

  1. Nov 12, 2008 #1
    If a string around a pulley is pulled without slipping, im wondering if there is any frictional force acting on the string by the pulley. If there is, will it appear in calculations? Thanks in advance
     
  2. jcsd
  3. Nov 12, 2008 #2

    Doc Al

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    Sure, otherwise the string couldn't turn the pulley.
    Not usually. (It's static friction and does no work.)
     
  4. Nov 13, 2008 #3
    but wouldnt the static friction appear in torque calculation? Also what if the string is assumed to be massless, doesnt that implies no friction and so the pulley shouldnt be able to turn?
     
  5. Nov 13, 2008 #4

    Doc Al

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    No, it just transmits the string tension to the pulley.
    No.

    Typical assumptions (in intro courses) are that strings are massless (and thus have the same tension throughout) and that pulleys are frictionless (not between string and pulley, but about the axle of the pulley).
     
  6. Nov 14, 2008 #5
     
  7. Nov 14, 2008 #6

    russ_watters

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    The pulley moves with the string because the pulley is on bearings and free to rotate. Thus it doesn't restrict the motion of the string except by its own inertia when accelerating. It is the static friction between the string and the pulley that makes the pulley rotate.
     
  8. Nov 15, 2008 #7

    tiny-tim

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    Hi boredaxel! :smile:

    If the pulley has mass, and if you need to calculate the energy or angular momentum of the pulley, then yes, the friction is an external force on the pulley, and will provide a torque to the pulley.

    But exam questions usually (though not always :wink:) say that pulleys are massless.
    Friction depends on the normal force on the pulley, and (if the string is bent round the pulley) there will always be a normal force, even if the string is massless.
    Hi Doc Al! :smile: May I qualify that?

    A massles string with no obstructions will have the same tension throughout (while a massive string hanging vertically will of course have more tension at the top) …

    but a string touching anything (like a pulley) with friction will have different tension on either side, even if the string is massless. :wink:
     
  9. Nov 15, 2008 #8

    Doc Al

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    Absolutely, but you generally don't need to explicitly consider static friction. The forces exerted on the pulley are represented by the tensions in the string.

    But not if the pulley is massless, a common assumption as you point out. I was waiting for the OP to bring up the issue of a massive pulley so I could qualify my statement. :wink: (One step at a time.)
     
  10. Nov 15, 2008 #9

    tiny-tim

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    ah … I guessed that's what you were thinking … but I noticed the OP had mentioned torque, so I assumed his pulley already had mass. :wink:
     
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