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I Is there a contradiction in Halliday and Resnick?

  1. Dec 25, 2016 #1
    In Chapter 11, section 11-4, subsection friction and rolling, it is stated that the static frictional force is along the same direction as the direction of motion because the point of contact of the wheel with the floor is moving in the opposite direction. Then, in the next subsection, the same situation but with an inclined plane is discussed, yet now the frictional force is opposing the direction of motion. Could someone please clarify this issue for me?
     
  2. jcsd
  3. Dec 25, 2016 #2

    NascentOxygen

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    Friction acts forwards on the rear wheel of a bicycle when you're peddling, and this allows you to speed up. But friction acts backwards on the front wheel all the time and on the rear wheel when you stop peddling and when you brake.

    Maybe the book is discussing the different situations?

    https://www.physicsforums.com/attachments/110502.gif
     
  4. Dec 25, 2016 #3
    Hello NascentOxygen
    Indeed the book is discussing a different situation. It is discussing the motion of a single wheel, not one that is related to another by the gears of the bike. If you have the book, extended 8th edition, please read it on page 279.
     
  5. Dec 25, 2016 #4

    NascentOxygen

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    No, I don't have the book. But I reiterate that the direction that friction acts differs according to whether the wheel is driving or is driven.

    Q: A heavy wheel rolling uphill is being kept turning by its inertia, so friction will be acting (a) in the direction of travel, or (b) opposite to the direction of travel?
    A: ???

    If this doesn't address your concerns, can you photograph the page and crop it to only what is needed, then attach it to your post here?
     
  6. Dec 25, 2016 #5
    The force of friction acts in the direction of motion of the surfaces in contact not the direction of motion of the whole object. In case of the bicycle the wheel in contact with the ground is moving opposite the bicycle motion whereas for the inclined plane the block and its contact surface are moving in the same direction.

    Oops the force of friction acts in the opposite direction of the moving surface.
     
  7. Dec 26, 2016 #6
    I've attached the relevant sections. Please read them and clarify the misunderstanding
    Thanks
     

    Attached Files:

  8. Dec 26, 2016 #7

    Stephen Tashi

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    Figure 11-7 isn't a complete free body diagram because there is no indication of a force that would cause the wheel to accelerate to the right. Assuming the wheel is a bicycle wheel we could draw an "applied torque" at the center of the wheel, but we could also imagine the torque is supplied by an applied force acting at point P. To give the applied torque the proper direction, this applied force would act to the left. The direction of the frictional force opposes the applied force.

    In Figure 11-8 , if you change the problem so the wheel is a bicycle wheel and a cyclist is trying to accelerate down the incline plane, then we can imagine that the the torque of the cyclist is produced by a force acting at P and pointing up the plane. In that case friction would act to oppose that force and it would point down the plane. In the problem showing in figure 11-8, there is no applied torque besides the frictional force and to explain the sense of rotation of the wheel, we have to point the force of friction up the plane.

    As to whether those figures are inconsistent with the words in the text, it's hard to say. (We don't see all of them!). I don't find it easy to apply the concept that "the direction of friction opposes the direction of slipping" when slipping would imply "standing still" as in figure 11-7.

    Friction is a peculiar force. It has a primitive sort of intelligence. If a block is stationary on an incline plane , friction "knows" to exert a force up the plane that exactly counteracts the component of gravity pointing down the plane. Is it Nature that knows how to do this ? - or is friction a force invented by human beings in order to balance the books?
     
  9. Dec 26, 2016 #8

    jtbell

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    One could say much the same thing about the reaction force that a wall exerts on you when you push against it. As the force that you exert increases, so does the reaction force... up to some limit, whereupon you push the wall over or you punch a hole through it.
     
  10. Dec 26, 2016 #9

    Stephen Tashi

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    And frictional forces and reaction forces are smarter than many physics students who sometimes draw them pointing the wrong way!
     
  11. Dec 26, 2016 #10
    Here is how Feynman explains friction.

    http://www.feynmanlectures.caltech.edu/I_12.html

    Perhaps you will get the impression that Feynman explains the topic much more thoroughly than those "other" textbooks and it actually makes sense? Thank you Professor Feynman, wish you were still around to enlighten us!
     
  12. Dec 26, 2016 #11

    Stephen Tashi

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    Part of his explanation is:
     
  13. Dec 27, 2016 #12
    This is one reason it's such a great explanation. Feynman was honest. I think it's great that he states what we don't know, instead of presenting a false picture of certainty. Among other things it's a challenge to people to figure things out. Elsewhere Feynman says not to worry if you don't understand QM, because no one does. Yet he helped develop QED.
     
  14. Dec 27, 2016 #13
    Thank you all for your interesting ideas :smile:
     
  15. Dec 28, 2016 #14

    Baluncore

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    The surfaces are not free to slide due to weak chemical bonds between them. The greater the force that pushes the surfaces together, the more bonds form. The force of friction is the tension needed to break those chemical bonds. Friction opposes movement because it is a “bond”.
     
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