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Homework Help: Direction of Friction force

  1. Aug 18, 2014 #1
    1. The problem statement, all variables and given/known data

    One surface moves over the other stationary surface.

    a. Is friction felt by both the surfaces?
    b. Is the friction force same in magnitude?
    c. What are the direction of friction forces felt by them?

    2. Relevant equations

    3. The attempt at a solution

    Since both the surface are in relative motion and touching each other, the friction force should be produced. But i could not imagine how the stationary surface feel the friction.

    This is my first attempt at physics forum. If someone can provide the clear example that lets me imagine, it will be greatly helpful.
  2. jcsd
  3. Aug 18, 2014 #2

    Doc Al

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    Staff: Mentor

    Take a shot at answering your own questions! Hint: Does friction help or hinder motion, in general?
  4. Aug 18, 2014 #3
    Why imagine when you can do an experiment: hold one hand still and use your other hand to move a book over it.
  5. Aug 18, 2014 #4
    consider a case:
    you're tied with a rope which is tied with the roof , and you're hanging , there is a platform (with small wheels) beneath you and you can walk on that platform . Now what will happen when you walk on that ( definitely you won't go forward ) ? now consider your first and third parts.
    For the second part observe and make cases .
  6. Aug 18, 2014 #5
    Thank you very much for this book and still hand example. I tried it. Now it is clear that if the hand is still and book sliding on the right side, produces two friction forces (i.e. two gripping forces), one that is felt by the book which tries to pull the book to the left and the other that is felt by the still hand which tries to pull the hand to the right. This frictional forces are of the same magnitude but in opposite directions.

    Please confirm and correct me if I am wrong.
  7. Aug 18, 2014 #6
    It can do both. It is the force of reaction, if I am not wrong. It can help you push forward as well as it can drag you from moving forward when sliding.
  8. Aug 18, 2014 #7


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    Homework Helper

    Yes that is correct. (Friction obeys Newton's third law.)
  9. Aug 18, 2014 #8
    Thus friction can help push you or drag you. Right?
  10. Aug 18, 2014 #9
    All correct!
  11. Aug 18, 2014 #10


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    Newton's third law just says that the force of friction from A on B is equal and opposite to the force of friction from B on A. It says nothing about which direction this is.

    As it turns out, dynamic friction acts to oppose the relative motion that exists between A and B.

    Static friction acts to oppose the relative motion that would exist between A and B if the static friction were eliminated or reduced.

    If you push your foot backwards against the ground, your foot would slip backwards were it not for the friction that exists. So the friction pushes your foot forward. Static friction in action.

    If you spin your tires forward, trying to get unstuck from the snow, the treads on your tires are moving backwards. What little friction there is pushes the treads forward. Dynamic friction in action.

    If you slide down a rope by not gripping tightly enough with your hands, your hands slide downward on the rope. So the friction pushes your hands upward. Dynamic friction in action.
  12. Aug 18, 2014 #11

    Doc Al

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    Staff: Mentor

    Yes. The key is that friction acts to oppose (or prevent) slipping between surfaces.
  13. Aug 18, 2014 #12
    There is lot of confusion about the ultimate cause of friction over the web. Some say it is the inter-molecular forces, others say it is caused by the roughness. What is it precisely? What actually grips the two surfaces?
  14. Aug 18, 2014 #13


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    Those are the same. "Roughness", looked at from the level of molecules is "inter-molecular forces".
  15. Aug 18, 2014 #14

    rude man

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    I am fortunate to have had, as my engineering tutor, Dr. Frederick Abernathy of Harvard, who showed that static friction always acts in a direction so as to oppose motion.
  16. Aug 18, 2014 #15
    I feel the roughness has to do with bumpy ride, whereas friction has to do with the stickiness.
  17. Aug 19, 2014 #16


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    Roughness implies that at some sufficiently detailed level molecules are being pushed into the body of the object of which they are part in order to get around bumps in the other body. While that does indeed involve intermolecular forces, one can imagine other such forces - maybe attraction across the interface.
    That said, it is more obvious how the roughness forces would be proportional to the normal force.
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