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Friction: Equal and Opposite Forces.

  1. May 2, 2012 #1
    From Newton's Third Law of Motion, I know that there is a reaction force--equal in magnitude and opposite direction--for every action force. But what I find a bit puzzling is what this author of an article says about friction, "Thus the force of friction has a remarkable property of adjusting its magnitude so as to become exactly equal to the applied force tending to produce motion. However, after a certain limit the force of friction cannot increase further. If the applied force exceeds this particular limit then the block starts moving as the two forces, i.e., the force of friction and the applied force are not balanced. " I thought forces were always equal in magnitude and opposite in direction. But if that were true, nothing would move. Observing motion in reality, it would make sense that, depending on the surface, friction has some threshold, which the allow the eventual motion.
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  3. May 2, 2012 #2


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    Action force and reaction force are applied to two different bodies. The two bodies that are interacting. So if you push on the block, the block pushes onto you with the same force. If there is friction acting from table onto block, there is friction acting from block onto table. But the force with which you push the block need not be equal to the force of friction applied by table on the block. Ergo, the net force on the block can be non-zero. What author points out is that if the block is initially at rest, then you must apply a certain amount of force to get it to move. If you apply less force, the friction will match it exactly, resulting in zero net force. This particular cancellation has nothing to do with Newton's 3rd law.
  4. May 2, 2012 #3


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    The applied force and the friction force on the book are not a Third Law action-reaction pair.

    There are two pairs of Third law forces here. To be more concrete, let's talk about using your finger to push a book across a table against friction.

    1. The contact force that your finger exerts on the book, and the contact force that the book exerts back on your finger (thereby compressing the flesh at your fingertip).

    2. The friction force that the table exerts against the book, and the friction force that the book exerts against the table (which would cause the table to move forward if there were no friction between it and the floor).
  5. May 2, 2012 #4


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    Be careful with Newton's third law. What it says if that if object 1 and object 2 are in contact, then the force felt by object 2 due to object 1 is equal in magnitude and opposite in direction to the force felt by object 1 due to object 2.

    Notice that these two forces act on *two different objects*, so your claim that "nothing would move" is false, because you assume that the balanced pair of forces is applied to the same object, when really each force in the pair is applied to a different object.

    What the author says about friction is something different still, and unrelated to Newton's third law. If I try to slide a box across a floor, then the friction force on the box will have the property that it will always try to oppose my push. This is a special property of friction.

    What Newton's third law says in this situation is that the friction force acting on the box due to the floor is equal in magnitude and opposite in direction to the friction force acting on the floor due to the box.
  6. May 2, 2012 #5

    Doc Al

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    The applied force and the friction force are not third law pairs.

    Imagine a block sitting on a surface. You push it with some force F(applied). If possible, the surface will generate just enough static friction F(friction) to balance out that applied force. But the static friction is not related to the applied force via Newton's 3rd law.

    The 3rd law pairs in this example are:
    - you push on the block (the applied force) and the block pushes back on you with an equal and opposite force;
    - the surface exerts a friction force on the block and the block exerts an equal and opposite friction force back on the surface.

    But only under certain conditions will F(applied) equal F(friction).
  7. May 2, 2012 #6


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    With the same explanation having appeared four times in slightly different ways, I am optimistic about the outcome ;)
  8. May 2, 2012 #7
    @cepheid: haha You are certainly right about the outcome. Thank you all, I understand now.
  9. May 2, 2012 #8
    OP, I think the author is referring to static friction specifically. There is also a constant coulomb friction, a viscous friction that is proportional to velocity, and another component called the stribeck effect. When the friction initially drops from the static friction point as velocity is non-zero and then rises again from the viscous friction, the friction is showing the stribeck effect.

    See the attachment, you might find it interesting.

    Attached Files:

    Last edited: May 2, 2012
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