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I have a simple question.

  1. Sep 23, 2011 #1
    Does friction come in pairs? Like if friction acts on something is there like another force due to Newton's 3rd law that acts in the opposite, so are like 3 forces acting upon things most times? Gravity, Friction and weight? I'm sorry for the vagueness of my question, I'm not really what sure to ask, I'm still a novice to Physics, I'm currently going through a textbook called Physics, By Abott.
  2. jcsd
  3. Sep 23, 2011 #2
    There are many more forces too, Centrifugal force of the earth spinning, air pressure, gravity of the moon, the sun too and we are also wizing through space on an arm of the milky way. Sure some of these are small undetectable forces but they are there. The molecules within the two substances are moving, causing brownian motion perhaps.
  4. Sep 23, 2011 #3
    :S I'm still confused, alright listen to my real question, I got from my teacher today.

    Okay let's say two coins are stacked upon each other on a table.

    A force was applied to the one of the bottom, considering there is friction between them, which way will the Top coin move?

    My teacher told me it would move forwards, because the friction acts on the bottom coin in the other direction, and I think the friction forces causes an opposite and equal force to act on the top coin THUS moving it with the bottom.

    The reason I asked if forces come in pairs, is because my teacher said something like that. But I'm not sure.
  5. Sep 23, 2011 #4


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    Staff: Mentor

    All forces occur in pairs; equal and opposite pairs. You can't push against something if it doesn't push back. If you want to drag something along a concrete pathway, the rough surface opposes the motion. It pushes back with the same force that you must exert to overcome it to achieve steady movement. If you exert a force that is greater than that needed to overcome friction, then the body speeds up. The force it pushes back with, as you accelerate it by pushing, is given by F=ma. Again, the forces are exactly equal (and opposite).
  6. Sep 23, 2011 #5
    Oh thank you, so much NascentOxygen and VolKl
  7. Sep 23, 2011 #6


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    Staff: Mentor

    Try it and see. :smile:
  8. Sep 23, 2011 #7
    I will.
  9. Sep 23, 2011 #8
    If the bottom coin is moved at the speed of light the top coin will simply drop, therefor in this case the correct answer is that it moves perpendicularly to the bottom coin. I'd like to see your teachers face when you explain that :)
  10. Sep 23, 2011 #9

    Ken G

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    Gold Member

    I think the point the teacher is making is a bit like when you move your hand through water, you feel a retarding force on your hand. That's a lot like friction on your hand (it's actually called viscosity, but it's just like friction). To see if your hand is exerting any opposite forces on the water, simply look at what happens to the water-- it ends up flowing in the same direction that you moved your hand. So the backward "friction" force from the water on your hand that makes it hard to move your hand through water, comes with a forward "friction" force of your hand on the water, and makes the water move forward. It's similar to the coins, the device is to help you visualize action/reaction pairs.
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