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Does friction oppose force or motion?

  1. Jun 5, 2013 #1
    I have always treated friction as 'opposing' force rather than motion, either at its limit or to maintain equilibrium. I have just realised, however, that if an object is at rest on an inclined plane, but is not at limiting equilibrium and you give it a nudge it will accelerate while you apply the force on to it but then (usually) decellerate to rest. If friction opposes force you would expect the object to maintain a constant velocity once it is moving, so there must be friction greater than the weight parallel to the plane.

    Does this mean that friction opposes motion rather than force and would you calculate the change in friction as the object deccelerates?
     
  2. jcsd
  3. Jun 5, 2013 #2

    mfb

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    Push an object, and let it go. There is no force acting on it (apart from gravity, downwards). If friction would oppose forces, in which direction would it point? ;)
    Push an object, now apply a force against the direction of motion. What do you expect from friction: does it "help" to decelerate it (friction against direction of motion), or will friction accelerate the object (friction against your force)?
     
  4. Jun 5, 2013 #3

    jedishrfu

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    for an object on an incline there is gravity trying to pull it down and friction holding it in place. If the gravity is greater or the incline is steep enough then that the sum of the two forces will favor gravity. If they were equal then the body will stay where it is.

    There's several kinds of friction. For the object on the incline there's static dry friction and dynamic dry friction. Static is usually greater then dynamic dry friction. Its like you must push a little harder to get it to move initially to overcome the static dry friction.

    Once you pushed the object if gravity is greater than the dynamic friction then the object will continue to slide and accelerate. If the dynamic friction and gravity were equal after you pushed it then it would slide with constant speed. If less then it would slow down. In the equals case the sum total of force on the object is zero and so it continue its motion at constant speed (Newton's first law)

    You can read more about the different types of friction here:

    http://en.wikipedia.org/wiki/Friction

    and Newton's laws here:

    http://en.wikipedia.org/wiki/Newton_first_law
     
  5. Jun 5, 2013 #4
    What I am asking is that once the object is pushed (assuming that limiting friction is greater that the force of gravity) if the friction opposes force shouldn't it be equal to gravity, keeping the object at constant velocity rather than greater than gravity, causing it to deccelerate?

    In other words if the object were at rest then friction could never be greater than gravity, so why can it be when the object is moving?
     
  6. Jun 5, 2013 #5
    The coefficient of kinetic friction is usually less than the coefficient of static friction, so that, once the block starts sliding down the incline, it will accelerate.
     
  7. Jun 5, 2013 #6
    I am talking about when friction is not limiting so the object decellerates (I've tested it)
     
  8. Jun 5, 2013 #7

    SteamKing

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    According to the Coulomb theory of friction, the magnitude of the force due to friction is proportional to the normal force of an object, and friction acts to retard motion.
     
  9. Jun 5, 2013 #8

    PhanthomJay

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    why not? Lets suppose the incline angle is very small, like practically flat. Assume m is 2 kg, the static coef of friction is 0.4, and the kinetic friction coef is 0.2. When the object is at rest, the gravity force down the plane is about 0, and the friction force is therefore also 0, for equilibrium. The limiting friction is .4mg = 8 N, so this meets your requirement. Now give it a good push so it starts moving, and release your pushing force. I think you will agree that the gravity force down the incline is still about 0, but the friction force up the incline is about 0.2mg = 4 N. The object must therefore decelerate, since the friction force is much greater than the gravity force down the incline.
     
  10. Jun 6, 2013 #9
    Ok thats really helpful thanks. So it is the case that when an object is in motion, friction will always be at a maximum? I.e. it responds to motion, not force?
     
  11. Jun 6, 2013 #10

    PhanthomJay

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    Yes, correct. Kinetic friction is opposite the relative motion (slipping) between the surfaces, always a single value equal to the kinetic coef of friction times the normal force. Static friction, which is less than or equal to the limiting value, is opposite to the pending relative motion between the surfaces.
     
  12. Jun 6, 2013 #11

    Dale

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    If I understand what you are asking then I would say that static friction opposes force and kinetic friction opposes motion.
     
  13. Jun 6, 2013 #12

    PhanthomJay

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    Not always, I would say, as in the case of a crate sitting on the bed of an accelerating truck, the crate being stationary with respect to the truck, but accelerating with respect to the ground. Static friction provides the force necessary to accelerate the crate at the same rate as the truck. There is no other force acting on the crate in that direction, so there is no force for the friction force to oppose. The direction of the friction force is opposite the direction it would move with respect to the truck bed if no friction was present.
     
  14. Jun 6, 2013 #13

    Dale

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    When you say "with respect to the truck bed" you are discussing a non-inertial reference frame where the truck bed is stationary. In such a frame there is a force which is opposed by the friction. So I don't really see that as a counterexample. I would rather use an inertial force than a hypothetical "direction it would move if...", but that is largely a matter of taste. I don't think that your approach is wrong, but I wouldn't use it.
     
  15. Jun 7, 2013 #14
    What do you mean by a "non-inertial reference frame" and what is the force opposing friction in the above case?
     
  16. Jun 13, 2013 #15
    The most general statement you can make is friction opposes the relative motion between two bodies.
    When you apply a push the object you apply force for very less time. So it attains a velocity. Friction tries to oppose relative motion between object & inclined plane.
    The object would attain a constant velocity if you would apply a continous force.
     
    Last edited: Jun 13, 2013
  17. Jun 13, 2013 #16

    mfb

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    Only in some cases. Usually, friction between solid objects does not depend significantly on the velocity, so its force is constant (if the object is moving) and we get constant acceleration. Not forever, of course, at some point friction will become nonlinear (or air resistance or heating gets relevant, or whatever).
     
  18. Jun 13, 2013 #17

    A.T.

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    I would specify that kinetic friction opposes the relative motion between the contact surfaces. Motion is relative, and you can find frames where the kinetic friction acting on an object is in the same direction as the velocity of the object, so it is not really opposing motion in that frame.
     
  19. Jun 13, 2013 #18

    Dale

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    Yes, good point.

    Maybe "opposes slipping".
     
  20. May 4, 2015 #19
    Ah... Awesome explanation... That's just what I wanted
     
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