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Difference between static friction and kinetic friction

  1. Oct 14, 2007 #1
    I have some confusions between static friction and kinetic friction:
    When the static friction equals to applied force on an object, an object doesn't move and stays at rest I think. How about if the static friction is larger than the applied force? Will the object still stay at the rest, that the object will not be moved to opposite direction of the applied force?
    When the applied force is larger than the static friction, then it starts to move?
    When the object is moving, it has a kinetic friction. If the kinetic friction is equal to the applied force, will the object come to rest or will move in a uniform motion?
    If the kinetic friction is larger than the applied force, will the object move in an opposite direction of the applied force?

    Thanks for help.
     
  2. jcsd
  3. Oct 14, 2007 #2

    rcgldr

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    Static friction indicates the maximum force that can be applied before an object starts to move. If the object is not accelerating, and is on a horizontal plane, then in all cases, friction force is equal in magnitude and opposes the net "horizontal" force on the object, for both static and kinetic forces.
     
  4. Oct 14, 2007 #3

    Doc Al

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    Let's keep it simple. Imagine an object resting on a horizontal surface. Apply a horizontal force. As long as the force is less that the maximum value for static friction (which is [itex]\mu N[/itex]), then the applied force will equal the friction and the object will remain at rest. When the force is greater than the maximum value for static friction, then the object starts moving.

    Once the object is moving, kinetic friction appears. If the applied force equals the kinetic friction, then there's no net force and thus no acceleration--it keeps moving at whatever speed it has. Realize that kinetic friction always opposes the slipping between the surfaces, so it always acts opposite to the way the object is moving. If the applied force is less than the kinetic friction, then the object will slow down.
     
  5. Oct 14, 2007 #4
    Static friction cannot move the object from the rest. Friction makes objects slower or come to the rest or keep their rest position. Friction is against the motion. That's what I can add Doc Al's great answer.
     
  6. Oct 14, 2007 #5
    So if the horizontal force=static friction, the object still doesn't move, right? As long as the static friction is not greater than horizontal force, then the object will remain at rest.
     
  7. Oct 14, 2007 #6

    Doc Al

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    Right, since the net force would be zero it would remain at rest.

    The point is that the static friction will never be greater than the applied force.

    Let's use an example to hammer this home. Assume that the maximum value of static friction is 100 N (thus [itex]\mu N[/itex] = 100 N). If you push with a force of 10 N, what will be the static friction? Answer: 10 N. Push with a force of 75 N. What's the friction? 75 N.

    Static friction will always be whatever it needs to be to prevent slipping, up to its maximum value.
     
  8. Oct 14, 2007 #7
    OHh, sorry, I was meant to say that as long as the horizontal force is not greater than the maximum value of static friction, not the static friction greater ... okay. I got it. But kinetic friction doesn't have maximum value, right? I think static friction has because when it gets over to the maximum, it will become kinetic friction, and the object starts to move.
     
  9. Oct 14, 2007 #8

    Doc Al

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    Right. Static friction is always [itex]\leq \mu_s N[/itex], while kinetic friction always [itex]= \mu_k N[/itex].
     
  10. Oct 15, 2007 #9
    Well, I have one more problem here about static friction that I just discovered today.
    I just posted a new topic. https://www.physicsforums.com/showthread.php?p=1467721#post1467721
    Or I just put here. Much easier to see.
    I know that static friction happens only when the object doesn't move. And if the maximum static friction=applied horizontal force, the object still remains at rest. I have already posted a topic about friction yesterday. But there is a problem here:
    Two crates are stacked on top of each other on a horizontal floor. The coefficient of static friction between Box A and Box B is 0.6. The coefficient of static friction between Box A and the floor is 0.2. A force of 100 N is applied parallel to the floor on Box B. The boxes move together along the floor. What's the ratio of the mass of Box A to Box B.
    My teacher said if there is net force 0 acted on an object, the object is either at rest or moves with constant velocity. So she said the force 100 = the static friction for Box B. I have two questions here: If an object is at rest, then zero net force will still make it rest, right? But is there a possibility that it will move with constant velocity according to what my teacher said? Second, if the applied force = static friction, then the object is not supposed to move? Only if the force is larger than the static friction, then it will move?
     
    Last edited: Oct 15, 2007
  11. Oct 15, 2007 #10

    Doc Al

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    I presume the teacher meant that the force of 100N was just enough to overcome static friction between block and floor. Once overcome, static friction is replaced by the lower kinetic friction.
     
  12. Oct 15, 2007 #11
    So when the applied force=static friction, the object may move? If 100 N was just enough to overcome the static friction, it can be equal to the static friction? I mean at least it should be slightly bigger than static friction, maybe as my friend said, we assumed it to be just equal to static friction?
     
  13. Oct 16, 2007 #12

    Doc Al

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    Yes, the applied force needs to be slightly greater than the maximum static friction, at least until it starts to move. But if the maximum static friction is 100 N, how much applied force do you need to overcome it? 100.1 N? 100.01 N? Just call it 100 N! (But I think you have the right idea.)
     
  14. Oct 21, 2007 #13
    Wish teachers knew that often they word things differently than they really mean to
     
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