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Friction and fictitious forces

  1. Nov 9, 2006 #1
    i have a question regarding friction and fictitious forces.

    if two crates are lying on a flat surface, while the smaller one is on top of the bigger one, and a force of 10N is acted upon the bigger one

    will the smaller one be pushed backwards because of fictitous forces?

    and because there is friction between the surfaces of the upper side of
    the bigger crate and the bottom side of the smaller crate, the smaller one should be slowed down accordingly by kinetic friction?
  2. jcsd
  3. Nov 9, 2006 #2

    Doc Al

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    Staff: Mentor

    Why would "fictitious" forces be involved? They appear when you view things from an accelerated reference frame.

    In this case, the only horizontal force acting on the smaller crate is due to the friction between the surfaces. If the 10N force acts to the right on the bigger crate and accelerates the crates to the right, the friction force (between the crates) would also act to the right on the smaller crate, dragging it along for the ride.
  4. Nov 9, 2006 #3
    yes but what happens if you push it a little bit stronger?

    intuitively it seems very right to me, that if you put an object on another object and you push the one on the bottom to the right, the upper object seems to be pushed to the left, and then the friction gets in motion, and attempts to stop it.

    what is the force which pushes the object to the left?
    because the same thing does not seem to apply when the speed is constant.

    when the speed is constant the only chance it would ever slip back, is if you tried to increase the speed, but an increase in speed also leads to a small time of acceleration.
  5. Nov 9, 2006 #4

    Doc Al

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    If for some reason you choose to view things from the frame of the accelerating lower crate, then yes the upper crate will seem to be pushed to the left with respect to the lower crate.

    But you can also--more simply--view things from the inertial frame of the floor. In that case, there are no fictitious forces to worry about. The only force acting on the upper crate is friction--a real force. So, with respect to the floor, the upper crate will accelerate to the right, never to the left. (Viewed from the floor, the upper crate never moves to the left, only to the right.)

    That "force" pushing to the left is just an artifact of viewing things from an accelerating frame. If the speed is constant, there is no accelerating frame and no fictitious forces.
  6. Nov 9, 2006 #5
    i tried conducting a little experiment testing this phenomenas. i took a paper clip and placed it on my notebook, and as i pushed the notebook to the right, from my frame of reference, which is the chair or the floor, the paper clip seemed to move backwards

    am i getting something wrong here?
    your explanation of the frames of reference really clears up a lot of things to me, but something still does not seem to sit right.

    and i have another question if we are already discussing this.
    an hypothetical situation -
    if i am in a non inertial frame of reference, and i fall back due to a fictitious force, how would it look to an outside observer? if fictitious forces only take place in my frame.
    Last edited: Nov 9, 2006
  7. Nov 9, 2006 #6


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    Staff: Mentor

    That simply isn't possible. Make a mark on the table and actually look to see if the paper clip moves to the left or right of the mark. The paperclip will move to the right.
  8. Nov 9, 2006 #7
    yes i think i understand where i got it wrong now.

    i went through my physics book, and if i understand it right, the lower object is dragging the upper object along with it due to electromagnetism, and reasons which supposedly will be explained later referring to other units of the course.

    what i do not understand though, why would the upper object deattached from the lower one in the first place, is the electromagnetic force not strong to keep up?

    and how can the electromagnetic force(friction) be affected by external forces that are pushing the lower object forward?

    the way i see it if this is all true, then everytime i touch something i change its internal composition.
  9. Nov 9, 2006 #8

    Doc Al

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    Exactly. The static friction force has a maximum strength:
    [tex]F_{static} \leq \mu mg[/tex]

    If the acceleration of the lower block is too great, the static friction will not be enough to give the upper block the same acceleration. In that case, the upper block will start slipping, being dragged along by kinetic friction but not enough to keep up with the lower block.
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