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Finding the Coefficient of Static Friction in the Real World

  1. Oct 21, 2011 #1
    I remember doing an experiment in Physics class to find the coefficient of friction with gravity.
    We set an object with a flat surface on top of a ramp, and adjusted the ramp angle so that the object would move down the ramp at a constant velocity. Then we measured the angle of that ramp to find the force of gravity, then solved for friction.

    I was just wondering... we solved for the coefficient of kinetic friction, right?

    But doesn't the experiment show that kinetic friction was actually greater than static friction? The force of static friction was unable to be great enough to prevent acceleration, yet the kinetic friction is great enough to prevent acceleration.

    Or were they both the same?
    No... that wouldn't work, would it? In that case, because kinetic friction = applied force by gravity, static friction would be just enough to prevent acceleration... but the block did move.

    It seems that in every case where an object moves down a ramp with constant velocity, kinetic friction is higher than the static friction threshold. I don't think it would matter what material we used, just as long as we don't use, let's say, something with velcro-strength friction.

    I don't understand: why is this so? Is there something we didn't account for in this experiment?
  2. jcsd
  3. Oct 21, 2011 #2
    kinetic fricton and static friction are the two usual categories...forces between surfaces in relative motion are called
    forces of kinetic friction.

    Once motion is started the frictional forces acting between surfaces usually decreases so a smaller force is required to maintain uniform motion.
  4. Oct 21, 2011 #3
    If the kinetic friction were higher than the static friction you would see as you increased the incline that the object would make a tiny jump and instantly stop and bunnyhop since if the kinetic friction were higher than the static friction the static friction would be overcome by gravity, putting the object in motion, then this higher kinetic friction would overcome the acceleration of gravity and stop the object from sliding, then since it isn't moving static friction would take over and be overcome by gravity and so on.
    Instead what we observe is that once the incline is high enough the object begins to smoothly slide suggesting that the kinetic friction is actually less.
  5. Oct 24, 2011 #4
    No, no, no! That's not what I'm asking at all...

    I'm asking why the object doesn't accelerate down a slope if the static friction is really higher than kinetic friction.

    Because some force greater than the static friction must have overcome the static friction force (in this case, gravity), the object MUST accelerate if the kinetic friction force is less, unless gravity somehow changes its force, because the net force will not be zero.

    In our experiment, the object did not accelerate down the ramp even after breaching the static friction threshold, and we decided that kinetic friction force as equal to the sine of the angle times the gravitation force.

    This says one of two things:
    1. The object DID accelerate. The difference between static and kinetic friction was so miniscule that the acceleration was also very tiny. (Does this mean that it's not possible to make an object move at a constant velocity along a rough plane unless the force is changed?)
    2. The object DID accelerate, I'm just crazy.

    I know we found kinetic friction through that experiment, but what happened to the static friction?

    Let me just sum it up in a few words very simply:

    For the object to move,
    Force (mg sin θ in this case) > Static friction.

    Static friction > Kinetic friction
    Force > Kinetic friction

    Force - Kinetic friction ≠ 0

    So Net Force is ≠ 0

    And F = ma...

    So the object must accelerate... but it didn't. It moved at a constant velocity (keep in mind this was an experiment to find the coefficient of friction, so we tilted the board until it was just enough to make the object to have 0 acceleration).

    And that's pretty much the question I'm asking here. I am nearly 100% sure that this is a logical error on my part.

    Also, another similar question I just thought of which might help to determine what I'm doing wrong....

    A person needs to exert a normal force of N Newtons on an object to keep it from falling.
    The person exerts a greater force, N+1, onto the object.
    The object will have a net force of 1 Newton, and will accelerate at 1 m/s2, right?
    Okay, so once the object reaches a velocity v, he abruptly decreases his force to N Newtons again. Will the object have velocity v?

    If this is so, all you have to do to lift an object is to give it an initial hard jerk, then proceed to carry it with just the force required to lift it and go up stairs and such?

    Is this what our bodies to subconsciously? I mean, do they apply just the right amount of force to an object to beat gravity and accelerate to the velocity of our choosing, then continue lifting the object with the same force gravity is pulling on it?

    See, the reason I thought this was similar to the friction question was because it's conceptually much alike, yet different. Both objects in both systems would require a "kick" to spur motion. However, they are different because gravity's force is constant on the same object, but friction has two modes: static and kinetic.

    Pretty much the same thing, though. In both systems, the object shouldcontinually accelerate if the same force is applied constantly.
    Last edited: Oct 24, 2011
  6. Oct 27, 2011 #5
    One important thing to note is that theoretically kinetic friction force is constant, i.e.m it does not depend neither on velocity nor time, as long as the surfaces maintain their properties. Static friction force is variable, depending on the input (external) tangential force which intends to put the system to move. Since static friction force implies rest condition, if you apply 10 newtons, the static fritcion force will be 10 newtons, if you apply 15 newtons, the static fritcion force will be 15 newtons, and so on.
  7. Nov 2, 2011 #6
    Hm, so really, the object would not move unless a force greater than what the static friction can apply is applied, right?

    So if the down-the-ramp force was 15N, static friction would exert 15N back (consider it is barely within its threshold).
    But suddenly, the down-the-ramp force increases to 16N (usually achieved by making the ramp steeper), and breaches static friction's threshold. Assuming the constant kinetic friction force is lower that static friction force, the object would accelerate until it reached the bottom of the ramp.

    So is there ANY way of making the object go at a constant velocity down the ramp without magically changing the kinetic friction force?
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