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A Friction Experiment

  1. Sep 27, 2007 #1
    During an experiment, a crate is pulled along a rough horizontal surface by a constant force F_vec and the magnitude of the acceleration along the x direction, a_x, is measured. View Figure The vector F_vec has a component along the x direction of magnitude F_x. The experiment is repeated several times, with different values of F_x each time.

    [​IMG]


    Create a plot of the force of static friction, f_s, versus the x component of the pulling force, F_x, for the experiment. Let the point F_min, along the horizontal axis, represent the minimum force required to accelerate the crate. Choose the graph that most accurately depicts the relationship among f_s, F_x, and F_min.

    [​IMG]

    Create a plot of the force of kinetic friction, f_k, versus the x component of the pulling force, F_x, for the experiment. Let the point F_min, along the horizontal axis, represent the minimum force required to accelerate the crate. Choose the graph that most accurately depicts the relationship among f_k, F_x, and F_min.

    [​IMG]

    After all the trials are completed, a graph of acceleration a_x as a function of force F_x is plotted. Assuming the presence of both static and kinetic friction, which of the following graphs View Figure is most nearly correct?

    [​IMG]


    I am too sure about these questions, but so far I have guessed.

    For the first one, I am thinking it is D, because the static friction to overcome is needed only for few seconds.

    For the second one, I am thinking it is D because force of kinetic is required after a few seconds.

    For the third one, I think it is B because the force applied must be equal to or greater than the force of friction for the block to move.

    Please comment on my answers, I am not too sure about them.
     
  2. jcsd
  3. Sep 27, 2007 #2

    Mentz114

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    Gold Member

    The force of kinetic friction remains constant, so any graph with it rising can be rejected.
    Also, a = F/m which is a straight line with slope 1/m, which should help with the third one.
     
  4. Sep 27, 2007 #3
    ok

    but still

    is my logic here ok?

    I mean for Parts A and B...

    for C I am assuming it is Graph A now.
     
  5. Sep 28, 2007 #4

    Mentz114

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    Gold Member

    For, C it is C. There is no acceleration until the static friction is overcome.
    I don't know about the others.
     
  6. Sep 28, 2007 #5
    can anyone help me with the first two parts?

    im thinking part b is D
     
    Last edited: Sep 28, 2007
  7. Sep 28, 2007 #6
    and for part A i think it is D but i need more clarification
     
  8. Sep 29, 2007 #7

    Doc Al

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    Staff: Mentor

    Realize that time is not being plotted in these diagrams, just forces. But D is correct because until the crate starts to move, the static friction equals F_x. (Note that the slope equals 1.)

    Again, only forces are being plotted, not time. But yes, D is correct. Once the crate starts moving, kinetic friction acts--and the kinetic friction is constant.

    Plot B implies that the acceleration is constant. That can't be true: Until F_x exceeds some value, you won't even be able to move the crate (acceleration = 0); Once it starts moving, as F_x increases the net force on the crate increases (thus acceleration increases).
     
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