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What is the coefficient of kinetic friction of the surface?

  1. Oct 20, 2005 #1
    I have a homework question (due tomorrow) that I cannot figure out how to answer. It is:

    This problem involves a single surface with a single unknown coefficient of kinetic friction that can be found. A box starting from rest, slides down along an incline a distance of 8.8 meters. The incline is at 35.5 degrees above the horizontal. After it reaches the bottom of the incline, it travels horizontally
    17. meters. What is the coefficient of kinetic friction of the surface?

    In class we derived an equation that I thought would work. I know that
    W = Fd or mgh [h being dsin(zeta)] and that
    Wtotal = Wgravity + Wfriction.
    (I'm going to say that the distance on the incline is di and the distance on the horizontal is dh) So I end up with Wtotal = mgh(di) - Coefficient Fk(Normal force on the incline)di - Coefficient Fk(Normal force on the horizontal)dh which is:

    Wtot = mg(di)sin(zeta) - Coefficient Fk(mg)di(cos)(zeta) - Coefficient Fk(mg)dh

    Is this even the right equation and if so, how then do I solve for Coefficient Fk?:uhh:

    I'm sorry that it's so hard to read.
    Thanks very much.
  2. jcsd
  3. Oct 20, 2005 #2
    As the box starts from rest you need to use an energy argument - the gravitational potential energy the box starts with has to completely be dissipated by the frictonal force, therefore the work done against friction (mu*normal reaction force) is equal to the gravitational potential energy and you can solve it to find mu (as the mass of the block cancels out).

    Remember the normal reaction force on the block is different when it's on the slope so the work done against friction will be split up into two parts: (mu*normal reaction force on slope + mu*normal reaction force on horizonal plane).

    Hope that helps.

    [edit]basically you have the right equation - just set Wtot equal to zero[/edit]
  4. Oct 20, 2005 #3
    Oh, okay, thanks very much. I'm getting the right answer now.

    I also have another problem that is exactly the same except that it deals with a frictionless surface, the box goes up another incline, and the only supplied number is the distance it travels down the first incline. Is it correct to use the same equation only setting the coefficient of friction equal to zero? I know that without friction the box will go up the second incline just as far as it came down the first, but I'm not exactly sure how to find that with an equation. I speculate:

    (d1 - 1st inlcine
    d2 - horizontal
    d3 - 2nd incline)

    Having set Wtot equal to zero I get d3sin(zeta) = d1sin(zeta) - 0 - 0 the
    sin(zeta)'s cancel and there's d3 = d1.

    Is this right?
  5. Oct 20, 2005 #4
    That's correct if both slopes are at the same angle, otherwise the sin(zeta) doesn't cancel out
  6. Oct 20, 2005 #5
    Thanks again. The problem doesn't specify, but the answer is the same distance so I'm assuming they're the same angle...
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