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Finding the coefficient of friction of a block on a table

  1. Feb 18, 2009 #1
    1. The problem statement, all variables and given/known data
    Block A is on a table and is connected by a pulley to block B.
    Block A has weight Wa and block B has weight Wb. Once block B is set into downward motion, it descends at a constant speed. Assume that the mass and friction of the pulley are negligible.

    Calculate the coefficient of kinetic friction between block A and the table top.
    Express your answer in terms of some or all of the variables Wa , Wb, and g (the acceleration due to gravity).

    2. Relevant equations

    I have gotten this hint (mastering physics problem):

    In this problem, blocks A and B are in dynamic equilibrium; their velocities are constant. This means that the net force on each,F net , is equal to zero.

    The tension is constant throughout the rope. By setting the sum of the forces acting on both blocks A and B equal to zero, you should be able to obtain two different expressions for the tension in the rope. Set these equal to each other and solve for [tex]\mu[/tex].

    3. The attempt at a solution
    I really don't know how to go about starting this. I know the equation above but don't know what to do with it. I feel like I need to know the acceleration. How would I find the tension?

    Please help! Thanks!!!
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 18, 2009 #2

    Tom Mattson

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    Staff Emeritus
    Science Advisor
    Gold Member

    You don't know what to do with it? :eek: Haven't you ever seen a free body diagram? Don't you think it would be a good idea to draw one?

    You DO know the acceleration! In your very own words:


  4. Feb 18, 2009 #3
    Ok so their accelerations were equal to zero and their net forces were also equal to zero. Thanks about the free boday diagram (duh). Well i figured out the answer. It ended up being the weight of block b divided by the weight of block a. I have tried looking in my book and looking through my notes but there were no examples for finding the coefficient of friction. (frustrating)
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