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Friction - static question

  1. Nov 14, 2011 #1
    1. The problem statement, all variables and given/known data


    A 4.0 kg block of wood sits on a table. A string is tied to the wood,
    running over a pulley and down to a hanging object. The greatest mass that can be
    hung from the string without moving the block of wood is 1.8 kg. Calculate the coefficient
    of static friction between the block of wood and the table.

    4.0 kg and 1.8 kg- mass

    2. Relevant equations

    Fs=μFn

    3. The attempt at a solution

    so i assume the block isnt moving so fnet would be 0, fn=fg so 4.0(9.8) ---> 39.2

    i dont have fs and mew so how am i supposed to solve this?
     
  2. jcsd
  3. Nov 14, 2011 #2

    PhanthomJay

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    There are 2 forces acting horizontally on the block on the table. One is the tension force that can be calculated by looking at the forces on the hanging mass.
     
  4. Nov 14, 2011 #3
    It is always best to create a diagram and Net force equations for each individual object. You are right in that F=0 but this also tells you that whatever force is pulling at the block of wood, there is an equal and opposite force that stops it from moving. This is crucial piece of information for your problem.
     
  5. Nov 14, 2011 #4
    so if i calculate fg=mg on the hanging mass, would the force of tension be the same as fg?
     
  6. Nov 14, 2011 #5

    PhanthomJay

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    Sure!
     
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