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Newtons laws question.

  1. Mar 18, 2010 #1
    1. The problem statement, all variables and given/known data

    Block A in the figure weighs 65.9N. The coefficient of static friction between the block and the surface on which it rests is 0.27. The weight w is 12.2N and the system is in equilibrium.

    block A is placed on a table, it is connected by a string to a wall block w is hung off that string to form a 90degree angle between A and w and a 45degree angle between w and the wall.

    A) Find the friction force exerted on block A.

    2. Relevant equations

    Fr = u Fn
    F = ma

    3. The attempt at a solution

    i tried to find the net force using f = ma but there is no horizonal acceleration... so i am confused. :S help??
     
  2. jcsd
  3. Mar 18, 2010 #2
    There is no horizontal acceleration, but there is an acceleration due to gravity on block W.

    First you need to calculate the static force of friction on block A:
    F=UsFn
    F=(0.27*65.9)
    F=17.793

    Then you need to determine if the tension on the string from block W has enough force to move block A:
    F(b)=MA
    F(b)=12.2(9.8)
    F(b)=119.56

    Therefore your answer is 18, keeping significant figures in mind.

    Conceptual/Additional Notes:
    Since the tension force on the block is more than that of the frictional force, all the force of friction will be applied. If there wasn't enough force to move the block then only the force needed to keep block A at rest would have been applied. (If it were possible to apply more frictional force, the block would move opposite to the tension force).
     
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