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Pulley question- What have I done wrong here?

  1. Jun 8, 2015 #1
    1. The problem statement, all variables and given/known data
    http://imgur.com/ymNWTOV
    Image isn't working, here is the question:
    http://imgur.com/ymNWTOV

    2. Relevant equations
    F=ma
    Weight force=mg
    Frictional force=μkN

    3. The attempt at a solution
    So for part (a):

    Force on 10kg block=mg=10x9.8=98N
    This is equal to the tension force in the string, which is pulling the 5kg block right

    Frictional force on the 5kg block (pulling it left):
    F=μkN
    =0.4*(mg)
    =0.4*9.8*5
    =19.6N

    Therefore the net force on the block is 98-19.6=78.4N
    F=ma
    Therefore accelaration(5kg block)=F/m=78.4/5
    This gave me 15.68m/s2, while the answer was actually 5.23...

    Part b
    I have no clue how to do this part!! Answer is 3.29J...
     
  2. jcsd
  3. Jun 8, 2015 #2
    No. This is incorrect. Have you drawn a free body diagram on the 10 kg block? Do you feel that you have advanced beyond the point where you need to draw free body diagrams? Let T represent the tension in the string. What is the net force acting on the 10 kg block?

    Chet
     
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