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Newton's Laws Of motion Please Reply

  1. Feb 26, 2006 #1
    This is my problem:

    A window washer pulls herself upward using the bucket-pulley apparatus The mass of the person plus the bucket is 70 kg.

    A) How hard must she pull downward to raise herself slowly at constant speed?................N
    (b) If she increases this force by 9%, what will her acceleration be?
    ..................m/s2

    I figured out part A) by taking 70 kg and multiplying it by 1/2 A which equals 4.9m/s2 and so the answer is 343 Newtons. But for part b i am confused as to how to use the the 9% and how to work it out..
     
  2. jcsd
  3. Feb 26, 2006 #2

    Hootenanny

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    Well what is 9% of 343? And what does an unbalanced force create?
     
  4. Feb 26, 2006 #3
    9 percent of 343 is 30.47 N ..so if her new force net equal 374 N where should i go from there?.. im sorry i just been working on this problem for a while and im still stuck
     
  5. Feb 26, 2006 #4

    Hootenanny

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    Ok, so what is the net force acting on her and the bucket?
     
  6. Feb 26, 2006 #5
    isnt the net force =343+/-30.97?
     
  7. Feb 26, 2006 #6

    Hootenanny

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    No. I mean the resultant force.
     
  8. Feb 26, 2006 #7

    Hootenanny

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    Think about what force is pulling the bucket and the girl down and what force is pulling them up.
     
  9. Feb 26, 2006 #8
    the force pulling on the bucket and the girl is mg= mass x gravity (9.8 m/s2)...the normal force-the force she is pulling the pulley down is the only other force i can think of
     
  10. Feb 27, 2006 #9

    Hootenanny

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    Hang on a minute, why did you half the acceleration due to gravity?
    Downwards force = [itex] 70 \times -9.8 = -686N [/itex].
    Therefore, the girl must be pulling 'up' with [itex] 686N [/itex] to travel at a constant speed. However, if she pulls with an addition 9% of force then the upward force will be [itex] 70 \times 9.8 \times 1.09 = 747.74N [/itex].
    This means there is an unbalanced force of [itex] 747.74-686 = 51.74N [/itex]. Using [itex] F = ma \Rightarrow a = \frac{F}{m} = \frac{51.74}{70} = 0.739 m\cdot s^{-2}[/itex]
     
    Last edited: Feb 27, 2006
  11. Feb 25, 2007 #10
    i was confused on this same problem but i dont understand where the 1.09 comes from...?
     
  12. Feb 25, 2007 #11
    1.09*x will give you the sum of the value of x plus 9% (0.09) of x

    (instead of doing 9% of x, then adding to x)
     
  13. Feb 25, 2007 #12
    ok i got it thanks so much!
     
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