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Pulley problem

  1. Oct 27, 2012 #1
    Can someone explain why the force P with which the man must pull on the rope to achieve an acceleration a m/s2 IS NOT (m+M)(a+g)/2 and is instead (m+M)(a+g). M+m is the combined mass of man and platform.

    Why does 2T-(M+m)g=(M+m)a not work here?
     

    Attached Files:

  2. jcsd
  3. Oct 27, 2012 #2
    when he pulls the rope, an equal and opposite force is exerted on the platform.
     
  4. Oct 27, 2012 #3
    Thanks for the response. But doesn't the platform have an equal and opposite normal force?
     
  5. Oct 27, 2012 #4
    Exactly why greater force is required to accelerate the mass of the man + platform.

    2P=(m+M)(g+a)+P
     
  6. Oct 27, 2012 #5
    but doesn't the tension of the rope on the other side contribute just as much as the man therefore doubling the total force upwards?
     
  7. Oct 27, 2012 #6
    Yes, and that is the only reason he can lift himself. This is why the left is 2P.
     
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