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Homework Help: Conservation of momnetum or newtons second law?

  1. Aug 20, 2011 #1
    conservation of momnetum or newtons second law?????????

    1. The problem statement, all variables and given/known data

    An acrobat of mass 'm' clings to a rope ladder hanging below a balloon of mass 'M'. the balloon stationary with respect to ground
    (a) If the acrobat begins to climb the ladder at a speed 'v' (with respect to the ladder), in what direction and with what speed (with respect to earth) will the balloon move?
    (b) If the acrobat stops climbing, what will be the velocity of the balloon.

    2. Relevant equations

    3. The attempt at a solution

    I put my answer and figure in attachment.

    you tell me where am i wrong

    Attached Files:

  2. jcsd
  3. Aug 20, 2011 #2


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    Homework Helper

    Re: conservation of momnetum or newtons second law?????????

    In the absense of any unbalanced external force, the centre of mass will remain stationary.
    If you alter the position of part of the mass of the system, you will probably alter the position of other parts of the system. If some part of the system began moving at some speed, the rest of the system would also move at a certain speed

    Imagine if the man and the balloon had the same mass, so the centre of Mass was exactly half way between them, and the ladder was 100m long.
  4. Aug 20, 2011 #3
    Re: conservation of momnetum or newtons second law?????????

    u r wrong in thinking that the man, as moving with const. velocity, exerts no force on balloon, where the actual case is the man starts from rest and acquires a velocity and in doing so acquires some accln. also, untill he reaches a const. velocity and hence applies force to the balloon also. this force is to be counted as an external force for the balloon, but there is no external force for the balloon+ man system. hence it follows that although the balloon moves downward as the man moves upward the centre of mass of man+ balloon does not move.
  5. Aug 20, 2011 #4
    Re: conservation of momnetum or newtons second law?????????

    thanks to both persons.
    I got the point.
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