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Angular momentum conservation principle

  1. May 23, 2009 #1
    Dear Friends.., I have something to share ..rather know about the mechanism of the angular momentum conservation principle still better..
    consider a space craft , far away in a gravity less outer space..the space craft contains a man and a wheel. intially the angular momentum of the entire system was "zero" ie., man+wheel+space craft.. but when the man gives an angular impulse to the wheel then this law is casted by the nature..ie., say if the wheel rotates in clockwise sense the space craft spins in anticlock wise sense and thus nullifying the effect caused by the internal mechanism...
    still to explain this we can take a shade from newton's 3rd law..that when the man had just given the angular impulse, his contact force with the surface of the space craft reduces and there by leads to a moment of force on the space craft which makes it spin. the wheel spins as a direct jerk is given by the man,,
    .....everything is okay if we prceed in this manner.. but..,
    consider the same space craft in a gravity less space ..but this time there is a "tape recorder" instead of a man and a wheel. Initially the the entire system was at rest angular velocity of either<space craft and tape recorder> is "zero" .now a person from the earth gives a signal to turn on the tape recorder by using a remote control. now as the wheels inside the taperecorder rotate, to nullify the effect the space craft also rotates <since the initial angular momentum is zero and should remain zero forever unless there is a non zero external torque> ...,.. but how can the space craft turn mysteriously with no internal forces or internal torques being acted..??
    Or, am I going wrong somewhere in choosing the things as system....??
  2. jcsd
  3. May 23, 2009 #2

    Doc Al

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    Staff: Mentor

    What makes you think there are no internal forces? What turns the tape wheel?
  4. May 23, 2009 #3
    If the tape recorder is rigidly attached to the spacecraft then you can anticipate that the spacecraft will rotate in the opposite direction, if the tape recorder is floating freely and not in direct contact with the space craft hull, then the space craft will remain at rest and the tape recorder housing will rotate in the opposite direction of the tape recorder wheels and you won't even need to consider the space craft at all.
  5. May 23, 2009 #4
    so you mean to say that there will be some "jerks" given by the tape recorder when it is turned on which make the space craft rotate..< ya, tape recorder is fixed to the space craft>.. I'm convinced,, thanks.
  6. May 24, 2009 #5
    The Newton third law of motion states the action and reaction forces occur in two different system that is non attached, isn't it?
  7. May 25, 2009 #6
    Consider when you stand on the floor. You are pushing down on the floor because of your weight, but the floor is pushing back up at you with equal and opposite force. Not unattached at all.
  8. May 27, 2009 #7

    Thanks for reply.But I didnot clearly mention what i wanted to say.i want to give an example for my sayings
    In the boat that runs with the wind power flowing through the ocean,can we set the boat in the motion by use of huge electric fan inside the boat?That is what I had refered
    Last edited: May 27, 2009
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