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....??