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Angular momentum trouble

  1. Sep 16, 2007 #1
    angular momentum trouble!!!!!

    since the angular momentum of a particle moving about a fixed point (axis) is given by
    L=r X p= r X mv
    where;
    L=angular momentum
    r= distance from the axis
    p=linear momentum of the particle
    m= mass of the particle
    v= linear velocity of the particle

    but if the distance 'r' decreases linear velocity 'v' should increase due to the law of conservation of angular momentum

    but if linear velocity 'v' of the particle decreases then the linear momentum 'p' would also decrease , but this is against the law of conservation of linear momentum.(there is no external force acting on the system)

    how is this possible .pls explain

    there r no external force acting on the system including torque (friction is neglected)
    eg. a planet revolving around the sun,as it comes closer to the sun its linear velocity increases bcuz angular momentum must be conserved but the increase in velocity means that its linear momentum wud increase
     
    Last edited: Sep 16, 2007
  2. jcsd
  3. Sep 16, 2007 #2

    Doc Al

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    Staff: Mentor

    Please give a specific example of what you're talking about. Angular momentum is only conserved in the absence of external torques; linear momentum is conserved only in the absence of external forces.
     
  4. Sep 16, 2007 #3

    Doc Al

    User Avatar

    Staff: Mentor

    In this case the system is "sun + planet". Neither the angular or linear momentum of the system changes. Of course, the planet's linear momentum changes as it nears the sun, but so does the sun's linear momentum; those changes are equal and opposite.
     
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