# Angular momentum trouble

1. Sep 16, 2007

### vivinisaac

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. Sep 16, 2007

### 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.

3. Sep 16, 2007

### 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.