concervation of angular momentum

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Fallingeagle - 30 Jul 2007 00:29 GMT

ok

a sylinder with rope around it is dropped so the rope unwailes and the
sylinder gains angular velocity.

it hits the floor.

and now in the impact with the floor, anguar momentum is concerved,
why?

isn't the sylinder rotating about the center of mass? and so doesn't
the friction force have torqe about the center of mass ( It does) so a
net external forqe acts on the system, and angular momentum should not
be concerved!??

aperarantly, A.M. is concerved, because there is no external torqe
with respect to the floor (the pont if impact)

please clairify? why use the point of impact as refrenence? why not
use center of mass, and why is is it correct????

help!!!!

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Paul Cardinale - 30 Jul 2007 19:27 GMT

> ok

No, it's not OK. You're a basement-dwelling fake.

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PD - 31 Jul 2007 16:31 GMT

> ok
>
[quoted text clipped - 5 lines]
> and now in the impact with the floor, anguar momentum is concerved,
> why?

Not necessarily. Angular momentum is conserved in *closed* systems,
where the interactions are internal to the system and do not cross the
system boundary. If you include the floor *and* the cylinder in the
system, then the angular momentum of that system is conserved.
However, if your system includes only the cylinder, then the
interaction with the floor is one that crosses the system boundary
(between floor and cylinder) and in that system angular momentum will
not be conserved.

> isn't the sylinder rotating about the center of mass? and so doesn't
> the friction force have torqe about the center of mass ( It does) so a
[quoted text clipped - 8 lines]
>
> help!!!!

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Natural Science Forum / Physics / General Physics / July 2007

concervation of angular momentum