Math Is Fun Forum

fusilli_jerry89

Member

Registered: 2006-06-23

Posts: 86

Quick Question About Vectors - Takes 2 seconds...maybe more

l = r x p for a rotating object

angular momentum = radius times perpendicular momentum

dl/dt = (dr/dt) x p + (dp/dt) x r
The book then says dr/dt equals the instantaneous velocity at that point, but how can the change in distance from the axis of rotation be the same as the change in circular distance?

JaneFairfax

Member

Registered: 2007-02-23

Posts: 6,868

Re: Quick Question About Vectors - Takes 2 seconds...maybe more

r is a vector, not a scalar. If r has a fixed magnitude, its direction will still be changing; hence it still makes sense to talk of the rate of change of r with respect to t (change of direction if not of magnitude).

NB: In the equation l = r × p, × is vector product, not ordinary multiplication. Vector product is only defined for vectors, not scalars.

Last edited by JaneFairfax (2007-10-30 14:02:06)

fusilli_jerry89

Member

Registered: 2006-06-23

Posts: 86

Re: Quick Question About Vectors - Takes 2 seconds...maybe more

Thanks!