Question

r(t) is a position vector. v(t) is r'(t) or the velocity vector, u(t) is a unit r(t) vector. Prove that:
d/dt ( r(t) x v(t) )=0

Answer 1

One question though, is the position vector always collinear to the velocity vector?

Answer 2

done by differentiating r(t) x r(t)