and 
physically for a non-rotating object?
and not
? Why is the position only defined by the radial direction?
, how is it perpendicular when
you take the limit
?
is in the direction of
?
come from when deriving
?
and
?
and
? Isn't 
in radians and 
in meters, so units are inconsistent?
?
for the skateboard example?
in the
final example with the skateboard?