,
a "polar angle" which measures the angle between r and k,
and polar coordinates as before in the xy plane, except now 
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Spherical coordinates uses a radial variable r, ,
a "polar angle" which measures the angle between r and k,
and polar coordinates as before in the xy plane, except now .
;
in other words our old r obeys 
)
We can now express r, v and a in spherical coordinates:
with
and now we have
and can deduce
