CT9-13.A planet is in elliptical orbit around the Sun. The zero of potential energy U is
chosen at r = , so
U r GMm
r
( )
.
How does the magnitude of U
U U
compare to the KE?
Pink:
U
> KE Green:
U
< KE Yellow:
U
=KE
Blue: depends on the position in the orbit.
Answer:
U
> KE. When v< vesc, the orbit is bound, as in the diagram, and the total
energy Etot is negative. [When v=vecs, Etot = 0. When v > vesc , Etot > 0.] Only if the
(+)KE is smaller than the magnitude of the (-)U, can we have Etot = KE + U < 0. A
diagram helps:
