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Did you complete the reading assignment? (Typically this means at least
20-30 minutes looking over the assigned sections of the book.)
Which of the following is
not one of Keplerís laws?
Planets all move in the same plane
Planets move in elliptical orbits
Equal areas swept out in equal time: faster
closer to sun
The period of orbit increases as r increases
The reason the moon does not fall into the Earth is that
the gravitational pull of the Earth on the
moon is weak
the moon has a sufficiently large orbital
the gravitational pull of the sun keeps the
the moon has less mass than Earth
none of the above
The "escape velocity" of a planet is the speed needed for a rocket to
go from the surface of the planet into orbit.
Ralph noticed the negative sign
in the general equation for gravitation potential energy, PE = -GMm/r,
and he read the book's statement (8th edition, chapter summary) that "this expression
reduces to PE = mgh close to the surface of Earth". He is very
confused, because one equation has a negative sign and the other one doesn't!
How can they possibly be equivalent? What can you tell Ralph to help him out?
The comments in the next next two boxes go into a big, mostly anonymous text
file that I skim through before the morning lecture, using the comments to help
me plan class discussion. ("Mostly anonymous", because I can track down who made
what comment, but it takes some effort on my part to do so.) Therefore, if you
really want to make sure I see your question/comment and answer it individually,
you should send it to me via email and not through this form.
Which part of today's assignment was particularly hard or confusing? What
would you like to spend extra time on in class?
Any other comments:
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