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Warm-Up Exercise for lecture 13

Due 8:00 am, Thurs, 13 Oct 2009

Physics 105, Fall 2009

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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.)
☐ yes ☐ no

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 speed
☐ the gravitational pull of the sun keeps the moon up
☐ 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.
☐ true
☐ false

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?

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