# Warm-Up Exercise for lecture 13

## Due 8:00 am, Thurs, 13 Oct 2009

Physics 105, Fall 2009

Which of the following is not one of Kepler’s 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?