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Homework Assignment #6
Please return this paper as your homework submission. You may attach additional pages if
necessary.
Reading: Read all of chapter 4.
Online Tutorials: visit the online tutorials on Kepler’s Laws and Motion and Gravity. There
may be others of interest as well.
1. If the earth rotated in the opposite direction (that is, clockwise as seen from the NP), but still
revolved around the sun counterclockwise, how would the solar day compare to the sidereal
day?
In this case, instead of needing to rotate one more degree to make up for its revolution
around the sun, it will have actually rotated one degree too much in its 23 hr, 56 min
sidereal day. Hence, the solar day would be 23 hrs, 52 min, or 4 minutes shorter (instead
of 4 minutes longer, as it is currently).
2. Defend or oppose the claim that “a theory in science is really just a guess.” Support your
answer. Hint: skim section 3.4 in your text.
While scientific theories can never be proven, they are robust in that they have been
scrutinized through years of peer review. Scientific theories are only replaced upon the
development of a more sophisticated one, and this process is continually occurring.
They are certainly not “just guesses.”
3. Of the three: Copernicus, Tycho, or Kepler, pick the one you feel contributed most strongly to
turning the Ptolemaic (ie, earth-centered, stationary) model of the solar system into the one we
know and love today. Give at least 2 solid reasons why you picked him. (You can get this right
regardless of which one you pick).
The answer to this is too long to type out. See class notes or section 3.3 in your
textbook.
4. The earth travels around the sun in a slightly elliptical path. In which season is it moving the
fastest through its orbit? Why?
Winter. It’s closest to the sun in the winter, and because of Kepler’s 2nd law, planets
move most quickly when they’re closest to the sun.
5. Look at Kepler’s 3rd law. If the mass of the earth were doubled, how would the period of
revolution around the sun change?
Doubling the mass has no first order effect on the period of revolution around the sun
(certainly, Kepler’s Law says that the period of revolution is independent of mass).
6. For each of the following, state whether the car is accelerating:
a. Traveling at 50 mph on a straight, flat road. Not accelerating.
b. Going around a bend at a constant speed of 25 mph. Accelerating (changing
direction).
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c. Stopping at a stop sign. Accelerating (changing speed).
d. Traveling uphill at 30 mph on a straight road. Not accelerating (constant speed and
direction).
7. According to Newton’s Laws, is there gravity on the moon? What about on Mars? How
about within the space between the planets?
According to Newton’s Laws (and all of the best observations scientists have made),
there IS GRAVITY EVERYWHERE. This includes “empty” space. When you’re far from
objects like the sun and earth, gravity is very small, but if it were zero we’d have no
comets or outer planets, and there would be no force creating stars and galaxies.
8. If you were suddenly on Mars, would you lose mass, weight, or both?
You’d only lose weight. Your mass wouldn’t change. Sorry.