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Quiz 3
Physics 55: Introduction to Astronomy
Professor Henry Greenside
Friday, September 30, 2005
This quiz is closed book. You can use a scientific calculator but no other electronic device. If you have any
questions during the quiz, please do not hesitate to ask me.
The following formulas and data may be useful:
p2 = a 3 ,
g=
GM
,
d2
G ≈ 7 × 10−11
m3
.
kg s2
True or False Questions (1 point each)
Please circle “T” for true or “F” for false to indicate the truth of the following statements.
1. T / F
At the equator, no star is circumpolar.
2. T / F
The side of the Moon facing away from the Earth is in perpetual darkness.
3. T / F The distance of an object to the Sun at its aphelion is always strictly bigger than the distance
of the object from the Sun at perihelion.
Multiple Choice Questions (4 points each)
Circle the letter that best answers each of the following questions.
1. If the tilt of the Earth’s axis were increased from its present value of 23.5◦ to a greater value of 40◦ ,
then summers in Durham
(a) would become warmer.
(b) would become cooler.
(c) would not change.
2. If a shrinking ray were accidently to shrink the Earth to one half of its current size without changing
its mass (think “Honey, I Shrunk the Earth”), then afterwards your weight on Earth’s surface
(a) would remain unchanged.
(b) would double.
(c) would quadruple.
(d) would half.
(e) would decrease by a factor of 4.
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3. The Kuiper belt object Sedna (named after an Inuit goddess) orbits the Sun with an average distance
of about 500 AU. The time for Sedna to orbit once around the Sun is about
(a) 100 years.
(b) 1,000 years.
(c) 10,000 years.
(d) 100,000 years.
4. Comets A and B are orbiting the Sun in such a way that their orbits have the same semimajor axes but
comet A’s orbit is more eccentric than comet B’s orbit. (Recall that a circle has eccentricity 0 while
a highly elongated ellipse has a bigger eccentricity approaching 1.) Then the comet that will have the
larger speed at perihelion is
(a) comet A.
(b) comet B.
(c) the comets will attain the same highest speed.
(d) this problem can’t be solved without further information.
5. In the above figure, two blocks of equal mass are moving toward the right, one above the horizontal line
and one below the horizontal line. The positions of the blocks at successive 0.2-second time intervals
are represented by the numbered squares. The following two multiple choice questions refer to this
figure.
(a) The total force acting on a block is zero for which block?
i.
ii.
iii.
iv.
The block above the line.
The block below the line.
Both blocks.
Neither block.
(b) Do the blocks ever have the same speed?
i.
ii.
iii.
iv.
v.
No.
Yes,
Yes,
Yes,
Yes,
at
at
at
at
about instant 2.
about instant 5.
about instants 2 and 5.
some time during the interval 3 to 4.
Please turn to the next page. . .
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Open Questions
(6 points) Six-year-old Calvin is holding his best friend, the tiger Hobbes, in his arms while standing on a
frozen pond when Hobbes spies a rabbit and jumps out of Calvin’s arms at high speed. Assuming that Calvin
and Hobbes have masses of 30 kg and 200 kg respectively (ok, ok, Calvin would have to be monstrously
strong to carry such a big tiger but anything can happen in a quiz problem), assuming that Hobbes jumps
away with a speed of 30 km/hr (about 20 mph), and assuming that the ice surface is slick so there is no
friction, calculate to one significant digit the speed in km/hr with which Calvin is sent tumbling across the
pond.
Extra Credit
For the following problems, you need to justify each answer briefly to get the extra credit (no credit for just
guessing).
The following figure shows the successive positions, over a period of several hours, of the Sun near the horizon
as observed by someone in Alaska.
1. (1 points) Approximately what compass direction was the observer facing?
2. (1 points) In what direction was the Sun moving, left to right or right to left?
3. (1 points) At roughly what time was the lowest altitude of the Sun observed?
4. (1 points) During which season was this figure recorded: spring, summer, fall, or winter?
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