Astronomy 170B1
Due in class Jan 29
Homework #2
Prof. M. Rieke and TA M. Halford
This homework is worth 10 points.
Enter your answers using a pencil on a Scantron form.
This portion of the homework investigates the relationship between how long it takes a
planet to orbit a star (orbital period) and how far away that planet is from the star (orbital
distance). We will start by investigating an imaginary planetary system that has an
average star like the Sun at the center. A huge Jupiter-like planet named Moto orbits
close to the star, while a small Earth-like planet named Spec is in a far away orbit
around the star. Use this information when answering the next four questions.
1.) Which of the two planets (Moto or Spec) do you think will move around the central
star in the least amount of time?
a.) Moto
b.) Spec
2. If Moto and Spec were to switch positions, would your answer to question 1 change?
a.) Not really as the orbital times would remain the same; Spec would orbit in the time
Moto used to and Moto would orbit in the time Spec used to
b.) Moto would orbit more slowly than Spec did before because it is bigger and more
massive
c.) Spec would orbit faster than Moto did before because it is small and light
3.) Do you think the orbital period for Moto would increase, decrease, or stay the same if
it were to move from being close to the central star to being much farther away?
b.) decrease c.) stay the same
The graph to the right illustrates how
the orbital period (expressed in
years) and orbital distance
(expressed in Astronomical Units, AU,
1 AU is the distance of the earth from
the sun) of a planet are related.
3
a.) Same
b.) Different
c.) it depends on other factors
2
4.) Imagine both Moto and Spec were in
orbit around the central star at the
same distance and that their orbital
positions would never intersect (so
that they would never collide). Do
you think the two planets would have
the same or different orbital periods?
Orbital Period (Years)
4 5 6 7 8 9 10 11 12
a.) increase
1
5.) According to the graph, would you
say that a planet’s orbital period
appears to increase, decrease, or stay the
same as a planet’s orbital distance is increased?
a.) increase b.) decrease
1
2
3
4
Orbital Distance (AU)
5
6
c.) stay the same
6.) How far from the central star does a planet orbit if it has an orbital period of 3 years?
a.) about 5 AU
b.) about 2.2 AU
c.) about 1.7 AU
7.) How long does it take Jupiter complete one orbit (it is 5.2 AU from the sun)?
a.) 3 years
b.) about 11.4 years c.) it is not possible to tell from this graph
8.) Based on your results from questions 6 and 7, which of the following best describes
how a planet’s orbital period will change (if at all) when its distance to the central star is
doubled?
a.) The planet’s orbital period will decrease by half.
b.) The planet’s orbital period will not change.
c.) The planet’s orbital period will double.
d.) The planet’s orbital period will more than double.
9.) From Newton’s law of gravity, when the distance of a planet from its star is tripled,
how does the gravitational force on the planet change?
a.) it doubles
b.) it is decreased by a three times
c.) it is decreased by nine times
d.) it stays the same
e.) it is increased by three times
10.) If the distance of a planet from its star is doubled, how much farther does it have to
travel to go all the way around its orbit? (you may assume a circular orbit, think about the
circumference of a circle)
a.) two times
b.) four times
c.) the same
11.) Copernicus developed his theory of the solar system because
a.) he wanted to see if he could stir up some controversy
b.) he just wanted to try something different
c.) he wanted to simplify the Ptolemaic system
d.) he had a vision that the sun was at the center of the solar system
e.) his measurements of the sizes of the earth, moon, and sun argued that the sun was
at the center
12.) Look at the expression for escape velocity given in lecture (section called “Physical
Laws”). If we are in orbit some distance away from the surface of the Earth, how does
the velocity needed to escape the Earth’s gravity change?
a. it stays the same regardless of where you are
c. the escape velocity will be lower
b. the escape velocity will be higher
d. other information is needed
13.) You are driving a car. You adjust your foot pressure on the accelerator so that the
speedometer indicates the same value as you go around a corner. Which of the
following statements is true:
a. because the car’s speed is constant, no forces are involved
b. because the car’s direction changes, a force is involved
c. because the car must overcome friction and wind resistance, a force is involved
d. both b. and c. are true
14.) Galileo’s telescopes
a. had really good lenses that allowed him to see fine details
b. were limited in resolution by poor lenses and the need to reduce their apertures
c. were the first ones ever made
d. were really no better than those made by others
15.) A container of air is resting on a table at room temperature. A hair dryer is turned
on and pointed at the container thus heating up the air in the container.
a. the air molecules in the container do not change their behavior
b. the air molecules in the container slow down
c. the air molecules in the container speed up
d. the air molecules emit more radiation
e. both c. and d.
16.) An object has a temperature of 500° K. An astronomer takes a spectrum and sees
that the object’s output is largest at a wavelength of 6000 nm (1 nm = 10-9 meters). The
object cools down to 250° K. At what wavelength does the output peak now?
a. 6000 nm
b. 12000 nm
c. 3000 nm
d. in visible light
e. in x-rays
17.) You and a friend are playing around on the university mall. One of you has a
flashlight. Initially you stand 10 feet apart. The one without a flashlight uses a light meter
to record the apparent brightness of the flashlight. The person holding the flashlight
moves away so she’s 100 feet away from the light meter. How does the reading on the
light meter change?
a. it gets 90 units smaller
b. it gets 100 times smaller
c. it doesn’t change
d. it gets larger
18.) A hotter object appears
a. brighter and bluer than a cooler one
c. brighter and redder than a cooler one
b. dimmer and redder than a cooler one
d. dimmer and bluer than a cooler one
19.) Tycho’s observation of an exploding star (a supernova) was significant because
a. it was an illustration that the Aristolean view of the sky as unchanging was incorrect
b. it helped him measure the distance to other stars
c. it provided proof that he was the best astronomer around
d. it actually was not important
20.) Which of the following were changes in the Copernican model of the Solar System
as compared to Ptolemy’s model?
a. the assumption that planets move on circular orbits
b. the assumption that planets move at constant velocity along their orbits
c. that the Sun is the center of the Solar System
d. both a. and b.
