ASTR 1P01 Exam 2 Practice Questions
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Brock University *
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1P01
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Astronomy
Date
Dec 6, 2023
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14
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Lecture 5 Practice Questions
What are the differences and similarities between Copernicus's model and Ptolemy's model?
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Copernicus's model was heliocentric, with the Sun at the center. Ptolemy's model was geocentric, with
the Earth at the center. However, other than that, both models had epicycles, circular orbits, movement at
constant speed along the orbit, and celestial spheres.
What did Copernicus's model get right? What did it not get right?
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It correctly put the Sun at the center of the solar system. However, the planets were still moving on
epicycles in circular orbits around the Sun, which is incorrect - they actually move along ellipses and
without epicycles. In addition, we now know that the speed of the planet varies along its orbit (per
Kepler's laws) and that there is not actual "celestial sphere", the planets move in empty space.
Why do we not feel the Earth move?
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Because it is moving at a constant speed (except for negligible variations). We only feel movement when
the speed changes, that is, when there is acceleration.
Does a planet move faster or slower in its orbit if it's closer to the Sun?
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Faster, according to Kepler's 2nd law.
What is an example of a prediction that scientists can use to determine experimentally whether
Copernicus's model or Ptolemy's model is more accurate?
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The phases of Venus are predicted correctly by Copernicus's model, but not by Ptolemy's.
If I push an object in empty space with no friction and no other objects to collide with, what will its
motion be like?
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It will keep moving at constant speed forever, since there will be no forces (like friction or air resistance)
to slow it down or speed it up.
I drop a 1 kg ball and a 2 kg ball to the ground. What does Aristotle predict about the speed in which
each of the ball falls? What does Galileo predict? Which one of them is right?
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Aristotle will predict that the heavier ball falls faster. Galileo predicts that both balls fall at the same
speed. Galileo is right, as can be demonstrated experimentally.
Why did the Catholic Church oppose Copernicus's heliocentric model?
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The Church claimed that the Earth is the center of the universe, as in Ptolemy's model, based on the
Bible. The heliocentric model claimed that the Earth is just one out of several planets orbiting the Sun,
so it's no longer the center of the universe. Copernicus's model challenged the authority of the Church,
and if people knew that the Church is wrong about astronomy, they might start questioning the Church's
teachings about other topics as well. (Religions often rely on the ignorance of their believers to maintain
power; this was true in Galileo's time and remains just as true today.)
What were telescopes used for before Galileo started using them for astronomical observations?
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For observing far away things on Earth, for example for military purposes.
What are the two reasons that Galileo's discovery of the moons of Jupiter strengthen the heliocentric
model?
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This discovery showed that there are celestial objects that revolve around other planets, not the Earth.
This weakened the geocentric claim that all celestial objects revolve around the Earth. It also proved that
the Earth can be moving without "losing" the Moon, since if Jupiter can do that, then the Earth also can.
What does the heliocentric model say about humanity's role in the universe?
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It shows that (despite what some of us would like to believe) we are not the center of the universe, we
are just one form of life living on one planet out of trillions, and there could be other forms of life on
other planets.
What data did Tycho Brahe collect?
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The positions of the Sun, Moon, planets, and stars over many years.
What did Kepler discover about the shape of the orbits of the planets that was different from Ptolemy's
and even Copernicus's models?
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That the orbits were ellipses, not circles, and there were no epicycles.
How do the foci of an ellipse compare to the center of a circle?
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In a circle, the distance from the center to any point on the circle is always the same, equal to the radius
of the circle. But in an ellipse, it's the SUM of distances from the two foci to any point on the ellipse that
is always the same.
What is the difference between the major axis and the semi-major axis of an ellipse?
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The major axis is the widest diameter of the ellipse. It is analogous to the diameter of a circle. The semi-
major axis is half the major axis (semi = half). It is analogous to the radius of a circle.
A circle is a special case of an ellipse with a specific eccentricity. What is its eccentricity?
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Zero.
According to Kepler's first law, where is the Sun located in relation to a planet's orbit?
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At one of the two foci of the ellipse.
In simple terms, what does Kepler's second law say?
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A planet moves faster in its orbit when it is closer to the Sun.
A planet is moving at constant speed along its entire orbit. What can you say about the shape of this
planet's orbit?
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It's a perfect circle, so it's always the same distance from the Sun, hence it moves at constant speed
according to Kepler's second law.
What is the orbital period of the Earth?
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By definition, this is exactly 1 year.
According to Kepler's third law, is the orbital period of Jupiter longer or shorter than that of Earth?
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The orbital period is larger if the semimajor axis is larger. Since Jupiter is farther away from the Sun
compared to the Earth, its orbital period must be larger. Indeed, the orbital period of Jupiter is around 12
years.
"Kepler's laws explain the fundamental forces of nature responsible for movement in the solar system."
True or false?
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False. Kepler's laws are just a set of observations; Newton's laws describe the actual underlying forces
responsible for the motion. Indeed, Kepler's laws can be mathematically derived from Newton's laws,
but not the other way around.
Lecture 6 Practice Questions
If I throw a ball in empty space with no gravity, it will keep moving forever at a constant speed in the
same direction. However, this won't happen if I throw a ball on Earth. Explain why using Newton's first
law.
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According to Newton's first law, an object will continue moving at a constant speed in a straight line
unless acted on by an outside force. On Earth, the force of gravity will act on the ball and pull it down
towards the ground.
According to Newton's second law, when does an object's momentum change?
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If a force is applied to it.
A proton is pulling an electron towards it by exerting an electric force on it. Does the electron exert a
force on the proton?
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Yes, according the Newton's third law the electron exerts an equal and opposite force, pulling the proton
towards it.
Explain how a rocket works using Newton's laws.
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The rocket pushes gas out behind it. According to Newton's third law, there must be an equal and
opposite force, pushing the rocket forward. According to Newton's second law, this force accelerates the
speed of the rocket.
I have a block of concrete and an inflated balloon. Both have the same volume. Which one will have a
larger mass, and why?
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The block of concrete is more dense, so it will have a larger mass, since density is mass per unit volume
and both objects have the same volume.
I have a block of lead and a bucket of water. Both have the same mass. Which one will have a larger
volume, and why?
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The water has a smaller density, so it will have a larger volume, since density is mass per unit volume
and both objects have the same mass. In other words, the water has less mass per unit volume, so it
needs more volume to achieve the same amount of mass.
If the Earth had an orbital radius twice as large, how will its angular momentum change?
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Angular momentum is mass times velocity times distance. If we increase the distance by a factor of 2,
then angular momentum will also increase by the same factor.
If the mass of the Earth doubled, how would the force of gravity between the Earth and the Moon
change?
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The force of gravity between two objects is proportional to the masses of both objects, so if we increase
one of the masses by a factor of 2, the force will also increase by the same factor.
If the distance between the Earth and the Moon tripled, how would the force of gravity between the
Earth and the Moon change?
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The force of gravity between two objects is INVERSELY proportional to the SQUARE of the distance
between the objects, so if we increase distance by a factor of 3, the force will DECREASE by a factor of
3 squared, or 9.
Does the Andromeda galaxy, 2.5 million light years away, exert any gravity on you personally?
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In principle, yes. Both the Andromeda galaxy and you have mass, and anything that has mass anywhere
in the universe attracts anything else with mass anywhere in the universe, so the Andromeda galaxy and
you attract each other gravitationally. However, this force is so tiny that you will never be able to
measure it.
"The astronauts on the International Space Station do not experience any gravity." True or false?
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False. They feel weightless, since they are in free fall, so technically they are falling all the time.
However, gravity is still acting on them, which is exactly why the space station stays in orbit.
"A planet moves fastest when it is at the perihelion (the point closest to the Sun) compared to any other
point, due to Kepler's second law". True or false?
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True. Kepler's second law says a planet moves faster in its orbit when it is closer to the Sun, and the
perihelion is the point where the planet is closest to the Sun.
Which planet has the most eccentric orbit? And what does this mean for the shape of its orbit?
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Mercury has the most eccentric orbit. This means that the shape of Mercury's orbit is the most different
from a circle (or "the most elliptic") compared to the other planets.
Where is the Kuiper belt located?
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30-50 AU away from the Sun.
If Superman throws a ball with enough force, can he send that ball into an orbit around Earth?
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Yes. More force means more acceleration and thus larger velocity. If thrown in just the right direction
with just the right force, the ball can be made to "fall" around the Earth, which is exactly what an orbit
is.
"Kepler's laws give a 100% precise description of the orbits of planets in the solar system." True or false?
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False. Kepler's laws only take the gravitational effects of the Sun into account. However, due to
Newton's law of gravitation, now we know that the planets also exert gravity on each other, and this is
not taken into account in Kepler's laws. This causes subtle differences in the orbits of the planets
compared to what Kepler's laws say. In addition, today we know that Newton's laws aren't 100% precise
either, and general relativity provides a much more precise description of the orbits.
What has more momentum: a 1 kg ball moving at 5 m/s or a 5 kg ball moving at 1 m/s?
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Momentum is mass times velocity. In both cases, the product of mass and velocity is 5, so in fact, both
ball have the same momentum.
A ball was moving at a velocity of 3 m/s. It then accelerated with an acceleration of 6 m/s
2
for 2 seconds.
What was its final velocity?
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3 + 2 × 6 = 15 m/s.
During a plane flight, when is the plane's frame an inertial frame and when is it not an inertial frame?
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Remember that an inertial frame is one that doesn't rotate or accelerate. When the plane is taking off and
landing, it's accelerating, so it's not an inertial frame. Once it's in the air, as long as it's maintaining
constant speed (no acceleration) and constant direction (no rotation), it's an inertial frame.
Lecture 7 Practice Questions
How many great circles are on a sphere?
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An infinite number. Any circle whose center is at the center of the sphere is a great circle, and you can
place that circle in any orientation you want.
The distance between me and the North Pole is equal to the distance between me and the South Pole.
Where am I?
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On the equator.
The coordinates of Brock University are 43.1176° N, 79.2477° W. What do these coordinates mean?
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The first number is latitude: the university is located 43.1176 degrees to the north of the equator. The
second number of longitude: the university is located 79.2477 degrees west of the Prime Meridian
(passing through Greenwich, England).
Is declination analogous to latitude or longitude? Is right ascension analogous to latitude or longitude?
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Declination is the angle to the north or south of the celestial equator, so that's analogous to latitude.
Right ascension is the angle to the east of the March equinox, so it's analogous to longitude.
What did Foucault prove with his pendulum?
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From the fact that the pendulum's direction of oscillation was slowly rotating over time, it follows that
the Earth is rotating around its axis. This is a great example of a simple experiment that literally anyone
can do, and yet can prove something that people have denied for thousands of years!
"The seasons are due to the distance between the Earth and the Sun changing throughout the year." True
or false?
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False. This is actually a common misconception. The real reason for the seasons is the Earth's axial tilt.
If the distance to the Sun was the cause for the season, then we would have two summers and two
winters during each year!
Which two effects are responsible for the seasons, and how?
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The Sun's rays hit the ground at different angles throughout the year, due to the Earth's axial tilt. When
the Sun hits the ground at a small angle, the rays spread out and each point on the ground gets less heat,
resulting in a winter. When the Sun hits the ground at a large angle, the rays hit the ground more directly
and each point on the ground gets more heat, resulting in a summer. When it's summer in the northern
hemisphere it's winter in the southern hemisphere, and vice versa, since when one hemisphere "leans
into" the Sun, the other "leans out".
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In addition to rays hitting the ground more directly, in the summer the Sun spends more time in the sky,
so it has more time to heat the ground before night falls.
Which solstice happens in June? What does it mean in each hemisphere?
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The June solstice happens every year around June 21. In the northern hemisphere, it's the summer
solstice: the day where the Sun is highest in the sky and the day is the longest. In the southern
hemisphere, it's the winter solstice: the day where the Sun is lowest in the sky and the day is the shortest.
What are the two tropics, and what do they mean?
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The Northern Tropic (Tropic of Cancer) is the northmost latitude where the Sun can be directly overhead
(at the zenith). The Southern Tropic (Tropic of Capricorn) is the southmost latitude where the Sun can be
directly overhead (at the zenith).
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