Astro 7n Exam 1 (Unit 1 & 2) Questions
and Answers
Newton's First Law
✓ ~~~An object at rest, or in motion in a straight line at a constant speed, will remain in that
state unless acted upon by a force
Ex) If the Sun suddenly disappeared, the Earth would continue in the direction that it
was traveling in its orbit at that time
Newton's Second Law
✓ ~~~The acceleration of a body due to a force will be in the same direction as the force,
with a magnitude directly proportional to its mass
-Force = Mass * Acceleration
-(A smaller mass will move faster, if the same force is applied to it.)
Newton's Third Law
✓ ~~~For every action, there is an equal and opposite reaction
-The Sun exerts force on planets, and they orbit it -- the planets exert equal force on
Sun, but it only moves slightly because of its very large mass relative to the planets
The strength of gravity, g, on the surface of a planet is given by:
✓ ~~~g=M/R^2
where M is the mass of the planet and R is its radius
What does more mass mean?
, ✓ ~~~-More gravity
If the mass of a planet were twice that of another, but they had the same radius, the
gravity felt on the surface of the more-massive one would be twice as strong
What does larger separation mean?
✓ ~~~-Less gravity
If the radius of a planet were twice that of another, the gravity is 1/(2)^2 = 1/4 as
strong
Figure out the surface gravity of Mars, relative to the Earth
Mars has 1/10 the mass of the Earth and 1/2 the radius of Earth. So for Mars,
✓ ~~~g= (1/10)/(1/2)^2 = (1/10)/(1/4) = 4/10
-Mars has a surface gravity 4/10 that of Earth
Newton's Universal Law of Gravitation
✓ ~~~This law gives the force of gravity between any two objects in the Universe. The force
of gravity is proportional to (Mass of object 1) * (Mass of object 2) divided by the distance
between the two objects squared:
F ∝ M1 x M2 / d^2
If the objects were moved 2 times closer (or, to half of their initial distance between)
...
✓ ~~~F ∝ M1 x M2 / (1/2)^2 = M1 x M2 / (1/4) = 4 (M1 x M2) so the gravitational attraction
would because 4 times as great
What causes day and night?
, ✓ ~~~Rotation of the Earth on its axis
How does the Sun appear to move in the sky in the course of a day?
✓ ~~~East to West, because of Earth's rotation. The stars and planets move in the same
way from our point of view, also, because of Earth's rotation
What happens to the Earth in one year?
✓ ~~~It orbits the Sun, once
Why do we have seasons?
✓ ~~~The tilt of the Earth's axis of rotation, with respect to the plane of its orbit around the
Sun (The tilt is about 23 degrees)
How is the Earth's axis tilted when we have summer in the Northern hemisphere?
✓ ~~~With the North pole toward the Sun
What season is it in the Southern hemisphere when it is summer in the Northern
hemisphere?
✓ ~~~Winter
When does Winter begin and what does that mean?
✓ ~~~On or about December 21 = in the Northern hemisphere, the nights are longer than
days
When does Spring begin and what does that mean?
✓ ~~~On or about March 21 = days and nights have equal length
, When does Summer begin and what does that mean?
✓ ~~~On or about June 21 = days longer than nights in the North
When does Fall begin and what does that mean?
✓ ~~~On or about September 21 = days and nights have equal length
What happens if the Earth's axis were not tilted?
✓ ~~~Seasons would not happen
What happens to the moon in 1 month?
✓ ~~~It moves once around the Earth
What causes the phases of the moon?
✓ ~~~The Sun is lighting up different fractions of the part of the Moon we see from Earth
What is the order of the phases of the Moon?
✓ ~~~New - Waxing Crescent - First Quarter - Waxing Gibbous - Full - Waning Gibbous -
Third Quarter - Waning Crescent - New
When is the full moon visible?
✓ ~~~Only at night. It transits at midnight, rises 6 hours earlier (at sunset) and sets 6 hours
later (at sunrise)
When is the new moon visible?
✓ ~~~The new moon is visible during the day. It transits at noon; it rises 6 hours earlier (at
sunrise), and sets 6 hours later (at sunset)
and Answers
Newton's First Law
✓ ~~~An object at rest, or in motion in a straight line at a constant speed, will remain in that
state unless acted upon by a force
Ex) If the Sun suddenly disappeared, the Earth would continue in the direction that it
was traveling in its orbit at that time
Newton's Second Law
✓ ~~~The acceleration of a body due to a force will be in the same direction as the force,
with a magnitude directly proportional to its mass
-Force = Mass * Acceleration
-(A smaller mass will move faster, if the same force is applied to it.)
Newton's Third Law
✓ ~~~For every action, there is an equal and opposite reaction
-The Sun exerts force on planets, and they orbit it -- the planets exert equal force on
Sun, but it only moves slightly because of its very large mass relative to the planets
The strength of gravity, g, on the surface of a planet is given by:
✓ ~~~g=M/R^2
where M is the mass of the planet and R is its radius
What does more mass mean?
, ✓ ~~~-More gravity
If the mass of a planet were twice that of another, but they had the same radius, the
gravity felt on the surface of the more-massive one would be twice as strong
What does larger separation mean?
✓ ~~~-Less gravity
If the radius of a planet were twice that of another, the gravity is 1/(2)^2 = 1/4 as
strong
Figure out the surface gravity of Mars, relative to the Earth
Mars has 1/10 the mass of the Earth and 1/2 the radius of Earth. So for Mars,
✓ ~~~g= (1/10)/(1/2)^2 = (1/10)/(1/4) = 4/10
-Mars has a surface gravity 4/10 that of Earth
Newton's Universal Law of Gravitation
✓ ~~~This law gives the force of gravity between any two objects in the Universe. The force
of gravity is proportional to (Mass of object 1) * (Mass of object 2) divided by the distance
between the two objects squared:
F ∝ M1 x M2 / d^2
If the objects were moved 2 times closer (or, to half of their initial distance between)
...
✓ ~~~F ∝ M1 x M2 / (1/2)^2 = M1 x M2 / (1/4) = 4 (M1 x M2) so the gravitational attraction
would because 4 times as great
What causes day and night?
, ✓ ~~~Rotation of the Earth on its axis
How does the Sun appear to move in the sky in the course of a day?
✓ ~~~East to West, because of Earth's rotation. The stars and planets move in the same
way from our point of view, also, because of Earth's rotation
What happens to the Earth in one year?
✓ ~~~It orbits the Sun, once
Why do we have seasons?
✓ ~~~The tilt of the Earth's axis of rotation, with respect to the plane of its orbit around the
Sun (The tilt is about 23 degrees)
How is the Earth's axis tilted when we have summer in the Northern hemisphere?
✓ ~~~With the North pole toward the Sun
What season is it in the Southern hemisphere when it is summer in the Northern
hemisphere?
✓ ~~~Winter
When does Winter begin and what does that mean?
✓ ~~~On or about December 21 = in the Northern hemisphere, the nights are longer than
days
When does Spring begin and what does that mean?
✓ ~~~On or about March 21 = days and nights have equal length
, When does Summer begin and what does that mean?
✓ ~~~On or about June 21 = days longer than nights in the North
When does Fall begin and what does that mean?
✓ ~~~On or about September 21 = days and nights have equal length
What happens if the Earth's axis were not tilted?
✓ ~~~Seasons would not happen
What happens to the moon in 1 month?
✓ ~~~It moves once around the Earth
What causes the phases of the moon?
✓ ~~~The Sun is lighting up different fractions of the part of the Moon we see from Earth
What is the order of the phases of the Moon?
✓ ~~~New - Waxing Crescent - First Quarter - Waxing Gibbous - Full - Waning Gibbous -
Third Quarter - Waning Crescent - New
When is the full moon visible?
✓ ~~~Only at night. It transits at midnight, rises 6 hours earlier (at sunset) and sets 6 hours
later (at sunrise)
When is the new moon visible?
✓ ~~~The new moon is visible during the day. It transits at noon; it rises 6 hours earlier (at
sunrise), and sets 6 hours later (at sunset)
