ASTRO 7N EXAM 1 INTRODUCTION TO
ASTRONOMY CORE CONCEPT REVIEW
AND STEP BY STEP EXPLANATIONS 2026
STUDY GUIDE
◉ Newton's First Law
Answer: An object at rest stays at rest and an object in motion stays
in motion with the same speed and in the same direction unless
acted upon by an unbalanced 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
Answer: The acceleration of a body due to a force will be in the same
direction as the force, with magnitude directly proportional to its
mass
Force = mass x acceleration
A smaller mas will move faster, if the same force is applied to it
,◉ Newton's Third Law
Answer: For every action there is an equal and opposite reaction
The Sun exerts force on planets, and they orbit it - he planets exert
equal force on Sun, but it only moves slightly because of its very
large mass relative to the planets
◉ Surface Gravity on a Planet
Answer: the strength of gravity: g = m/r^2
where M is the mass of the planet and R is its radius. Note the radius
is squared but the mass is not.
More mass = 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.
Larger separation = less gravity.
- If the radius of a planet were twice that of another, the gravity is 1
/ (2) 2 = 1/4 as strong.
Putting the effects of mass and radius together, you should be able to
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. You will need to be
able to do this type of estimate for the test, given the numbers for
mass and radius.
◉ If I apply exactly the same amount of force to a pebble and a
boulder, what will happen
Answer: the pebble will move faster
◉ If the Earth were moved to half its current distance from the sun,
how would the force of gravity by the sun on the earth change?
Answer: it would become 4 times weaker
◉ Mass
Answer: As the mass of the planet increases, the jump height does
decreases by the same factor
As the mass of the planet decreases, the jump height does increases
by the same factor
, ◉ According to Newton's Second law of motion, if the net force
acting on the object increases while the mass of the object remains
constant, what happens to acceleration
Answer: acceleration increases
◉ If the earth were moves to half its current distance from the Sun,
how would the force of gravity by the Sun on the Earth change
Answer: it would become four times weaker
◉ What did newton's law of universal gravitation tell us about how
gravity works
Answer: the force of gravity from the sun will be stronger on object
with more mass
◉ If you were to sit on the ground, there would be a force due to
gravity pulling you toward the earth. which of the following is true
according to newton's third law
Answer: you are pushing the earth away from yourself with the
same force
◉ what would happen to the earth sun's gravity somehow turned off
instantaneously
Answer: it would continue to move in the line in the same direction
it was moving when the gravity turned off
ASTRONOMY CORE CONCEPT REVIEW
AND STEP BY STEP EXPLANATIONS 2026
STUDY GUIDE
◉ Newton's First Law
Answer: An object at rest stays at rest and an object in motion stays
in motion with the same speed and in the same direction unless
acted upon by an unbalanced 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
Answer: The acceleration of a body due to a force will be in the same
direction as the force, with magnitude directly proportional to its
mass
Force = mass x acceleration
A smaller mas will move faster, if the same force is applied to it
,◉ Newton's Third Law
Answer: For every action there is an equal and opposite reaction
The Sun exerts force on planets, and they orbit it - he planets exert
equal force on Sun, but it only moves slightly because of its very
large mass relative to the planets
◉ Surface Gravity on a Planet
Answer: the strength of gravity: g = m/r^2
where M is the mass of the planet and R is its radius. Note the radius
is squared but the mass is not.
More mass = 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.
Larger separation = less gravity.
- If the radius of a planet were twice that of another, the gravity is 1
/ (2) 2 = 1/4 as strong.
Putting the effects of mass and radius together, you should be able to
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. You will need to be
able to do this type of estimate for the test, given the numbers for
mass and radius.
◉ If I apply exactly the same amount of force to a pebble and a
boulder, what will happen
Answer: the pebble will move faster
◉ If the Earth were moved to half its current distance from the sun,
how would the force of gravity by the sun on the earth change?
Answer: it would become 4 times weaker
◉ Mass
Answer: As the mass of the planet increases, the jump height does
decreases by the same factor
As the mass of the planet decreases, the jump height does increases
by the same factor
, ◉ According to Newton's Second law of motion, if the net force
acting on the object increases while the mass of the object remains
constant, what happens to acceleration
Answer: acceleration increases
◉ If the earth were moves to half its current distance from the Sun,
how would the force of gravity by the Sun on the Earth change
Answer: it would become four times weaker
◉ What did newton's law of universal gravitation tell us about how
gravity works
Answer: the force of gravity from the sun will be stronger on object
with more mass
◉ If you were to sit on the ground, there would be a force due to
gravity pulling you toward the earth. which of the following is true
according to newton's third law
Answer: you are pushing the earth away from yourself with the
same force
◉ what would happen to the earth sun's gravity somehow turned off
instantaneously
Answer: it would continue to move in the line in the same direction
it was moving when the gravity turned off
