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Activity 1 What Causes Gravity?
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GRAVITATIONAL INTERACTIONS
Activity 1: What Causes Gravity?
In Activity 1, your class came up with several ideas about what causes gravity. You and
your class probably thought that these might be causes of gravity:
a) the Earth’s magnetic field
b) the rotation of the Earth
c) air pressure from the Earth’s atmosphere
Pick one of these ideas, and design a quick experiment to test whether it could be the
cause of gravity.
1. Write a paragraph describing your experiment. Your paragraph must be at least
five complete sentences.
2. Draw and label a sketch of your experiment.
3. What experimental result would show that you might have found the
cause of gravity?
Activity 2: Testing Ideas about Gravity
Suppose you were visiting a third-grade class, and some students were talking about
their ideas on what causes gravity.
1. One of the students, Jacob, asks you, “Since we learned that the Earth is a big
magnet, isn’t that what causes gravity?” Write how you would simply demonstrate
to Jacob why the Earth’s magnetic field is not the cause of gravity. Include any
drawings or diagrams on your answer sheet that you would make to help Jacob
understand.
2. Danielle asks, “I read that the Earth spins around on its axis. Wouldn’t its spinning
cause gravity?” Write how you would simply demonstrate to Danielle why the
Earth’s rotation is not the cause of gravity. Include any drawings or diagrams that
you would make to help Danielle understand.
3. Leticia asks, “Doesn’t the air around us push down on us, causing gravity?” Write
how you would simply demonstrate to Leticia why air pressure from the Earth’s
atmosphere is not the cause of gravity. Include any drawings or diagrams that you
would make to help Leticia understand.
Activity 3: More on Gravitational Interactions
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Activity 1 Practice
Activity 2 Practice
1. Students will have various
responses. Check to see if the
experiment tests the hypothesis the
student chose, and follows the rules
on “How To Evaluate an
Experimental Design for a Fair
Test.”
1. (Example 1) I would show Jacob
that objects like pens and pennies
are not attracted to a magnet, but
both fall toward the ground, so
gravity cannot be caused by the
Earth's magnetism.
2. Students will have various
responses.
3. Students will have various
responses.
3. (Example) If I could, I would show
Leticia the video that showed the
mass placed on the mass scale.
The video showed that the reading
on the mass scale did not change
when the air pressure around the
mass and scale was greatly
reduced. If air pressure was the
cause of gravity, the reading on the
scale would have reduced as the
air was taken out of the bell jar.
Activity 3 Practice
(Questions 1-3) In the video of the Cavendish experiment, a meter stick is
suspended by a fine thread. Bottles are attached to each end of the meter stick. To
test Newton’s idea about what causes gravity, heavy boxes of sand are brought close
to the bottles, but the bottles and boxes do not touch each other. There is an
observable gravitational interaction between each box and its bottle.
Unit 3 • Chapter 2
2. (Example) I would attach a string
to a plastic toy soldier, and tape
the string to a rotating globe, so
that the soldier is standing up on
the Earth in the northern
hemisphere (but not at the North
Pole). I would ask Danielle,
“Which way does the Earth's
gravity pull on the soldier?” She
might say it holds the soldier on
Earth, but after spinning the globe,
Danielle would see the toy soldier
will fly off the Earth. So, because
the spinning tends to have objects
on the outside surface fly off, the
Earth's rotation cannot be the
cause of gravity.
(Example 2) I would have two
magnets. I would show Jacob the
two magnets, the Earth being like
one magnet. If I turn around the
other magnet, it is repelled from
the first magnet. However, either
way the second magnet is turned,
it still is attracted to the Earth when
you let it fall. Thus, the Earth's
magnetism is not the cause of
gravity.
1. If the boxes of sand had much
more sand in them, we would
observe a greater rotation of the
meter stick. The meter stick would
rotate more because the
gravitational interaction between
the bottles and the boxes of sand
would be greater. As the mass
increases the interaction strength
increases.
2. If the boxes of sand were not
brought so close to the bottles, the
meter stick would not have rotated
as much. The meter stick would
not have rotated as much because
the gravitational interaction
between the boxes and bottles
would have been less. The
gravitational interaction strength
decreases as the distance between
the interacting objects increases.
3. If the bottles were replaced by
bigger bottles with more water in
them, we would observe a greater
rotation of the meter stick. The
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meter stick would rotate more
because the gravitational
interaction between the bottles and
the boxes would be greater. As
the mass increases the interaction
strength increases.
thread
meter stick
bottles
box of sand
Activity 4 Practice
box of sand
1. What would you observe if the boxes of sand had much more sand in them? Why?
Answer this question by writing at least two complete sentences.
2. What would you observe if the boxes of sand were not brought so close to the
bottles? Why? Answer this question by writing at least two complete sentences.
1. (See forces drawn on diagram
below.)
3. What would happen if the bottles were replaced by bigger bottles with more water
in them?
The apple would weigh the most
on Jupiter.
Activity 4: Weight
2. The Moon is less massive than the
Earth and hence has the weaker
gravitational interaction. The Moon
wouldn't pull you down as hard as
the Earth. The result of this is that
you could jump higher on the
Moon because it has a weaker
gravitational interaction on you
than the Earth does.
1. In the diagram below, astronauts on different planets are dropping apples.
force exerted by
Mercury on apple
Mercury
force exerted by
Venus on apple
Venus
force exerted by
Earth on apple
Earth
force exerted by
Jupiter on apple
Jupiter
The planets appear in order of their gravitational pull. Mercury is the least
massive planet and has the weakest gravitational pull. Jupiter is the most massive
4
and has the strongest gravitational pull. Mercury’s gravity = 10
of Earth’s
9
gravity. Venus’ gravity = 10
of Earth’s gravity, and Jupiter’s gravity = 2 12 times
Earth’s gravity.
a) Copy the drawings, then draw and label force arrows to show the force exerted
by each planet on its apple. Pay attention to the lengths of your force arrows!
One is done for you.
b) On which planet would the apple weigh the most?
2. Mercury, a planet in our Solar System, and Callisto, a moon
of the planet Jupiter, have about the same volume.
However, Mercury’s mass is about 3 times greater than
Callisto’s mass. Why could a future astronaut (perhaps
you!) jump higher on Callisto than on Mercury? Use
gravitational interaction ideas to answer this question.
Write at least three complete sentences.
Callisto
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Mercury
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Activity 5 Putting Together Gravitational Interaction Ideas
GRAVITATIONAL INTERACTIONS
3. Copy and complete the table below.
person slows
down as
she moves
upward
person’s speed
increases
as she falls
Other than drag,
does an interaction
affect the person‘s
motion?
If so, what kind of
interaction(s) affect
the person‘s motion?
If so, what are the
interacting objects?
Is a force (or forces)
being exerted on the
person? If so, what is
your evidence?
On the pictures in the
top row, label force
arrows to represent any
forces on the person.
Label force arrows
on diagram.
3
Label force arrows
on diagram.
Activity 5: Putting Together Gravitational Interaction Ideas
Analyze and explain the following situations using How To Write an
Analysis and Explanation. You will need to copy the diagrams onto
your answer sheet. All of the analyses should include the interacting
objects and their interaction type and labeled force arrows showing
the forces being exerted on the object named in the task. You will not
need to draw energy diagrams. Be sure that you would get a good
evaluation using How To Evaluate an Analysis and Explanation.
Activity 5 Practice
1. (Why does the softball slow down
as it moves upward away from the
bat?)
Analysis:
There is a gravitational interaction
between the ball and the Earth.
Explanation:
The ball slows down because there
is an unbalanced force acting on
the ball, opposite to the ball's
direction of motion. The force
acting on the ball is exerted by the
Earth and is due to the
gravitational interaction between
the Earth and the ball. This force
points toward the Earth, which is
opposite to the direction of motion
of the ball.
What goes up…
1. A softball player hits a pop fly straight up into the air. After the
softball leaves the bat, why does the softball slow down as it moves
upward away from the bat? (You may ignore the drag interaction,
since it is very small.)
Write an analysis and explanation.
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305
3.
Table: Density and Buoyancy
Other than drag, does
an interaction affect the
person’s motion?
© It’s About Time
If so, what kind of
interaction(s) affect the
person’s motion?
If so, what are the
interacting objects?
Is a force or forces being
exerted on the person?
If so, what is your evidence?
Yes
Yes
Gravitational
Gravitational
The earth and the person
The earth and the person
The jumper’s motion changes,
which means there is an
unbalanced force. Since the
jumper slows down, the
force must be opposite
to her motion.
The jumper’s motion changes,
which means there is an
unbalanced force acting
on her. Since she is speeding
up, the force is in the
direction of her motion.
On the pictures in the
top row, label force arrows
to represent any forces on
the person.
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2. (Why does the softball speed up as
it moves downward toward the
ground?)
Analysis:
… must come down!
There is a gravitational interaction
between the ball and the Earth.
2. After the softball reaches the highest point of its path, it falls back to
the field. Why does the softball speed up as it moves downward
toward the ground? (You may ignore the drag interaction, since it is
very small.)
Explanation:
Write an analysis and explanation.
Force exerted
by Earth on
ball
The ball speeds up
because there is an
unbalanced force
acting on the ball in the ball's
direction of motion. The force
acting on the ball is exerted by the
Earth and is due to the gravitational
interaction between the Earth and
the ball. This force point points
toward the Earth, which is in the
same direction that the ball is
moving.
3. Two planets have the same size, but Planet A is more massive than
Planet B. How does the weight of a book on Planet A compare with
the weight of the same book on Planet B?
Write an analysis and explanation.
Activity 6: Orbital Motion
(Questions 1-3) Analyze and explain the situations by following How To Write an
Analysis and Explanation. You will need to copy the diagrams below onto your
answer sheet. All of the analyses should include the interacting objects and their
interaction type, and labeled force arrows in the diagrams showing the forces being
exerted on the object named in the task. You will not draw energy diagrams. Be sure
that you would get a good evaluation using How To Evaluate an Analysis and
Explanation.
1. As a rotating space station simulates the feeling of gravity, the men and women
inside the space station also move around in a circle.
Analyze and explain why the people move around in a circle.
3. (How does the weight of a book on
Planet A compare with Planet B?)
2. A satellite is an object that is launched into space to orbit the Earth. Some satellites
are used to reflect radio waves from distant points on Earth that cannot send and
receive the radio waves directly. These radio waves are used for radio and TV, cell
phones, and computer navigation systems like you might find in your car. Someday,
satellites may be used to collect solar energy and transmit energy to Earth.
Analysis:
There is a gravitational interaction
between Planet A and the book,
and between Planet B and the book.
Analyze and explain why a satellite orbits the Earth rather than moves off into
outer space.
3. Another satellite that orbits the Earth (though not
a man-made one!) is the Moon.
See diagram below.
Analyze and explain how the Moon is kept
in its orbit.
Explanation:
Since the planets are the same size,
but Planet A is more massive than
306
force exerted by
Planet A on book
Planet B
InterActions in Physical Science
force exerted by
Planet B on book
Planet B, there is a stronger
gravitational interaction between the
book and Planet A than between
the book and Planet B. A stronger
gravitational interaction means
greater force (or weight) exerted on
the book from Planet A than from
Planet B.
Activity 6 Practice
1. (Why do the people on a rotating
space station move around in a
circle?)
Analysis:
There are applied interactions
between the floor and each person.
(See force arrows on diagram.)
Explanation:
There is an
Force
unbalanced,
exerted by
constant
floor on
inward force
person
exerted by the
floor on each
person
associated with applied interactions.
This constant inward force exerted
by the floor on the people causes
them to continuously change their
direction of motion and move
around in a circle.
2. (Analyze and explain why a
satellite orbits
the Earth
Force exerted by Earth
rather than
on satellite
moving off
into outer
space.)
Analysis:
There is a gravitational interaction
between the Earth and the satellite.
(See force arrow on diagram.)
(Note: From
another view, the
force arrow
diagram may be
drawn as such:)
Explanation:
There is an
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Force exerted by
Earth on satellite
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Activity 7 Terminal Speed
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GRAVITATIONAL INTERACTIONS
Activity 7: Terminal Speed
The Parachute Inflates! (Analyze and Explain)
(Questions 1-2) In the activity, you analyzed and explained how a sky diver
reaches terminal speed. As the sky diver approaches the ground, she will, of
course, need to open her parachute! What do you think happens to the speed
of the sky diver when her parachute inflates in the air?
Immediately after the sky diver’s parachute catches the air and inflates, she and the
parachute will slow down until they reach another slower terminal speed, allowing
the sky diver to make a gentle landing on the ground.
1. Analyze and explain why the sky diver slows down immediately after her parachute
inflates. In your analysis, include the interacting objects and their interaction type,
and a diagram with labeled force arrow(s) showing the force(s) being exerted on
the sky diver. You do not need to draw energy diagrams.
2. Does the mass of the sky diver increase, decrease, or stay the same after the
parachute inflates? Explain your answer.
Multiple Choice
3. Which of these variables affects the strength of a gravitational interaction?
a) how fast the Earth rotates
b) the atmospheric (air) pressure
c) the distance between the objects
d) the volume of the objects
e) the strength of the Earth’s magnetic field
4. Which of these statements is not true?
a) There is a gravitational interaction between all objects in the universe.
b) Gravitational interactions can happen between objects that are not touching.
c) Gravitational interactions are very difficult to observe unless one of the objects is
very massive, like a planet.
d) The weight of an object is the force exerted by a planet on the object.
e) Increasing the mass of an object has no effect on its weight.
5. A rocket drops off a camera to take pictures of the Earth. Which arrow (a), (b), (c), (d),
or (e) best represents the direction of the force exerted by the Earth on the camera?
4
(Questions 6-7) Mercury has a weaker gravitational pull than Earth (about 10
as much).
6. The mass of an apple
a) will be the same on both planets.
b) will be less on Mercury than on Earth.
A
c) will be greater on Mercury than on Earth.
d) cannot be compared because Mercury has a very thin
atmosphere around it compared to Earth.
B
C
camera
D
E
© It’s About Time
Unit 3 • Chapter 2
unbalanced, constant inward force
acting on the orbiting satellite
toward the Earth. The force exerted
by the Earth on the satellite is
associated with the gravitational
interaction. This constant inward
force exerted by the Earth on the
satellite causes the satellite to
continuously change its direction of
motion and circle the Earth.
Analysis:
There is a gravitational interaction
between the Earth and the moon.
(See force arrow on diagram.)
(Note: From
another view, the
force arrow
diagram may be
drawn as such:)
Force exerted
by Earth
on Moon
Explanation:
3.
Force exerted by Earth on Moon
Earth
Moon
307
There is an
unbalanced,
constant inward force acting on the
orbiting moon toward the Earth.
The force exerted by the Earth on
the moon is associated with the
gravitational interaction. This
constant inward force exerted by
the Earth on the moon causes it to
continuously change its direction of
motion and orbit the Earth.
Activity 7 Practice
1. Analysis:
There is a gravitational
interaction between the
ball and the Earth.
There is a drag
interaction between the
parachutist (with
parachute) and the air.
Explanation:
Force
exerted by
air on
parachutist
Force
exerted by
Eartth on
parachutist
The parachutist slows down
immediately after the parachute
inflates because the upward force
exerted by the air on the parachutist
(drag) is greater than the
downward force exerted by the
Earth on the parachutist
(gravitational). This means there is
an unbalanced force on the
parachutist in the direction opposite
her motion. This unbalanced
upward force causes her to slow
down as she falls toward the
ground below.
2. The mass of the parachutist
(including her parachute) stays the
same, because both she and the
parachute have the same amount of
material whether the parachute is
inflated or not.
(There is a very slight increase in
the weight (not the mass) of the
parachutist as she gets closer to the
ground because she gets closer to
the center of Earth. However, this
increase is negligible, because the
distance that she falls (maybe 4000
meters) is very small compared to
the radius of Earth (about
6,400,000 meters).)
3. (c) the distance between the objects
4. (e) (not true) Increasing the mass of
an object has no effect on its
weight.
5. (b) (The direction of gravitational
force exerted by Earth on camera is
toward the center of Earth.)
6. (a) will be the same on both
planets.
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7. (b) will be less on Mercury than
on Earth.
8. (d) (The direction of gravitational
force exerted by Saturn on the
piece of ice is toward the center of
Saturn.)
7. The weight of an apple
a) will be the same on both planets.
Weight
b) will be less on Mercury than on Earth.
? Weight
c) will be greater on Mercury than on Earth.
d) cannot be determined because Mercury
has a very thin atmosphere around it
compared to Earth.
Activity 8 Practice
Earth
Mercury
8. In Activity 6, you read about the rings around Saturn consisting of pieces of ice.
Another view of Saturn is shown below. At the position shown, which force arrow
(a), (b), (c), (d), or (e) best shows the direction of the force exerted by Saturn on the
piece of ice?
1. The Newton, named after Sir Isaac
Newton, is the standard unit of
force strength.
2. 20 N to the right
A
E
3. 0, no direction (The forces are
balanced.)
D
B
C
Activity 8: Unbalanced and Balanced Forces
1. What is the standard unit of force strength, and who is it named after?
2. Suppose a bicycle experiences forces like those shown by the force arrows with the
force strengths in the picture below. What is the strength and direction of the
unbalanced force on the bicycle?
30 N
70 N
20 N
3. Later, the forces on the bicycle are like those shown by the force arrows in the
picture below. What is the strength and direction of the unbalanced force on the
bicycle?)
30 N
50 N
20 N
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Activity 9 Buoyancy
GRAVITATIONAL INTERACTIONS
4. In the Yukon, a team of four dogs heading west across a snow-covered plain exerts
a force of strength 2000 N on a loaded sled. As the sled is moving, there is a wind
blowing toward the east that exerts a force of strength 400 N on the sled. In
addition, there is a friction force of strength 700 N resisting the motion of the sled.
What is the strength and direction of the unbalanced force on the sled?
5. Consider a situation where you are pushing to the left with a force of strength
10 N against a heavy chair, but the chair is not moving. Because the chair is not
moving, the multiple forces acting on it must be balanced. Assume that a friction
force is preventing you from moving the chair. Find the strength and direction of
the friction force.
6. Three boys and two girls engage in a tug of war. Simon, Antonio and Chan exert
forces of 30 N, 45 N, and 20 N on the rope, all directed to the left. Rosalie and
Luisa exert forces of 20 N and 30 N on the rope, both directed to the right.
a) What is the strength and direction of the unbalanced force on the rope?
b) Elizabeth joins the girls in the tug of war and balances the forces on the rope.
What is the strength and direction of the force Elizabeth exerts on the rope?
Activity 9: Buoyancy
bag of water
To answer some of the problems below, you will need to refer to
these density values:
• aluminum – 2.7 g/cm3
• brass – 8.0 g/cm3
• steel – 7.6 g/cm3
• gold – 19.3 g/cm3
• mercury – 13.0 g/mL
• water – 1.0 g/mL
3
• oxygen – 0.0013 g/cm
• air – 0.0012 g/cm3
• helium – 0.00017 g/cm3
3
?
tank of water
1. As you saw in the activity, a plastic bag filled with water immersed in a tank
of water remains motionless in the tank after being let go. If the weight of the
bag is 5 N, is the buoyant force greater than 5 N, less than 5 N, or equal to 5 N?
Explain your answer.
2. A plastic bag filled with vegetable oil weighing 5 N is immersed in a tank of water
and let go. The oil-filled bag rises to the top and floats.
a) When the bag is let go, is the buoyant force greater than 5 N, less than
5 N, or equal to 5 N? Explain your answer.
b Is the density of the vegetable oil greater than, less than, or
equal to the density of water? Explain your answer.
buoyant force
exerted on balloon
by surrounding air
= 200 units
3. For her friend Maya’s birthday party, Sofia wants to inflate a
few dozen balloons so that they rise up toward the ceiling.
a) Why should Sofia use helium gas to fill up the balloons
rather than other gases like oxygen or air?
b) Look at the force arrow diagram beside the balloon. There
are 50 units of force from the combined weight of the
helium gas and balloon, and 200 units of upward buoyant
force exerted on the balloon. How many units of force are
needed from a downward pull of the string to hold the
balloon in place? Explain your answer.
weight of
balloon &
helium gas
= 50 units
force exerted by
string on balloon
= ? units
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4. Choose east as positive direction.
Force exerted by dogs = -2000 N
Force exerted by wind = + 400 N
Force exerted by friction =
+ 700 N
Total unbalanced force = -2000 N
+ 400 N + 700 N = -900 N
The unbalanced force on the sled
has a strength of 900 N and points
to the west.
direction
Forces exerted by boys = - 30 N 45 N - 20 N = -95 N
Forces exerted by girls = + 20 N +
30 N = 50 N
Total unbalanced force = -95 N +
50 N = -45 N
The unbalanced force on the rope
has a strength of 45 N and points
to the left.
5. Forces on chair to right = Forces on
chair to left
Friction force = Push force on chair
= 10 N
The strength of the friction force is
10 N, and it is pointing to the right.
B. Since the unbalanced force
points to the left, to balance the
forces, Elizabeth must exert a force
to the right.
Force exerted by Elizabeth to right
= Unbalanced force to left
Force exerted by Elizabeth to right
= 45 N
6. A. Choose right as positive
direction, left as the negative
The strength of the force Elizabeth
exerts on the rope is 45 N, and the
force points to the right.
Activity 9 Practice
1. The upward buoyant force is equal
to 5 N because it must balance the
downward weight force for the
baggie to remain motionless.
2. A. The upward buoyant force must
be greater than 5 N for the oilfilled baggie to rise to the top from
rest.
B. The density of the vegetable oil
must be less than the density of
water because the oil-filled baggie
rises to the top from rest. (Knowing
that the oil-filled baggie floats
means that the weight of the oil in
the baggie must be less than the
water it displaces. Since the volume
of the oil-filled baggie and the
water it displaces are the same, the
density of the oil-filled baggie must
be less than the density of the
water.)
3. A. Luisa should use helium gas to
fill the balloons so that they will
rise up to the ceiling because the
density of helium is less than the
density of air. (Since the inflated
balloon and the volume of air
displaced are equal, the lesser
density of helium than air means
the helium-filled balloon weighs
less than air and will float in air.)
B. 150 units. In order to hold a
helium-filled balloon in place, the
forces must be balanced. Thus, an
additional downward pull of 150
units along with the 50 units
weight force will balance an
upward buoyant force of 200
units.
4. (d) All three cubes experience the
same buoyant force (because they
displace the same volume (and
therefore weight) of water).
5. (d) gold
6. (b) The buoyant force strength is
equal to the block's weight.
(Note: Students may find this problem
a bit tricky. The reason the answer is
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(b), and not (c), is because when an
object is floating, it is at rest (or more
precisely, it isn't moving in the vertical
direction up or down). Therefore, the
forces are balanced. For those who
answer (c), point out that the block is
not completely submerged. Unless the
object is completely submerged,
comparing densities isn't enough to
determine whether the buoyant force
is greater than the weight.)
Activity 10: Potential Energy
1. Julie stands on a diving board 1 meter above a diving pool. Her twin sister Diana,
who weighs the same as Julie, stands on a high-dive platform 10 meters above
the pool.
a) Which system has more gravitational potential energy, the Julie-and-Earth
system or the Diana-and-Earth system? Explain your answer.
b) Diana dives off of the 10-meter platform. As she falls toward the pool, does her
gravitational potential energy increase, decrease, or stay the same? As she falls
toward the pool, does her kinetic energy increase, decrease, or stay the same?
c) Later, Julie’s big brother Bill, who weighs more than Julie, joins her standing on
the 1-meter diving board. Which system has more gravitational potential energy,
the Julie-and-Earth system or the Bill-and-Earth system? Explain your answer.
Multiple Choice
Part 2 (In Practice Book)
2. The bookcase in Evelyn’s bedroom has four shelves. Suppose the book with a mass
of 0.5 kg fell off a shelf. From which shelf would the fall of the book convert the
most potential energy into kinetic energy?
a) bottom shelf (located 0.1 m above the floor)
b) second shelf (located 0.5 m above the floor)
c) third shelf (located 0.9 m above the floor)
d) top shelf (located 1.3 m above the floor)
1. See Table Below.
Activity 10
Practice
3. André’s teacher gives him an aluminum spring that has a mass of 800 g. After a
lesson about potential and kinetic energy, André’s teacher asks him how he would
go about increasing the potential energy of the spring. André considers the four
ideas listed below. Which response does not increase either the spring’s potential
energy or the potential energy of the spring-and-Earth system?
a) compressing the spring
b) putting the spring on the floor
c) stretching the spring
d) tossing the spring up into the air
1. A. The Diana-and-Earth system has
more gravitational potential energy
than the Julie-and-Earth system.
Even though they both weigh the
same, Diana is higher above the
pool along the Earth's surface.
4. Choose the Earth-and-bird system with the most gravitational potential energy.
a) Earth and an eagle with a mass of 4.5 kg flying at a height of 1000 m
b) Earth and a gull with a mass of 0.5 kg flying at a height of 100 m
c) Earth and a falcon with a mass of 1.0 kg flying at a height of 1000 m
d) Earth and a raven with a mass of 1.0 kg flying at a height of 100 m
B. As Diana falls toward the pool,
her gravitational potential energy
decreases (because she is getting
closer to the Earth), and her kinetic
energy increases (because of the
simple relationship between
gravitational potential energy and
kinetic energy).
2. (d) Top shelf (located 1.3 m above
the floor)
3. (b) putting the spring on the floor
(does not increase spring's
potential energy)
4. (c) Earth and a falcon with a
mass of 1.0 kg flying at a height of
1000 m
5. (d) Ledge located 53 m below the
top of the waterfall
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Table: The Buoyancy of Objects
Object
Mass
Volume
Density
Water:
Sink or
Float?
Salt Water:
Sink or
Float?
Baseball
145 g
232 cm3
0.63 g/cm3
Float
Float
Can of Cola
(12 oz)
384 g
379 cm3
1.01 g/cm3
Sink
Float
Can of Diet Cola
(12 oz)
371 g
379 cm3
.98 g/cm3
Float
Float
Milky Way™ Bar
60 g
50 cm3
1.20 g/cm3
Sink
Neither
Toy Wagon
55 g
63 cm3
0.87 g/cm3
Float
Float
(includes open part)
Part 2 (In Practice Book)
1. (b) (greatest elastic potential
energy)
2. (d) (greatest kinetic energy after
spring released)
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UNIT 3: INTERACTIONS AND FORCES
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C. The Bill-and-Earth system has
more gravitational potential energy
than the Julie-and-Earth system.
Even though they are both the
same height above the pool along
the Earth's surface, Bill weighs
more than Julie.
5. A person living in the wilderness near a waterfall 60 m high wants to utilize the
waterfall to generate electricity for his small wooden cabin. He has a turbine and
generator. There are four ledges in the cliff behind the waterfall where he could
place the turbine and generator he has built for this purpose. On which ledge
would the turbine and generator produce the most electrical energy?
a) ledge located 15 m below the top of the waterfall
b) ledge located 23 m below the top of the waterfall
c) ledge located 36 m below the top of the waterfall
d) ledge located 53 m below the top of the waterfall
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Activity 12 Distance in Space
3
GRAVITATIONAL INTERACTIONS
Activity 11: The Solar System
1. a) Make a close sketch of (or trace over) the picture of the Sun and planets in our
Solar System in Activity 11. Label each planet and the Sun.
b) On the sketch, indicate the terrestrial planets and the gas giants.
c) On the sketch, label where the Asteroid Belt and the Kuiper Belt are located.
2. a) Which planet is the largest in size (excluding rings)?
Which planet is the smallest?
b) Which planet has the greatest mass?
Which planet has the least mass?
c) Which planet has the greatest density?
Which planet has the least density?
3. a) Which planet do some people think could have supported life at one time?
b) What are some reasons they have for this idea?
4. a) How is a planet different from a dwarf planet, other than in size?
b) How is a moon different form a planet or a dwarf planet?
Activity 12: Distances in Space
a) astronomical unit
b) light-year
2. Identify the following terms as either a unit of distance, time, or speed.
b) year
c) light-year
d) mph
e) astronomical unit
3. Rank the following distances from least to greatest: astronomical unit, light-year,
100,000,000 kilometers.
(least distance) _____________________________________________ (most distance)
Unit 3 • Chapter 2
Jupiter
Terrestrial Planets
Activity 11
Practice
1. (A through C)
2. A. Jupiter is the largest planet;
Mercury is the smallest planet.
B. Jupiter has the greatest mass;
Mercury has the least mass.
C. Earth has the greatest density;
Saturn has the least density.
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Neptune
Uranus
Pluto
Kuiper Belt
Asteroid Belt
Sun
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Saturn
Mars
Gas Giant Planets
6. (a) ice and rock
Activity 12
Practice
3. (least distance) 100,000,000
kilometers, astronomical unit, lightyear (greatest distance)
4. Planets and other Solar System bodies have elliptical orbits. How are elliptical orbits
different than circular orbits? Include a sketch of these orbits with your answer.
Earth
5. (b) Venus
2. A. kilometers per hour: (unit of)
speed
B. year: time
C. light-year: distance
D. mph: speed
E. astronomical unit: distance
a) kilometers per hour
Venus
4. A. A planet dominates its orbital
neighborhood, while a dwarf planet
does not.
B. Planets and dwarf planets orbit
the Sun, while moons orbit planets
and dwarf planets.
1. An astronomical unit is the average
distance between the Earth and
Sun, about 150,000,000 km (150
million kilometers).
1. Define the following terms:
Mercury
have proposed that the layered rock
outcrops that have been interpreted
as signs of past water could have
been left by explosive volcanic ash
or an ancient meteorite impact.)
KBO
3. A. Mars
B. Like the Earth, Mars has
relatively mild temperatures
(between -100°C and 27°C), a day
nearly the same as Earth's, and an
atmosphere (though thinner and of
different composition than Earth's).
Dry channels similar to the Earth's
river systems invite speculation that
they once carried water, a
substance critical for supporting life
on Earth. (However, recent studies
4. In a planet's elliptical (oval or “eggshaped”) orbit, the planet's distance
from the Sun varies depending on
where it is in its orbit. In a circular
orbit, an object keeps the same
distance (called the radius) from a
central point.
Part 2 (In Practice Book)
1. (d) 660 (= 165 yr/ 0.25 yr)
(number of Mercury orbits around
Sun for every one of Neptune's
orbits)
2. (c) 12,000 (= 149,600 km/
12.756 km) (Earth diameters fit into
1 AU)
3. (e) 500,000 AU (= (8.6 ly) x
(63,240 AU/ly) (distance of Sirius
from Sun)
4. (a) Betelgeuse (At 100 km/hr, it
takes 11,000,000 years to travel 1
ly distance. Over 4.5 billion years,
the distance is 4,500,000,000/
11,000,000 = 409 ly, closest to
Betelgeuse (427 ly))
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PRACTICES—ANSWERS
INTERACTIONS AND FORCES
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CHAPTER 2 GRAVITATIONAL INTERACTIONS
Activity 13
Practice
1. (b) in the center of the disc where
a star forms
Activity 13: Gravity, Stars, and Planets
2. (c) the part of the disc far from
the star is much colder, so some
gases can freeze.
1. When a large cloud pulls together to form a solar system, it flattens out into a disc.
Most of the cloud’s mass ends up
Multiple Choice
a) far out in the disc where gas giants form.
b) in the center of the disc where a star forms.
c) on the edge of the disc where icy objects like KBOs form.
3. (d) cores of gas giants are more
massive than terrestrial planets,
which means they have stronger
gravity.
d) close to the center of the disc where terrestrial planets form.
2. Ice is more likely to form on bodies far from the star in the center of a solar system
than on bodies close to the star because:
a) bodies far from the star are more massive and have stronger gravity.
b) bodies far from the star exert stronger magnetic forces.
4. (c) they are too small.
c) the part of the disc far from the star is much colder, so some gases can freeze.
d) the part of the disc far from the star just has more ice.
5. (a) the force produced by hot gas
and nuclear radiation balances
gravity.
3. Gas-giant planets have thicker atmospheres than terrestrial planets because
a) gas giants have lower densities than terrestrial planets.
b) gas giants are cooler than terrestrial planets.
6. You never see the “far side of the
Moon” because the Moon had
become locked into position so that
one side of the Moon always faces
Earth, and one side always faces
away. Its rotation period is exactly
equal to the period of its orbit
around earth.
c) gas giants exert stronger magnetic forces on gases than terrestrial planets.
d) cores of gas giants are more massive than terrestrial planets, which means they
have stronger gravity.
4. Gravity doesn’t pull some bodies in space into round shapes because
a) they are made of rock and metal.
b) they are made of ice and rock.
c) they are too small.
d) they are too cold.
5. A star has a round shape because
a) the force produced by hot gas and nuclear radiation balances gravity.
7. (d) Pluto (always keeps its same
face toward its moon Charon.)
b) its magnetic forces balance gravity.
c) the solid interior of the star stops gravity from making it contract (get smaller
and more dense).
d) nuclear reactions can only occur if the star is spherical.
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SCIENTISTS’ CONSENSUS IDEAS—ANSWERS
Scientists’ Consensus Ideas
Unit 3 Chapter 2 Answer Keys
(Unit 3 Chapter 2, Activities 2–6)
9. The Gravitational Interaction
General Description of the Gravitational Interaction
1. The video we saw of Cavendish’s experiment. There was a meter stick suspended by a string. On each end
of the meter stick was an attached bottle. There was also a mirror attached to the meter stick and a beam of
light reflected off the mirror onto a wall. A large box of sand was brought near each end of the meter stick,
but on opposite sides. We observed that the light beam on the wall moved. This is because the mirror
attached to the meter stick rotated slightly. The only thing that seemed to cause the meter stick to rotate
was the attraction between the boxes of sand and the bottles.
3. Two small magnets placed on a table can be observed to interact (as in Unit 1), overcoming the friction
interaction. However, two non-magnetic objects (assume same size, shape, and mass) are not observed to
interact because the gravitational interaction is weaker than the friction interaction.
4. An object will change its motion if there is an unbalanced force acting on it. If the unbalanced force is in the
direction of motion, the object will speed up. The evidence for the gravitational force is that the apple’s
motion changes – it speeds up toward Earth.
6. In the experiment we watched, an object was placed on a scale in a bell jar. The air was then evacuated out
of the bell jar. The object did not begin to float and its scale measurement did not change. Because there
was no change observed, the gravitational interaction cannot be due to air pressure.
7. Example Response: In the experiment we did, the string straightened up (pointed outward) when the
bucket rotated. Gravity is not caused by Earth’s rotation because if it were the string would not have moved
outward, pointing away from the bucket, when the bucket was rotated. Rather, it should have moved
inward, toward the bucket and rested itself on the bucket.
8. Example Response: Gravity always attracts objects (objects are pulled toward each other); magnetism
sometimes repels objects. Also, there is a magnetic attraction between a magnet and some metals, but
there is a gravitational attraction between Earth and ALL objects.
(Unit 3 Chapter 2, Activities 8–10)
10. Other Ideas about Forces, Motion,
and Energy–Part 2
Buoyancy
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5. In the video, when the force exerted on the floating object by the scale was subtracted from the weight of
the object, the result was the buoyant force on the object, which was 1.4 N. The weight of the water
displaced by the object was also 1.4 N, equal to the buoyant force.
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Sinking and Floating
9. A baggie of salt water, which is denser than water, sinks when submerged in a container of water.
10. A baggie of alcohol, which is less dense than water, rises to the surface and then floats when submerged
in a container of water.
11. A baggie of water neither rises nor sinks when submerged in a larger container of water.
(Unit 3 Chapter 2, Activities 11–13)
11. Earth and Space Science–Part 1
The Solar System
4. Eclipses of the Moon, which occur when the Moon passes through Earth’s shadow. Earth blocks sunlight
from reaching all or part of the Moon’s surface, and the Moon doesn’t shine.
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Scientists’ Concensus Ideas – Answers
NOTES
3
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CHAPTER 2
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