Gravity
Earth and Space
Gravity
Background
All of the celestial bodies in the solar system move in
predictable patterns known as orbits, and this motion is
controlled by gravity. Gravity is a force of attraction
between two or more masses. Everything that has mass
has gravity. The more mass you have, the stronger the
gravitational pull. For example, Earth has more mass than
the Moon, so its gravitational field is stronger. The Sun, as
the most massive object in our solar system, governs the
motion of all other bodies in the system by the force of
gravity. Bodies of the solar system remain in orbital paths
due to a balance between gravitational forces and the
constant forward motion of the celestial bodies.
The strength of the gravitational attraction not only depends on the mass of the objects involved
but also the distance between the two objects. As the distance between two masses increases, the
gravitation attraction between them decreases. For example, Earth's gravitation pull on us is
strongest when we are standing on its surface compared to if we were located in space, thousands
of miles away. The celestial body in our solar system with the most mass and hence strongest
gravitational force is the Sun. Because the Sun is so massive, its gravitational field is able to attract
all of the objects within the solar system but due to the great distance, the Sun does not attract
objects outside of the solar system.
Answer the Background questions in your Student Journal.
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Gravity
Earth and Space
Part I: The Bigger, The Better
Gravity is a force of attraction between two or more masses. Two things must be considered when
thinking about the strength of the force of gravity:
•  The distance between the masses that are attracted by gravitational force.
•  The amount of mass present in the masses that are attracted by gravitational force.
This activity will demonstrate how increasing or decreasing mass affects the force of gravity.
Procedure:
1.  The teacher will divide the class into two groups of equal size. Group A will stand in the
designated Group A location and Group B will stand in the designated Group B location as
marked on the floor. Groups A and B represent two large masses such as two planets of equal
mass.
2.  The teacher will select one student to be an object positioned at a location between the two
masses/planets. This student represents an object such as a Moon. The student will choose
which floor marking is the most appropriate location for the object with respect to the effects of
gravitational force.
3.  Half of the students in Group A will now move to become members of Group B.
4.  The student or object/moon will choose a new location appropriate for the change in the
masses of Group A and Group B.
Complete Part I in your Student Journal.
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Gravity
Earth and Space
Part II: Big, Bigger, Biggest
The scientific law concerning gravity is named Newton's Universal Law of Gravitation because the
law is true for all objects in our universe. Newton concluded that any two objects in the universe
exert gravitational attraction on each other in a universal manner: He reasoned that every object in
the universe attracts every other object with a force that is directly related to the mass of the
objects and inversely related to the distance between the objects.
In other words:
(1) Direct relationship: the greater the mass of the objects, the greater the force of gravity; and, the
smaller the mass of the objects, the smaller the force of gravity.
(2) Inverse relationship: the greater the distance between the objects, the smaller the force of
gravity; and, the smaller the distance between the objects, the greater the force of gravity.
This is true for every object in the universe, not just planets, stars, and moons. Gravitational
interaction exists between ALL objects in the universe, even between you and your desk, the
person next to you or the computer sitting on the teacher's desk. These gravitational forces are so
small they are not noticed as compared to those exerted by massive objects like the Sun or Earth.
In Part I, you modeled the direct relationship between increasing mass and increasing gravitational
force. In Part II, the direct relationship of between mass and gravitational force is explored by
ranking celestial objects in our solar system by mass.
Procedure:
1.  Look at the Student Reference Sheet: Planet Sizes.
2.  Use the comparative sizes for each of the objects to arrange them in an ordered list from
largest gravitational force to least gravitational force in your Student Journal.
3.  After completing the table, answer the questions in your Student Journal.
Complete Part I and Part II in your Student Journal.
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Gravity
Earth and Space
Part III: How Far is Too Far?
Background
You have explored the gravitational interaction in the direct
relationship between mass and force of gravity and learned that
the Universal Law of Gravitation applies to all objects in the
universe although some gravitational interaction are so small
they are not noticed. It would also be difficult to measure small
gravitational interactions. Magnetism, like gravity, has an inverse
relationship between distance and force of attraction. Magnets
can be used in the classroom to model the force of gravitational
attraction. In this experiment, you will test the relationship
between distance and the strength of magnetic force to model
that of gravitational force.
Force
Force
mass
mass
All objects attract all other objects.
mass
mass
mass
The greater the mass of an object,
the greater the attraction.
mass
There is a relationship between distance and the strength of the gravitational
force. As the distance between objects increases, the strength of attraction
between the two objects decreases.
Complete Part III of your Student Journal.
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Gravity
Earth and Space
Part IV: Plan Your Investigation
You will now develop an investigation to test the relationship between distance and the strength of
the force of gravity. Since the force of gravity between two masses is difficult to observe in a
classroom and magnetic force has the same relationship between distance and strength, you will
use magnets to observe magnetic force as a model of the relationship between distance and the
strength of gravitational force.
Question of Inquiry: With your class and teacher, discuss the Question of Inquiry and list the
materials that you will need to conduct your investigation.
Safety Precautions
There are no special safety concerns in this investigation; however, lab safety rules and
procedures should always be followed.
Complete Part IV in your Student Journal.
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Gravity
Earth and Space
Part V: Implement Your Investigation
Procedure:
1. 
Take four books of the same size and build two stacks,
two books in each stack.
2. 
Place the stacks 25 cm apart.
3. 
Take a sturdy piece of cardboard and place it on top of
both stacks. To keep the cardboard from moving, secure
it with a large rubber band wrapped around the top book
and the cardboard.
4. 
Place a paperclip on top of the cardboard.
5. 
Take a magnet and hold it under the cardboard and
paperclip.
6. 
Move the magnet along the bottom of the cardboard so
that the attracted paperclip also moves along the top of
the cardboard.
7. 
Remove the paperclip and the magnet.
8. 
Place a second piece of cardboard on top of the first
piece of cardboard. Use two binder clips to clip the
cardboard in place.
9. 
Repeat steps 3 through 7 until the paper clip is no longer
attracted to the magnet (does not move).
1. 
2.
3.
4.
7.
10.
10.  Use a ruler to measure the thickness of the cardboard
plus the added pieces of cardboard.
Complete Part V and the Reflections and Conclusions in your Student Journal.
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Gravity
Earth and Space
Technology Connections:
Use Video and Image Capture to Increase Your Understanding!
Make a video recording of the movement of the paperclip in response to the movement of the
magnet. Upload your video to a computer and capture a few specific image frames of the
paperclip. Save the images and using an image editor, label the images to demonstrate your
understanding of the Universal Law of Gravitation.
Instructions
1.  Place the paperclip on top of the single layer of cardboard.
2.  Set up the camera so that the movement of both the magnet held under the cardboard and the
movement of the paperclip on top of the cardboard may be observed. You may have to film
from two different positions.
3.  Start recording and direct your partner to move a magnet along the underside of the cardboard
so that the attracted paperclip also moves.
4.  Stop filming.
5.  Place a second piece of cardboard on top of the first piece. Repeat steps 1 to 4.
6.  Continue adding pieces of cardboard and repeating steps 1 to 4 until the paperclip is no longer
attracted to the magnet and does not move.
7.  Upload your video to a computer and open it with video editing software.
8.  Capture and save the video clips showing each test of the attraction of the paperclip after
adding each layer of cardboard.
9.  Put a title at the top of each video clip.
10. Write a description underneath each clip to relate your investigation to the Universal Law of
Gravitation.
Present your findings to your teacher/class.
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