Teacher Information 

Teachers: This material explains Newton’s
Law of Gravitation in a way that will help you teach the law to your students.
The photocopyready Student Activity pages will give students the opportunity
to learn aspects of the Law of Gravitation in a way that they will find
interesting and fun. Notes about each activity appear in the Notes to
Teachers section. Each activity can be tailored for the level of your
students, and can be completed individually or in groups. In addition,
students will create a logbook, called Newton’s Lawbook, in which
they can take notes and track their findings from the scientific experiments
offered in the Student Activity pages. 

Newton’s Law of Gravitation 

The Second Law of Motion and The Law of GravitationSir Isaac Newton (16421727) established the scientific laws that govern 99% or more of our everyday experiences. He also explained our relationship to the Universe through his Laws of Motion and his universal theory of gravitation  which are considered by many to be the most important laws of all physical science. Newton was the first to see that such apparently diverse phenomena as a satellite moving near the Earth’s surface and the planets orbiting the Sun operate by the same principle: Force equals mass multiplied by acceleration, or F=ma. Our everyday lives are influenced by different forces. As you know, the Earth exerts a force on us that we call gravity. We feel the force required to lift an object from the floor to a table. We can see and feel a magnet’s pull on a pile of metal paper clips. But exactly how does Newton’s Second Law of Motion relate to gravity? To understand Newton’s theories, you must first know about the nature of force and acceleration when applied to circular motion, rather than motion in a straight line. Newton’s First Law of Motion tells us that, without the interaction of some sort of force, everything travels in a straight line forever. This means that an object traveling in a circular path must be influenced by a net (outside) force. The circulating object has a velocity that is constantly changing, not because its speed is changing but because its direction is changing. A change in velocity is called an acceleration. Newton’s Second Law explains it this way: A net force changes the velocity of an object by changing either its speed or its direction. Therefore, an object moving in a circle is undergoing an acceleration. The direction of the acceleration is toward the center of the circle. The magnitude of the acceleration is equal to, where “v” is the constant speed along the circular path and “r” is the radius of the circular path. This acceleration is called centripetal (literally, “centerseeking”) acceleration. The force needed to produce the centripetal acceleration is called the centripetal force , which is equal to “ma” according to Newton’s Second Law (F=ma). But since “a” is , the centripetal force is equal to . Majestic examples of circular motion can be found throughout our Universe: Planets orbit around the Sun in nearly circular paths; moons orbit around their planets in nearly circular paths; and satellites orbit the Earth in nearly circular paths. Newton was smart enough to realize that his Second Law (F=ma) must apply to the force of gravity. He theorized that the Sun must be providing a centripetal acceleration to the Earth (and all the other planets in our solar system) in order for it to maintain its roughly circular orbit. Newton figured out that the force of gravity is the source for the acceleration. He calculated that,
where “mp” is the mass of the planet and “R” is the distance from the planet to the Sun. Newton learned from his predecessor, Johannes Kepler (15711630), that the square of the orbital period for a planet is proportional to the cube of its orbital radius, or T^{2} ~ R^{3} (known as Kepler’s Third Law). Since “T” is the time taken to complete one orbit, by definition: So Putting these equations together, we see that . Thus, the force that the Sun exerts on a planet must take the form of: Newton’s Third Law of Motion says: For every force, there is an equal and opposite force. From this, Newton calculated that a planet must exert an equal (but oppositely directed) force on the Sun that the Sun exerts on the planet. Due to this symmetry, he concluded that the two forces must depend on the masses of both objects in the same way. This means they take the form of:
In his final equation, Newton added the Universal Gravitational Constant, or “G,” which accounts for all of the constants of proportionality. His final equation reads:
The gravitational constant cannot be derived or predicted by theory. It must be determined by experimental measurement. The value of “G” was first measured by Henry Cavendish in 1798. The currently accepted value of “G” is 6.672 x 1011 N^{2}/kg^{2}. Newton’s model of gravity is one of the most important scientific models in history. It applies to everything in the Universe, from apples falling from trees to baseballs soaring into the outfield; from the Earth orbiting the Sun to a moon orbiting a planet. It applies to the motion of all of the objects in our solar system, as well as to the distant stars and galaxies. Interesting Side NoteIf we put the above equations together, we learn something very interesting! We know that:
In this case, “a” is the centripetal acceleration of an object
in circular motion. From our discussion, we know that, Combining these equations, we see that:
In other words, the mass of an object does not influence the orbit of the object. At a given distance, a baseball or a moon will orbit in exactly the same way at exactly the same speed! Student Activities
Activity #1: Round and Round They Go!As you learn about the orbits of the planets around our Sun, it is important to learn more than just the order of the planets based on their distances from the Sun. Think about the amount of time it takes to complete one orbit. Why do the outer planets take so much longer to complete an orbit than the inner planets? Is it just because they have so much farther to go? Or is some other factor involved? This lesson investigates these questions. Perhaps you’ve seen a scale model of the solar system with pieces of string representing the orbital paths of each of the nine planets. Stretching the strings out from a common point will show you that the outer planets indeed have much farther to go to complete one revolution. However, this by itself does not explain the orbital periods. There is something more to it. The following activity will show you what else is involved. Materials You will need the following items for this experiment: • one sturdy plastic drinking straw (cut in half) or one piece
of 1/2” plastic tubing about 5” long Procedure In the following exercise you will build a model of the solar system. 1. Tie one end of the string to one of the washers. Next, run the string
through the straw and tie it
As the orbit of the washer gets shorter, what do you notice about the speed at which the spinning washer is traveling? Use your Newton’s Lawbook to make notes about your discovery.

Planet  Mean Distance From Sun (km)  Sidereal Orbital Period (days) 
Mercury  57,910,000  87.97 
Venus  108,200,000  224.70 
Earth  149,600,000  365.26 
Mars  227,940,000  686.98 
Jupiter  778,300,000  4,332.71 
Saturn  1,429,400,000  10,759.50 
Uranus  2,870,990,000  30,685.00 
Neptune  4,504,300,000  60,190.00 
Pluto  5,913,520,000  90,800.00 
When a right circular cone is intersected by a plane, a conic section is formed. Any plane that’s perpendicular to the axis of the cone cuts a section that is a circle. Incline the plane a bit and the section forms an ellipse. Tilt the plane still more until it is parallel to an outside edge of the cone and a parabola is formed. Continue tilting until the plane is parallel to the axis of the cone and the section is a hyperbola. So what do conic sections have to do with Newton’s Laws?
Conic sections play a fundamental role in space science. Any object under
the influence of an inverse square law force (such as gravity) must have
a
trajectory (curved path) that is one of the conic sections. For example,
when talking about orbits, a celestial body (such as a planet, comet,
or artificial satellite) moves according to its gravitational attraction
to a primary celestial body. The primary body’s center of mass is
one focus of the conic section along which the satellite moves. The conic
section you follow is determined by your energy (or velocity). Starting
with a lowvelocity, you follow an ellipse (oval). As your velocity (speed)
increases, you eventually follow a circle. As your velocity increases
some more, your path becomes an ellipse again. As your velocity continues
to increase, your path becomes a parabola, and finally, with an even higher
velocity, a hyperbola. The specific values of the velocities that transition
you between the conic sections are determined by the gravitational pull
you are experiencing and
by your kinetic energy 1/2mv^{2}
Note that all closed orbits are either circles or ellipses.
Part 1
You can model conic sections by simple paper folding. Try to predict which conic sections you will make from each of the following setups.
Materials
You will need the following items for this exercise:
• wax paper or tracing paper
• a pencil
Procedure
1. Cut out or trace a circle on wax paper or tracing paper. Make the circle at least 7 cm in radius. 2. Locate and mark the center of your paper circle. In the illustration below, the center of the circle lies at Point C. 3. Choose and mark a point within the interior of your circle (Point F in the example). The location of this point is not important. For the most dramatic results, however, pick a point that is not too close to the circle’s center. 
4. As shown above, fold the circle so that any point on the circle’s
circumference (edge) intersects
Point F. Make a sharp crease so that you have evidence of your fold. When
you open your fold, you
will have a straight line on the circle where you made the crease.
5. Make several more creases so that different points on the circle’s
circumference are folded through
Point F.
6. You should start seeing a curve appear on your paper circle.
A. The creases on your circle form the outline of what appears to be a/an______________. Where
is/are the focus/foci? __________________________________________________________
B. If you moved Point F closer to the edge of the circle and folded another curve, describe how you
think the curve’s shape would change. ____________________________________________
___________________________________________________________________________
C. If you moved Point F closer to the center of the circle and folded another curve, describe how you
think the curve’s shape would change. ____________________________________________
__________________________________________________________________________
D. How does this paper folding construction work? ___________________________________
___________________________________________________________________________
Part 2
In this exercise, you will create another conic section. The paperfolding activity will show you the mathematical characteristics found with the focus and directrix.
Materials
You will need the following items for this exercise:
• wax paper or tracing paper
• a pencil
Procedure
1. Cut out a rectangle from the wax paper or tracing paper. You need
at least a 7 cm by 7 cm area.
2. Mark a point near the bottom edge of the rectangle, roughly in the
center (Point F in the illustration).
3. Fold the paper so that a point on the bottom edge of
the paper lands on Point F. Make a sharp crease so
that you have evidence of your fold. When you open
your rectangle, you will have a straight line where
you made the crease.
4. Continue to make creases from the bottom edge of
the paper through Point F. After you’ve made several
creases, predict what kind of curve these creases are
forming.
A. Describe any symmetries you see in your curve.
_________________________________________
________________________________________________________________________
________________________________________________________________________
B. How do you think the appearance of your curve would have been different
if Point F had been
closer (or farther away from) the bottom edge? ____________________________________
________________________________________________________________________
C. How does this paperfolding construction work? Use mathematical vocabulary
such as directrix and
focus in your explanation. _____________________________________________________
_________________________________________________________________________
Part 3
In this exercise, you will fold yet another piece of paper in order to
discover a third type of conic section.
Materials
You will need the following items for this exercise:
• a blank sheet of paper
• a pencil
• a compass
Procedure
1. Use a compass to draw a circle on your piece of
paper. Make the circle as dark as you can. The
location and size of your circle does not matter,
but try to keep the radius no larger than 3 cm.
Mark the center of your circle as Point C.
2. Mark a Point F outside the circle’s circumference
anywhere on the paper.
3. Use the sharp end of your compass to punch a
small hole through Point F. The hole should be
large enough so that you can see through it.
4. Fold the paper so that the hole at Point F
lands somewhere on the circle’s edge. You should
be able to see the circle’s edge through the hole.
Hold the paper up to the light to see it even
better. Make a sharp crease so that you have evidence of your fold. When
you open the paper, there
will be a straight line where you made the crease.
5. Fold more creases, making certain that Point F falls somewhere on the
circumference of the circle.
After you’ve made several creases, predict what kind of curve these
creases are forming.
A. What similarities and differences are there between this construction and the previous two?
____________________________________________________________________
____________________________________________________________________
B. The curve that you made looks like a(n) ____________________. Where is/are the focus/foci?
____________________________________________________________________
C. Describe how you think the shape of the curve would change if you varied the location of Point F.
_____________________________________________________________________
_____________________________________________________________________
Have students spin the washer at a quick but steady rate before starting to pull the string. Tell them to keep spinning as they pull the washer, until the point that the spinning becomes selfsustaining. Students will wonder why the washer orbits faster as its orbit radius gets smaller.
Shorter string lengths produce a more manageable assembly for younger or smaller students. They can even spin their “planet” in a vertical circle if they can’t maintain the horizontal circle. While the science of this experiment is not as technically correct with a vertical circle, this may be the only way younger students can enjoy the experience independently. Make each student describe what they feel in their hand that is holding the straw and also what they see as the orbital circle gets smaller. This will help them to focus on their orbiting washer.
Remind students to pull the string slowly enough for the washer to make complete orbits at each radius before pulling more on the string. This is hard to do at larger orbits, but easy to do as the orbits get smaller.
Be forewarned that students will inevitably hit themselves with their orbiting washers. This is why rubber washers are suggested for this exercise, something soft and not hard and heavy. Also, washers have enough mass to be effective orbiters.
Discussion Point: As you know, planets farthest from the Sun have a longer orbital path AND move more slowly around that path than the planets closer to the Sun. It is important to understand that there are two reasons the outer planets have longer orbital periods than the inner planets.
The elliptical shape of planetary orbits was first asserted by German astronomer Johannes Kepler (15711630). His declaration was based on the painstaking observations he made in conjunction with those of his predecessor, Danish astronomer Tycho Brahe. However, it took Isaac Newton’s great genius years later (between 166567) to establish mathematically that the inverse square law of gravitation must produce a trajectory that is one of the conic sections. (The proof required the use of calculus and will not be shown here.) Newton concluded that the nature of a trajectory depends on the total energy, E, such that:
If E=0, the trajectory is a parabola If E<0, the trajectory is an ellipse If E>0, the trajectory is a hyperbola 
Recall that E, the total energy, is defined as:
Note that 1/2mv^{2} is the kinetic energy of the system and is the gravitational potential energy of the system.
At a meeting of the Royal Society in 1684, some of the great thinkers of the time took up the question of the relationship between the inverse square attraction of the Sun for a planet and the nature of the planet’s elliptical orbit. The connection between the two had to be mathematical, which was a profoundly different way of thinking for the time. Until then, most understanding of the physical world came from human experience: We could feel it, hear it, or see it. To complicate things, the force of gravity acting on a planet is not something a human can directly experience. It is even hard for the brain to comprehend: It acts at a distance. It acts immediately. And it acts everywhere!
Scientists searched for mathematical proof of why the properties of a force led to the orbit of a planet. Looking for answers, in 1684 Edmund Halley went to visit Isaac Newton in Cambridge, England, and posed the problem to him. A famous account of this meeting has been provided by mathematician Abraham De Moivre. In it, De Moivre wrote that after Halley posed the problem, Newton immediately responded that the planet must move in an ellipse if it moves under the inverse square force of gravitational attraction to the Sun. Halley asked how he knew this to be so, and Newton purportedly replied, “I have calculated it.”
Halley asked Newton to write down his proof. Later that year Newton delivered his treatise, “On the Motion of Bodies in an Orbit.” In the space of nine pages, Newton calculated that if the planets are moving in elliptical orbits, they must be under the control of an inverse square force directed toward one focus of the ellipses in which they move. He went on to show that under the influence of an inverse square force, the orbits of all objects (whether moving slowly or swiftly) must describe a conic section.
The relationship between physical science and mathematics was changed forever. There was now no doubt that mathematical methods can provide a comprehensive description of the world around us.
For the sake of understanding the answers, it should be assumed that a conic section is made from a locus of points whose distance from a fixed point to a fixed line is in a constant ratio. The fixed point is the focus of the conic and the fixed line is the directrix.
A. The creases on your circle form the outline of what appears to be
a/an ellipse.
Where is/are the focus/foci? The center (point C) and point F.
B. If you moved Point F closer to the edge of the circle and folded another
curve, describe how you
think the curve’s shape would change. The ellipse would be “stretched”
and become less circular.
C. If you moved Point F closer to the center of the circle and folded
another curve, describe how you
think the curve’s shape would change. The ellipse would become more
circular, until both “F” and
“C” coincide, where the ellipse would have one focus and actually
be a circle.
D. How does this paper folding construction work? Explain. Each crease
that is created is a line from
which the distance to the focus AND the directrix are equal. In this case,
when an ellipse is made,
the focus is point F and the directrix is the circumference of the original
circle.
A. Describe any symmetries you see in your curve. There is a line of symmetry through the vertex of the parabola that extends vertically with the left and right branch of the parabola on either side of it.
B. How do you think the appearance of your curve would have been different if Point F had been closer (or farther away from) the bottom edge? The farther point F is from point C, the more elongated or the 
C. How does this paperfolding construction work? Use mathematical vocabulary such as directrix and focus in your explanation. Like the ellipse made in Part 1, each crease that is created is a line from which the distance to the focus AND the directrix are equal. In this case, when a parabola is made, the focus is point F and the directrix is the bottom edge of the rectangle. 
A. What similarities and differences are there between this construction
and the previous two?
Answers may vary. For example, in all of the constructions, point F is
a focus or foci and all
curves are created by folding the directrix onto the foci or focus. However,
if the shape of the
directrix changes, the shape of the conic changes as well.
B. The curve that you made looks like a(n) hyperbola. Where is/are the focus/foci? The hole at point F.
C. Describe how you think the shape of the curve would change if you
varied the location of Point F.
As point F moves farther and farther away from point C, the two branches
of the hyperbola
become wider and the “bend” in the hyperbola becomes less
sharp.
Resources
Copies of these materials, along with additional information on Newton’s
Laws of Motion and Law of Gravitation, are available on the Swift Mission
Education and Public Outreach Web site:
• NASA Web sites:
NASA’s official Web site  http://www.nasa.gov
Swift Satellite  http://swift.gsfc.nasa.gov
• NASA Education Resources:
Swift’s Education and Public Outreach Program  http://swift.sonoma.edu
SpaceLink, Education Resources  http://spacelink.nasa.gov
Imagine the Universe!  http://imagine.gsfc.nasa.gov
StarChild  http://starchild.gsfc.nasa.gov
• NASA’s Central Operation of Resources for Educators (CORE):
http://core.nasa.gov/index.html
Check out these videos:
“Liftoff to Learning: Newton in Space” (1992), $15.00
“Flight Testing Newton’s Laws” (1999), $24.00
• Newton’s Laws of Motion:
http://www.grc.nasa.gov/WWW/K12/airplane/newton.html
• Newton’s Law of Gravitation:
http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html
• Conic Sections:
http://www.keypress.com/sketchpad/java_gsp/conics.html
http://cs.jsu.edu/mcis/faculty/leathrum/Mathlets/conics.html
• Newton in the Classroom:
http://www.physicsclassroom.com/Class/newtlaws/newtltoc.html
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1a.html
Acknowledgments
Creators:
Kara Granger, Maria Carrillo High School, California
Laura Whitlock, NASA’s Swift Mission, California
Science and Education Reviewers:
Thomas C. Arnold, State College Area High School, Pennsylvania
Margaret Chester, The Pennsylvania State University, Pennsylvania
Alan Gould, Lawrence Hall of Science, California
Bruce H. Hemp, Ft. Defiance High School, Virginia
Derek Hullinger, University of Maryland, Maryland
James Lochner, NASA Goddard Space Flight Center, Maryland
Jane D. Mahon, Hoover High School, Alabama
Ann Parsons, NASA Goddard Space Flight Center, Maryland
Original Artwork and Design: Aurore Simonnet, Sonoma State University,
California
Painting of Sir Isaac Newton by Enoch Seeman, 1726
Editor: Stacy Horn, San Francisco, California