Newton's Third Law

6/29/2015 Newton's Third Law

http://swift.sonoma.edu/education/newton/newton_3/html/newton3.html 1/10

Teacher InformationTeachers: This material examines Newton’s Third Law of Motion in a way that will help you teachthe law to your students. The photocopyready Student Activities pages will give students theopportunity to learn aspects of the Third Law in a way that they will find interesting and fun. Notesabout each activity appear in the Notes to Teachers section. The activities can be tailored for thelevel of your students, and can be completed individually or in groups. In addition, students willcreate a logbook, called Newton’s Lawbook, in which they can take notes and track their findingsfrom the scientific experiments offered in the Student Activities pages.

Newton's Third Law of MotionFor every force, there is an equal and opposite force.

Newton’s First Law of Motion explains the Law of Inertia, the connection between force andmotion. Newton’s Second Law of Motion describes quantitatively how forces affect motion. AndNewton’s Third Law of Motion addresses the nature of force.



Our daily experiences might lead us to think that forces are alwaysapplied by one object on another. For example, a horse pulls abuggy, a person pushes a grocery cart, or a magnet attracts a nail. Ineach of these examples a force is exerted on one body by anotherbody. It took Sir Isaac Newton to realize that things are not sosimple, not so onesided. True, if a hammer strikes a nail, thehammer exerts a force on the nail (thereby driving it into a board).Yet, the nail must also exert a force on the hammer since thehammer’s state of motion is changed and, according to the FirstLaw, this requires a net (outside) force. This is the essence ofNewton’s Third Law: Whenever one object exerts a force on asecond object, the second object exerts an equal and opposite forceon the first object. This law is often stated: For every action there isan equal and opposite reaction. However, it is important tounderstand that the action force and the reaction force are acting ondifferent objects.

Try this: Press the side of your hand against the edge of a table.Notice how your hand becomes distorted. Clearly, a force is beingexerted on it. You can see the edge of the desk pressing into yourhand; you can feel the desk exerting a force on your hand. Now pressharder. The harder you press the harder the desk pushes back on yourhand. Remember this important point: You can only feel the forcesbeing exerted on you, not the forces you exert on something else. So,it is the force the desk is exerting on you that you see and feel in yourhand.

It is often difficult to visualize how an inanimate object (such as a desk, floor, or wall) can exertforce. How do they do it? The fact is that all objects, to some degree, are elastic. It is easy tovisualize a stretched rubber band exerting a force on a wad of paper and causing it to fly across theroom. Other materials may not stretch as easily as a rubber band, however all objects stretch (orcompress) when a force is exerted on them, and in return they react.

The Swift Satellite

Swift is a spacebased multiwavelength observatory dedicated to the study of gammaray bursts. Itspurpose is to determine the origin and nature of these powerful cosmic explosions; determine howthe blastwaves from the bursts evolve and interact with their surroundings; and determine if thesebursts can be used as effective probes of the early Universe. Scheduled for launch in Fall 2003,Swift is a collaboration between the United States, the United Kingdom, and Italy.

Newton’s Third Law and the Swift Satellite

Swift was created to study gammaray bursts, which are brief cosmic bursts of electromagneticradiation. When Swift detects a gammaray burst, it must be able to turn and point to it veryquickly in order to gather information about the burst before it is over. This means Swift must beable to rotate to point at a burst, then stop rotating. This is called slewing. In order to start and stopslewing, the satellite must “push” against something. In this case, it will push against a set of smallwheels (called reaction wheels or flywheels) inside the satellite.



To begin slewing, the satellite “pushes” against one of the wheels. This push rotates the wheel inone direction, forcing the satellite to rotate in the other direction. Once Swift is pointing in the rightdirection, it pushes against the wheel again this time in the opposite direction. This brings thesatellite to a stop. By pushing against three different wheels, all oriented in different directions, thesatellite can turn and point in any direction.

Newton’s Third Law of Motion explains the physics behind this technical maneuver: For everyforce, there is an equal and opposite force.

Demos and Thought Problems

Teachers: Use the following demonstrations to introduce Newton’s Third Law to your class.

Remind students that a force is necessary to start something moving when it is at rest, or to changeits motion from one speed or direction to another.

might be baseball. The force exerted on the ball by the bat causes the ball tochange directions. As an added brainteaser, ask students to picture a game ofcroquet. The mallet hits Ball A, which, in turn, hits Ball B. Ask students toexplain this chain of events.

Lastly, throw a ball straight up and catch it as it comes down. Ask studentswhat force caused the ball’s direction to change. The answer, of course, isgravity. This may lead to a discussion that not all forces come from obvioussources like a wall or a bat, but may be “action at a distance,” like gravity ormagnetism.

Gather a selection of balls which are roughly the same size, but very different masses. Forexample, you might have a small beachball and a basketball, or a whiffleball and a softball. Givenwhat you know about Newton’s Second Law of Motion (F=ma), you would expect to apply agreater force to kick the basketball than you would need to kick the beachball the same distance.Furthermore, Newton’s Third Law of Motion tells you to expect the basketball to exert a largerforce on your foot than the beachball would exert.

Have your students kick the balls and find out. Tell them that if they“feel” a force, it is because one is being exerted on them by the ball, notbecause they are exerting one on the ball! Alternately, in the case of asoftball and whiffleball, Newton’s Second Law tells us that the softball(which has greater mass) will generate a greater force as it falls than willthe whiffleball. Remember that in this case of F=ma, “a” is theacceleration due to gravity and is a constant for both balls.

Have your students take turns dropping the softball at least one meter into the hand of anotherstudent. Next, have them drop the whiffleball from the same height. Ask them what they felt. Askwhich ball required their hand to exert the most force in order to stop the fall. The sensation in theirhands will give them the answer!



Have a student sit on a skateboard (facing one end of the board) with his or her legs up off theground. Throw a basketball to the student. Ask the student to catch the ball against his or her body,and not by stretching out their arms. Someone should stand behind the student to stop theskateboard from rolling too far. Next, tell the student on the skateboard to throw the basketball tosomeone standing directly in front of them. What happens?

To continue the activity, have a student stand on the skateboard. Ask him or her to jump or step offone end of it. What happens to the skateboard as a result of this action?

Try variations of this activity. Throw the basketball harder then softer. Throw a ball of greater thenlesser mass. How do these changes affect the rolling of the skateboard? What if the skateboard ison carpet or a tile or wooden floor? How does friction come into play?

Student Activities

Students: These activities will help you learn all about Newton’s Third Law of Motion. Use thenotebook, which you have designated as your Newton’s Lawbook, to take notes, track yourprogress, and evaluate findings from the experiments you will conduct. Start by writing downNewton’s Third Law of Motion.

Newton’s Third Law of Motion

For every force, there is an equal and opposite force.

Activity #1: A Day at the Races

In this experiment you will create a balloon rocket! You will figure out how to shoot the balloonfrom the back of your classroom and hit the blackboard with it at the front of the room. You will dothis using a fishing line as a track for the balloon to follow.

Materials

You will need the following items for this experiment:

• balloons (one for each team)• plastic straws (one for each team)• tape (cellophane or masking)• fishing line, 10 meters in length• a stopwatch• a measuring tape

Procedure

This is a race. The race will be timed and a winner determined.

1. Divide into groups of at least five students. 2. Attach one end of the fishing line to the blackboard with tape. Have one teammate hold theother end of the fishing line so that it is taut and roughly horizontal. The line must be held



steady and may not be moved up or down during the experiment. 3. Have one teammate blow up a balloon and hold it shut with his or her fingers. Have anotherteammate tape the straw along the side of the balloon. Thread the fishing line through the straw and hold the balloon at the far end of the line.4. Assign one teammate to time the event. The balloon should be let go when the time keeper yells “Go!” Observe how your rocket moves toward the blackboard.

5. Have another teammate stand right next to the blackboard and yell “Stop!” when the rocket hits its target. If the balloon does not make it all the way to the blackboard, “Stop!” should becalled when the balloon stops moving. The timekeeper should record the flight time. 6. Measure the exact distance the rocket traveled. Calculate the average speed at which theballoon traveled. To do this, divide the distance traveled by the time the balloon was “inflight.” Fill in your results for Trial 1 in the Table below. 7. Each team should conduct two more races and complete the sections in the Table for Trials 2 and 3. Then calculate the average speed for the three trials to determine your team’srace entry time.

Distance (cm) Time (sec) Speed (cm/sec)Trial 1 Trial 2 Trial 3 Average:

The winner of this race is the team with the fastest average balloon speed.

Think About It

1. What made your rocket move? 2. How is Newton’s Third Law of Motion demonstrated by this activity? 3. In your Newton’s Lawbook, draw pictures using labeled arrows to show the action and reactionforces acting on the inside of the balloon before it was released and after it was released.

Things to Discuss

Remember, Newton’s Third Law of Motion says that whenever one object exerts a force onanother object, the second object exerts an equal and opposite force on the first object. However,note that the two forces do not act on the same object.

Sir Isaac Newton recognized that all objects are covered by his Laws of Motion. This includesobjects with a mass that changes, even though these situations are less common. One example ofthis changingmass situation is a rocket, which loses fuel and other matter as it travels. Rockets areperfect for space travel because they carry their fuel and oxygen with them. In fact, most of themass of an unlaunched rocket is in the form of fuel and oxidizer. In space, the burning fuel isejected from the rear of the rocket. This action produces a reaction force on the rocket body andpropels it forward. Note that there is no need for air to push against the rocket for it to work.Newton’s Third Law of Motion assures us that ejection of an object from a system must propel thesystem in the opposite direction (the ejected fuel goes one way, the rocket goes the other). Thispropulsive force is referred to as the thrust of the rocket.



Activity #2: Reacting to Action

Newton’s Third Law of Motion makes sense when you think about it in connection with his firsttwo laws. The First Law says an object remains at rest or moves in a straight line at a constantspeed unless acted on by an outside force. The Second Law says an object acted on by an outsideforce experiences an acceleration (in other words, a change in its speed or direction of motion). TheThird Law says for every force there must be some sort of reaction force.

Think about what happens when you hit a nail with a hammer. It is clear that the hammer exerts aforce on the nail. But since the hammer motion was stopped (it did not drive the nail through thewall and keep on going indefinitely at a constant speed), the nail must have exerted a force back onthe hammer in order to stop its motion. Let’s think about some of the forces we experience everyday and how they might be explained in terms of Newton’s Third Law of Motion.

Materials

You will need the following items for these experiments:

• Experiment 1: one rubber ball• Experiment 2: two doughnut magnets; a small plastic toy car or truck; 10 cm of thread; tape orglue• Experiment 3: a wooden block; a bucket of water

Procedures

Perform the following activities. In your Newton’s Lawbook, describe what you did and what youobserved. Then write your answers to the questions that follow.1. Drop a rubber ball from a height of one meter. Catch it when it bounces back up to itsmaximum height. Name the force which caused the ball to start moving. What must havehappened in order for the ball to bounce back to your hand? After the ball bounces andstarts to move upward, what happens to its motion? Why? Discuss your observations in termsof Newton’s Third Law of Motion. 2. With tape or glue, attach a doughnut magnet to the back of a small plastic toy car or truck.Slowly bring the other magnet close to the back of the vehicle until the vehicle starts to rollforward (the magnets should be of similar polarity, meaning that they repel each other).Quickly pull away the magnet in your hand and let the car roll to a stop. What force made thecar move? What force made it stop? Describe all of the action/reaction pairs in this case and draw a diagram to illustrate them. Next, suspend a magnetfrom a 10 cm length of thread. Hold the end of the thread and bring thehanging magnet toward the back of the vehicle (again, the magnets should be of similar polarity). Describe what happens this time and why. How does thisrelate to Newton’s Third Law of Motion.3. Place a wooden block in a bucket of water so that it floats. Push the blockdown into the water and release it. What happens? Why? What do you noticeif you push the block down to greater and greater depths in the bucket? Howcan you explain this in terms of forces?



Notes to Teachers

Activity #1: A Day at the Races

The air inside the balloon rocket pushes on the rocket, sending it forward. But at the same time therocket (balloon) is pushing back on the air inside it! This is what accounts for the air coming outthe back.

Many students will have difficulty with this concept. The air outside the balloon pushes on the wallof the balloon, forcing out the air inside the balloon. Newton’s Third Law explains why the aircoming out the back causes the balloon to move forward. A common misconception is that theforward movement is due to the molecules rushing out the rear of the balloon and pushing on theoutside air molecules.

Activity #2: Reacting to Action1. When the ball is dropped, the force of gravity causes it to move and drop toward the ground. The floor exerts a force on the ball to make it change its direction of motion and move back up. Asit travels upward, the ball slows down. This is because the force of gravity is now acting on it inthe direction opposite to its motion. If we think about this in terms of Newton’s Third Law, wewould say that the floor exerts an equal and opposite force on the falling ball, so it stops the fall ofthe ball and then provides and upward acceleration of magnitude “g” to the ball. Without a loss ofenergy when it bounces (for example when using a superball), the ball would bounce back up tothe same height from which it was originally dropped.

2. Magnetic force is what starts the toy vehicle moving. Make sure your students understand that,just like with gravity, magnetism is a force that “acts at a distance.” In other words, direct contactbetween the objects is not necessary. Point out to your students that it is their hand that pushes themagnet forward and it is the magnetic force between the magnets that causes the toy vehicle tomove. Once the magnetic force is removed, the car would (according to Newton’s First Law)continue to move indefinitely in a straight line at a constant speed. But it doesn’t, it slows downand stops. It experiences a negative acceleration, or deceleration. According to Newton’s SecondLaw, for deceleration to take place a force must be applied. In this case, it is the force of frictionbetween the vehicle’s wheels and the table top or floor and between the vehicle’s body and the airwhich caused the vehicle to change its motion.



In the second part of the experiment, with the magnet hanging from a thread, students will discovera different result. (Make sure your students orient the magnets so that they repel each other.) Asyou slowly bring the suspended magnet closer to the magnet on the vehicle, the magnet on thethread will be pushed backward with a force equal to the force it is exerting on the vehicle. Thevehicle may move forward a little, or may not move at all, depending on the weight of the vehicleand the type of surface it is trying to roll on. But the magnet on the thread will definitely be pushedbackward, away from the vehicle. Students may have felt this force in their hand on the first trial,although they may not have realized its significance.

3. It is the buoyant force which causes the block in the bucket to rise to the top of the water andfloat. The pressure of the liquid increases with depth. Therefore, the upward force of the water onthe bottom of an object is greater than the downward force of the water on the top of the object.Objects only sink if their weight is larger than the net upward force of the water (recall that weightis the force equal to the object’s mass times the acceleration due to gravity).

Note that it is harder to push the block deeper into the bucket because the force of the waterincreases with depth. How can you tell? You can feel the block pushing harder on your hand as youtry to push it deeper into the water. Have your students try to push the block to various depthsusing only one or two fingers. The results become very clear!

National Science and Mathematics Standards for the Newton’s Laws Materials

PHYSICAL SCIENCE (Grades 58, 912)• Motions and Forces

UNIFYING SCIENCE CONCEPTS AND PROCESSES • Systems, order, and organization • Evidence, models, and explanation • Change, constancy, and measurement

SCIENCE AS INQUIRY • Understanding of scientific concepts• Understanding of the nature of science• Skills necessary to become independent inquirers about the natural world

ALGEBRA (Grades 612)• Understand patterns, relations, and functions • Represent and analyze mathematical situations• Use mathematical models

GEOMETRY (Grades 612)• Use geometric modeling to solve problems

MEASUREMENT (Grades 612)• Understand and use measurable attributes of objects • Apply appropriate techniques, tools, and formulas

DATA ANALYSIS (Grades 612)• Select, create, and use appropriate graphical representations of data • Develop and evaluate inferences and predictions that are based on data



MATHEMATICS PROCESS STANDARDS• Reasoning• Problem Solving• Representing Mathematical Relationships• Connections to Science and the Outside World• Communication of Mathematics and Science

ResourcesCopies of these materials, along with additional information on Newton’s Laws of Motion and Lawof Gravitation, are available on the Swift Mission Education and Public Outreach Web site:

http://swift.sonoma.edu/education/

• NASA Web sites:NASA’s official Web site http://www.nasa.govSwift Satellite http://swift.gsfc.nasa.gov

• NASA Education Resources:Swift’s Education and Public Outreach Program http://swift.sonoma.eduSpaceLink, Education Resources http://spacelink.nasa.govImagine the Universe! http://imagine.gsfc.nasa.govStarChild http://starchild.gsfc.nasa.gov

• NASA’s Central Operation of Resources for Educators (CORE): http://core.nasa.gov/index.htmlCheck 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.htmlhttp://cs.jsu.edu/mcis/faculty/leathrum/Mathlets/conics.html

• Newton in the Classroom:http://www.physicsclassroom.com/Class/newtlaws/newtltoc.htmlhttp://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1a.html

Acknowledgments

Creators: Kara Granger, Maria Carrillo High School, CaliforniaLaura Whitlock, NASA’s Swift Mission, California

http://starchild.gsfc.nasa.gov/docs/StarChild/StarChild.html

http://www.grc.nasa.gov/WWW/K-12/airplane/newton.html

http://swift.sonoma.edu/

http://swift.gsfc.nasa.gov/

http://core.nasa.gov/index.html

http://www.physicsclassroom.com/Class/newtlaws/newtltoc.html

http://www.keypress.com/sketchpad/java_gsp/conics.html

http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1a.html

http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html

http://cs.jsu.edu/mcis/faculty/leathrum/Mathlets/conics.html

http://www.nasa.gov/

http://swift.sonoma.edu/education/

http://imagine.gsfc.nasa.gov/

http://spacelink.nasa.gov/



Science and Education Reviewers:Thomas C. Arnold, State College Area High School, PennsylvaniaMargaret Chester, The Pennsylvania State University, PennsylvaniaAlan Gould, Lawrence Hall of Science, CaliforniaBruce H. Hemp, Ft. Defiance High School, VirginiaDerek Hullinger, University of Maryland, MarylandJames Lochner, NASA Goddard Space Flight Center, MarylandJane D. Mahon, Hoover High School, AlabamaAnn Parsons, NASA Goddard Space Flight Center, Maryland

Original Artwork and Design: Aurore Simonnet, Sonoma State University, CaliforniaPainting of Sir Isaac Newton by Enoch Seeman, 1726 Editor: Stacy Horn, San Francisco, California

http://swift.sonoma.edu

http://swift.sonoma.edu/

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Newton's Third Law