Teacher Information
Teachers: This material examines Newton’s Third Law of Motion in a way that will help you teach the law to your students. The photocopy-ready Student Activities pages will give students the opportunity to learn aspects of the Third Law in a way that they will find interesting and fun. Notes about each activity appear in the Notes to Teachers section. The activities 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 Activities pages.
Newton’s First Law of Motion explains the Law of Inertia, the connection between force and motion. Newton’s Second Law of Motion describes quantitatively how forces affect motion. And Newton’s Third Law of Motion addresses the nature of force.
Our daily experiences might lead us to think that forces are always applied by one object on another. For example, a horse pulls a buggy, a person pushes a grocery cart, or a magnet attracts a nail. In each of these examples a force is exerted on one body by another body. It took Sir Isaac Newton to realize that things are not so simple, not so one-sided. True, if a hammer strikes a nail, the hammer exerts a force on the nail (thereby driving it into a board). Yet, the nail must also exert a force on the hammer since the hammer’s state of motion is changed and, according to the First Law, this requires a net (outside) force. This is the essence of Newton’s Third Law: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. This law is often stated: For every action there is an equal and opposite reaction. However, it is important to understand that the action force and the reaction force are acting on different 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 being exerted on it. You can see the edge of the desk pressing into your hand; you can feel the desk exerting a force on your hand. Now press harder. The harder you press the harder the desk pushes back on your hand. Remember this important point: You can only feel the forces being 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 your hand.
The Swift Satellite
Swift is a space-based multiwavelength observatory dedicated to the study of gamma-ray bursts. Its purpose is to determine the origin and nature of these powerful cosmic explosions; determine how the blastwaves from the bursts evolve and interact with their surroundings; and determine if these bursts 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 gamma-ray bursts, which are brief cosmic bursts of electromagnetic radiation. When Swift detects a gamma-ray burst, it must be able to turn and point to it very quickly in order to gather information about the burst before it is over. This means Swift must be able to rotate to point at a burst, then stop rotating. This is called slewing. In order to start and stop slewing, the satellite must “push” against something. In this case, it will push against a set of small wheels (called reaction wheels or flywheels) inside the satellite.
To begin slewing, the satellite “pushes” against one of the wheels. This push rotates the wheel in one direction, forcing the satellite to rotate in the other direction. Once Swift is pointing in the right direction, it pushes against the wheel again this time in the opposite direction. This brings the satellite to a stop. By pushing against three different wheels, all oriented in different directions, the satellite can turn and point in any direction.
Newton’s Third Law of Motion explains the physics behind this technical maneuver: For every force, 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 change its motion from one speed or direction to another.
Roll a ball at a wall with enough velocity to make it roll back or bounce off at an angle. Have students describe what they saw. Once you establish that the ball changed directions, ask why that happened. In an open discussion, establish that the wall had to exert a force on the ball.
Ask for other examples of a force changing the direction of an object’s motion. One example might be baseball. The force exerted on the ball by the bat causes the ball to change directions. As an added brainteaser, ask students to picture a game of croquet. The mallet hits Ball A, which, in turn, hits Ball B. Ask students to explain this chain of events.
Lastly, throw a ball straight up and catch it as it comes down. Ask students what force caused the ball’s direction to change. The answer, of course, is gravity. This may lead to a discussion that not all forces come from obvious sources like a wall or a bat, but may be “action at a distance,” like gravity or magnetism.
Gather a selection of balls which are roughly the same size, but very different masses. For example, you might have a small beachball and a basketball, or a whiffleball and a softball. Given what you know about Newton’s Second Law of Motion (F=ma), you would expect to apply a greater 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 larger force 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, not because they are exerting one on the ball! Alternately, in the case of a softball and whiffleball, Newton’s Second Law tells us that the softball (which has greater mass) will generate a greater force as it falls than will the whiffleball. Remember that in this case of F=ma, “a” is the acceleration 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 another student. Next, have them drop the whiffleball from the same height. Ask them what they felt. Ask which ball required their hand to exert the most force in order to stop the fall. The sensation in their hands 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 the ground. 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 the skateboard from rolling too far. Next, tell the student on the skateboard to throw the basketball to someone 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 off one 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 then lesser mass. How do these changes affect the rolling of the skateboard? What if the skateboard is on 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 the notebook, which you have designated as your Newton’s Lawbook, to take notes, track your progress, and evaluate findings from the experiments you will conduct. Start by writing down Newton’s Third Law of Motion.
Activity: A Day at the Races
In this experiment you will create a balloon rocket! You will figure out how to shoot the balloon from the back of your classroom and hit the blackboard with it at the front of the room. You will do this 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 the
other 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 another
teammate 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 be
called 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 the
balloon traveled. To do this, divide the distance traveled by the time the balloon was “in
flight.” 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’s
race entry time.
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 reaction
forces 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 on another 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.
Newton’s laws of motion apply to all objects familiar to us in our everyday world. This includes objects with a mass that changes, even though these situations are not common. One example of this changing-mass situation is a rocket, which loses fuel and other matter as it travels. Rockets are perfect for space travel because they carry their fuel and oxygen with them. In fact, most of the mass of a rocket before launch is in the form of fuel and oxidizer. In space, the burning fuel is ejected from the rear of the rocket. This action produces a reaction force on the rocket body and propels 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 the system in the opposite direction (the ejected fuel goes one way, the rocket goes the other). This propulsive force is referred to as the thrust of the rocket.
Notes to Teachers
Activity: A Day at the Races
The air inside the balloon rocket pushes on the rocket, sending it forward. But at the same time the rocket (balloon) is pushing back on the air inside it! This is what accounts for the air coming out the back.
Many students will have difficulty with this concept. The air outside the balloon pushes on the wall of the balloon, forcing out the air inside the balloon. Newton’s Third Law explains why the air coming out the back causes the balloon to move forward. A common misconception is that the forward movement is due to the molecules rushing out the rear of the balloon and pushing on the outside air molecules.
National Science and Mathematics Standards
for the Newton’s Laws Materials
PHYSICAL SCIENCE (Grades 5-8, 9-12)
• 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 6-12)
• Understand patterns, relations, and functions
• Represent and analyze mathematical situations
• Use mathematical models
GEOMETRY (Grades 6-12)
• Use geometric modeling to solve problems
MEASUREMENT (Grades 6-12)
• Understand and use measurable attributes of objects
• Apply appropriate techniques, tools, and formulas
DATA ANALYSIS (Grades 6-12)
• 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
Acknowledgments
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