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 Third Law of Motion
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.
It is often difficult to visualize how an inanimate object (such as a desk, floor, or wall) can exert force. How do they do it? The fact is that all objects, to some degree, are elastic. It is easy to visualize a stretched rubber band exerting a force on a wad of paper and causing it to fly across the room. Other materials may not stretch as easily as a rubber band, however all objects stretch (or compress) when a force is exerted on them, and in return they react.
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.
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 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?
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 #1: 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.
You will need the following items for this experiment:
This is a race. The race will be timed and a winner determined.
1. Divide into groups of at least five students.
5. Have another teammate stand right next to the blackboard and yell
“Stop!” when the rocket
The winner of this race is the team with the fastest average balloon speed.
Think About It
1. What made your rocket move?
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.
Sir Isaac Newton recognized that all objects are covered by his Laws of Motion. This includes objects with a mass that changes, even though these situations are less 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 an unlaunched rocket 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.
Activity #2: Reacting to Action
Newton’s Third Law of Motion makes sense when you think about it in connection with his first two laws. The First Law says an object remains at rest or moves in a straight line at a constant speed unless acted on by an outside force. The Second Law says an object acted on by an outside force experiences an acceleration (in other words, a change in its speed or direction of motion). The Third 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 a force on the nail. But since the hammer motion was stopped (it did not drive the nail through the wall and keep on going indefinitely at a constant speed), the nail must have exerted a force back on the hammer in order to stop its motion. Let’s think about some of the forces we experience every day and how they might be explained in terms of Newton’s Third Law of Motion.
You will need the following items for these experiments:
• Experiment 1: one rubber ball
Perform the following activities. In your Newton’s Lawbook, describe
what you did and what you observed. Then write your answers to the questions
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 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.
Activity #2: Reacting to Action
1. When the ball is dropped, the force of gravity causes it to move and
drop toward the ground.
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 contact between the objects is not necessary. Point out to your students that it is their hand that pushes the magnet forward and it is the magnetic force between the magnets that causes the toy vehicle to move. 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 down and stops. It experiences a negative acceleration, or deceleration. According to Newton’s Second Law, for deceleration to take place a force must be applied. In this case, it is the force of friction between the vehicle’s wheels and the table top or floor and between the vehicle’s body and the air which caused the vehicle to change its motion.
In the second part of the experiment, with the magnet hanging from a thread, students will discover a different result. (Make sure your students orient the magnets so that they repel each other.) As you slowly bring the suspended magnet closer to the magnet on the vehicle, the magnet on the thread will be pushed backward with a force equal to the force it is exerting on the vehicle. The vehicle may move forward a little, or may not move at all, depending on the weight of the vehicle and the type of surface it is trying to roll on. But the magnet on the thread will definitely be pushed backward, 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 and float. The pressure of the liquid increases with depth. Therefore, the upward force of the water on the 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 weight is 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 water increases with depth. How can you tell? You can feel the block pushing harder on your hand as you try to push it deeper into the water. Have your students try to push the block to various depths using only one or two fingers. The results become very clear!
UNIFYING SCIENCE CONCEPTS AND PROCESSES
SCIENCE AS INQUIRY
ALGEBRA (Grades 6-12)
GEOMETRY (Grades 6-12)
MEASUREMENT (Grades 6-12)
DATA ANALYSIS (Grades 6-12)
MATHEMATICS PROCESS STANDARDS
Science and Education Reviewers:
Original Artwork and Design: Aurore Simonnet, Sonoma State University,