Newton's Laws of Motion

There was this fellow in England named Sir Isaac Newton. A little bit stuffy, bad hair, but quite an intelligent guy. He worked on developing calculus and physics at the same time. During his work, he came up with the three basic ideas that are applied to the physics of most motion (NOT modern physics). The ideas have been tested and verified so many times over the years, that scientists now call them Newton's Three Laws of Motion.

First Law

The first law says that an object at rest tends to stay at rest, and an object in motion tends to stay in motion, with the same direction and speed. Motion (or lack of motion) cannot change without an unbalanced force acting. If nothing is happening to you, and nothing does happen, you will never go anywhere. If you're going in a specific direction, unless something happens to you, you will always go in that direction. Forever.

You can see good examples of this idea when you see video footage of astronauts. Have you ever noticed that their tools float? They can just place them in space and they stay in one place. There is no interfering force to cause this situation to change. The same is true when they throw objects for the camera. Those objects move in a straight line. If they threw something when doing a spacewalk, that object would continue moving in the same direction and with the same speed unless interfered with; for example, if a planet's gravity pulled on it (Note: This is a really really simple way of descibing a big idea. You will learn all the real details - and math - when you start taking more advanced classes in physics.).

Second Law

The second law says that the acceleration of an object produced by a net (total) applied force is directly related to the magnitude of the force, the same direction as the force, and inversely related to the mass of the object (inverse is a value that is one over another number... the inverse of 2 is 1/2). The second law shows that if you exert the same force on two objects of different mass, you will get different accelerations (changes in motion). The effect (acceleration) on the smaller mass will be greater (more noticeable). The effect of a 10 newton force on a baseball would be much greater than that same force acting on a truck. The difference in effect (acceleration) is entirely due to the difference in their masses.

Third Law

The third law says that for every action (force) there is an equal and opposite reaction (force). Forces are found in pairs. Think about the time you sit in a chair. Your body exerts a force downward and that chair needs to exert an equal force upward or the chair will collapse. It's an issue of symmetry. Acting forces encounter other forces in the opposite direction. There's also the example of shooting a cannonball. When the cannonball is fired through the air (by the explosion), the cannon is pushed backward. The force pushing the ball out was equal to the force pushing the cannon back, but the effect on the cannon is less noticeable because it has a much larger mass. That example is similar to the kick when a gun fires a bullet forward.

Motion Commotion

When you kick a soccer ball up in the air, why does it come back down? Should it not just keep going?

Keywords: mechanics, Newton, acceleration, force, inertia, kinetic energy, mass, potential energy, laws of motion, motion

Summary: Students learn why and how motion occurs and what governs changes in motion, as described by Newton's three laws of motion. They gain hands-on experience with the concepts of forces, changes in motion, and action and reaction. In an associated literacy activity, students design a behavioral survey and learn basic protocol for primary research, survey design and report writing.

Engineering Connection:

Whether they design moving objects (scooters, boats, compact disk players, blenders) or stationary objects (dams, bridges, stoves, sunglasses, picture hangers), understanding Newton's laws of motion helps engineers of all disciplines quantify the "invisible" forces acting on the objects.

Learning Objectives

After this lesson, students should be able to:

• Understand the concept of forces.
• Explain why an airplane is able to fly in the sky.
• Identify Newton's three laws and explain what each law physically describes with respect to motion.
• Understand the vocabulary presented in this lesson and be able to explain how these terms apply to Newton's three laws.
• Predict results from the various motions presented in the activities and be able to explain why these motions occurred.
• Give examples of why Newton's laws are important to engineering.
• Understand how the variables in the F = ma equation are related to one another.

Introduction/Motivation

When Isaac Newton was 23 years old, he identified three traits of moving objects. His identification of these traits, now accepted as Newton's laws of motion, revolutionized science and transformed human understanding of the natural world. Newton's laws are universal, describing the motion of everything, everywhere!

Scientific application of Newton's laws led to advancements in every aspect of engineering, from building machines and structures to the functioning of airplanes and rockets. Sir Isaac Newton is the founder of the modern study of movement and balance because of his development of the three laws of motion.

Newton's laws hold true everywhere and at all times. Understanding the laws of motion helps us to understand what causes every movement we make throughout the day. The laws apply to ALL movement, from you, to a running stream, to a falling leaf, to a bird's flight. Having three laws that describe the why and how of all motion is an incredibly useful tool!

Lesson Background & Concepts for Teachers

Simply stated, Newton's three laws of motion are:

Law #1: Objects at rest will stay at rest, and objects in motion will stay in motion in a straight line unless they are acted upon by an unbalanced force. (law of inertia)

Law #2: Force is equal to mass multiplied by acceleration. (F = ma)

Law #3: For every action, there is always an opposite and equal reaction.

Newton's first law is also known as the law of inertia. It says that if you were to kick a ball and there were no forces acting on the ball, it would keep going in a straight line forever! This law is somewhat abstract because on Earth, invisible forces are always at work. Gravity, friction and air pressure are examples of "invisible" forces that act on objects everywhere. Therefore, objects on Earth are constantly changing direction, speeding up and slowing down — a ball does not keep going forever because there are forces acting to slow it down. Scientists and engineers must always keep in mind these "invisible" forces acting on the object's motion.

Newton's second law means that if you kick two balls that weigh the same, the ball you kick harder will go farther (that is, for a constant mass, exerting a greater force yields a greater acceleration). The second law also says that if you have a heavy ball and a light ball, you have to kick the heavy ball harder to make it go as fast as the lighter ball (that is, for a constant acceleration, a greater mass requires a greater force). The mathematical way to state this law is:

F = m x a

(Force = mass times acceleration)

If you hit a golf ball and a baseball with the same amount of force, which one would go farther? The golf ball! Why? Because the golf ball has less mass than the baseball, therefore less force is needed for the golf ball to achieve the same distance as the baseball.

Newton's third law is possibly the most widely known — for every action, there is an equal but opposite reaction. There is always a partner of forces at play: an action force and a reaction force. Even though this is possibly the most famous of his three laws, it is not necessarily the most intuitive. For example, when you walk on the ground (action force), the ground pushes up on you with an equal reaction force. You cannot see the force, and we are so accustomed to walking on the ground, we do not even realize there must be a reaction force that keeps us from sinking into the ground. Imagine sinking into the ground with every step we take! That's exactly what would happen if this third law were not true.

Vocabulary/Definitions

 Force: Something that acts from the outside to push or pull an object. For example, an adult pulling a child in a wagon exerts a force upon the wagon. Mass: The amount of material (matter) present in an object. Acceleration: Rate of change in velocity with respect to magnitude, direction or both.

Lesson Closure

Ask the students to explain Newton's three laws of motion. Have them give some examples of what life on Earth would be like if these laws were not true. Ask the students why Newton's laws are so important to engineers. Have them write on the board at least three reasons why Newton's laws are important to engineers, or what has become possible with the understanding of these laws. (Possible answers: Has made it possible to build airplanes that fly, elevators that move, amusement park rides and roller coasters, cars that drive safely, seatbelts in cars, bridges and buildings that do not collapse; basically, Newton's laws are the foundation for all structures that move or are stationary.)

Lesson Extension Activities

Inertia Zoom Ball

In this hands-on demonstration of Newton's first law of motion, students use plastic bottles and string to see how force causes an object to change in motion.

Inertia Zoom Ball

More Power to You

Newton's third law of motion is illustrated in this hands-on activity in which students fuel a plastic bottle boat to move on water.

More Power to You

Library research project

Have the students research Sir Isaac Newton, write a book report and present their findings to the class.

References

Gittewitt, Paul. Conceptual Physics. Menlo Park, CA: Addison-Wesley, 1992.

Hauser, Jill Frankel. Gizmos and Gadgets: Creating Science Contraptions that Work (and Knowing Why). Charlotte, VT: Williamson Publishing, 1999.

Kagan, Spencer. Cooperative Learning. San Juan Capistrano, CA: Kagan Cooperative Learning, 1994. (Source for the Flashcards assessment.)

Newton's laws of motion: http://id.mind.net/~zona/mstm/physics/mechanics/forces/newton/newton.html

Newton's laws of motion: http://www.usoe.k12.ut.us/curr/science/sciber00/8th/forces/sciber/newtons.htm

Owner: Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder

Contributors: Sabre Duren, Ben Heavner, Malinda Schaefer Zarske, Denise Carlson