Laws of Motion

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Laws of motion

What makes a bird fly? A person run? A judo expert flip a heavier opponent? Earth orbit the Sun ? These and any other motions are governed by three deceptively simple laws first stated by Isaac Newton in the seventeenth century. These three laws of motion when coupled with Newton's law of gravity form the basis for explaining both the motions we see on theearth and the motions of the heavenly bodies.

History

In the sixteenth century, Copernicus suggested that Earth and other planets orbited the Sun, but his model contained no physics . It did not say why the planets should orbit the Sun. Galileo was censured by the Catholic Church and forced to recant his belief in the Copernican model. He then realized that to ultimately win the Copernican model needed a physical basis. Galileo therefore started to quietly develop the new physics needed to explain planetary motions. Isaac Newton, who was born the year Galileo died, built on the foundation laid by Galileo. The resulting edifice, Newton's laws, was a grand synthesis that for the first time explained motions both on Earth and in the heavens with a unified set of laws.

Newton's three laws

Slide a block of wood across a level uncarpeted floor and notice its behavior . The block continues to move as long as you apply a force . When the force stops, the block stops moving. The block will continue to slide for a while after you stop applying a force. Your pushing is not the only force acting on the block. There is also a frictional force opposing the motion. The block sliding across the floor stops because this frictional force acts on it. The block on an icy surface takes longer to stop because there is less frictional force. If you could slide the block across a surface with absolutely no friction , it would never stop. The block would keep moving until some outside force, such as the wall of the room, stopped it. A block on a level surface, without application of forces will not move unless something applies an outside force; it will remain there at rest forever.

The first of Newton's laws states an object will continue its motion at a constant velocity until an outside force acts on it. The block has a tendency to continue in its state of motion, whatever that state might be, until some force changes that state of motion. This tendency to continue in a state of motion is called the object's inertia. An object at rest simply has a constant velocity of zero , so it needs an outside force to start moving. The physicist's definition of velocity includes both speed and direction, so any deviation from straight line motion is a change in velocity and will require an outside force. The inertia of any object will cause it to continue to move at a constant (in a straight line) velocity (or stay at rest) until an outside force acts on it.

A block will slide more easily than, for instance, a refrigerator because it has less mass . Newton's first law says that a force is needed to change the velocity of an object; the second law tells us how much force. Any change in velocity (speed up, slow down, or change direction) is an acceleration . For the common case where the mass does not change, Newton's second law states that the force required is equal to the accelerated mass times the acceleration (Force = Mass × Acceleration). It's harder to slide the refrigerator than the block across the floor because the greater mass requires a greater force to accelerate it from rest. A force in the same direction as the velocity increases the velocity; in the opposite direction, decreases it. A force perpendicular to the velocity changes the direction of motion. Occasionally the accelerated mass changes. In this case, the force is equal to the rate at which the momentum changes with time.

Newton's third law states that for every action there is an equal and opposite reaction. The action and reaction are equal and opposite forces forming an action reaction pair. If you are sitting in a chair, Earth's gravity pulls you down. The reaction is that you pull Earth up with exactly the same amount of force. The action reaction pair is: you on Earth, Earth on you. The reaction is NOT as is often thought the floor or chair holding you up. Not all equal and opposite forces form an action reaction pair.

Newton's three laws of motion revolutionized physics. For the first time the same simple set of laws explained a wide variety of apparently unrelated types of motion both on Earth and in the heavens. Not until the twentieth century were these laws surpassed by quantum mechanics and relativity for the special cases of subatomic particles , motion near the speed of light and strong gravitational fields.

Resources

books

Feynman, Richard P., Robert B. Leighton, and Mathew Sands. The Feynman Lectures on Physics. Vol.1. Reading, MA: Addison-Wesley, 1963.

Hewitt, Paul. Conceptual Physics. New York: Prentice Hall, 2001.

Ostdiek, Vern J., and Donald J. Bord. Inquiry into Physics. 2nd ed. St. Paul: West Publishing, 1991.

Serway, Raymond, Jerry S. Faughn, and Clement J. Moses. College Physics. 6th ed. Pacific Grove, CA: Brooks/Cole, 2002.

Paul A. Heckert

KEY TERMS

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Acceleration: —The rate at which the velocity of an object changes over time.
Force: —Influence exerted on an object by an outside agent which produces an acceleration changing the object's state of motion.
Inertia: —The tendency of an object in motion to remain in motion, and the tendency of an object at rest to remain at rest.
Mass: —A measure of the amount of matter in kilograms. Related both to the resistance to the change in motion and to the amount of gravitational force.
Velocity: —The speed and direction of a moving object.