# Newton's Laws of Motion

## Examples Of The Third Law

(1) What enables us to walk? To move forward parallel to the floor we must push backward on the floor with one foot. By the third law, the floor pushes forward, moving us forward. Then the process is repeated with the other foot, etc. This cannot occur unless there is friction between the foot and floor and on a frictionless surface we would not be able to walk.

(2) How can airplanes fly at high altitudes and space crafts be propelled? High altitude airplanes utilize jet engines; that is, engines burn fuel at high temperatures and expel it backward. In expelling the burnt fuel a force is exerted backward on it and it exerts an equal forward force on the plane. The same analysis applies to space crafts.

(3) A father takes his eight-year-old daughter to skate. The father and the girl stand at rest facing each other. The daughter pushes the father backwards. What happens? Whatever force the daughter exerts on her father he exerts in the opposite direction equally on her. Since the father has a larger mass his acceleration will be less than the daughter's. With the larger acceleration the daughter will move faster and travel farther in a given time.

## Resources

### Books

Cohen, I. Bernard. Introduction to Newton's Principia. Lincoln, NE: iUniverse, 1999.

Hagen, Robert M. and Trefil, James. Science Matters. New York: Doubleday, 1991.

Hewitt, Paul. Conceptual Physics. Englewood Cliffs, NJ: Prentice Hall, 2001.

Hobson, Art. Physics Concepts and Connections. Englewood Cliffs, NJ: Prentice Hall, 1995.

Kirkpatrick, Larry and Gerald Wheeler. Physics: A World View. 2nd ed. Chicago: Saunders, 1995.

Munson, Bruce, et al. Fundamentals of Mechanics. 4th ed. New York: John Wiley and Sons, 2002.

Teller, Edward, Teller, Wendy, and Talley, Wilson. Conversations On The Dark Secrets Of Physics. New York: Plenum Press, 1991.

Billy W. Sloope

## KEY TERMS

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Centripetal acceleration

—Produces a change in the direction of velocity and always perpendicular to the velocity vector. This, in turn is caused by a centripetal force.

Force

—Influence exerted on an object by an outside agent which produces an acceleration changing the object's state of motion.

Inertia

—Property of matter whereby any change in state of motion is opposed. Quantitatively measured by mass.