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Newton’s Third Law

Newton’s third law of motion states, "to every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts." In other words forces acting on a body originate in other bodies that makeup its environ-ment. Any single force is only one aspect of a mutual interaction between two bodies.


Work is done when a force succeeds in over-coming a body’s inertia and moving the body in the direction the force is applied. The formula is

where W is work, F is force and d is the distance moved. The amount of work done is the product of the magnitude of the force and the distance moved.

Work is measured in the English system by the foot-pound; that is, if 1 pound of force acts through a distance of 1 foot, it performs 1 foot-pound of work. In the metric CGS system, force is measured in dynes, distance is measured in cen-timeters, and work is denoted in ergs. An erg is the work done by a force of one dyne exerted for a distance of one centimeter. Another unit used to measure work is the joule. It is simply 10,000,000 ergs, and is equivalent to just under three-fourths of a foot-pound.


Energy is defined as the ability to do work. Energy is conservative, meaning it may be neither created nor destroyed. It is defined in two forms—potential energy and kinetic energy. As its name implies, potential energy is the amount of energy that MAYBE AVAILABLE to a body due to its position. It is primarily due to the force of gravity. The higher a body is raised above the surface, the greater its POTENTIAL energy. Kinetic energy is the energy available to a body due to its motion through a field. The total amount of energy a body possesses is the sum of its potential and kinetic energies. The total amount of energy available to a body determines how much work it can accomplish.


There are two types of forces the AG deals with—contact force and action at a distance force. Contact force is the force that occurs when pressure is put on an object directly through physical contact. An example of contact force is the force your hand exerts when you push your coffee cup across a table. Contact force may act in several different directions at once as well. For example, the force exerted by water in a can is equally exerted on the sides and the bottom of the can. In addition, an upward force is trans-mitted to an object on the surface of the water.

Forces that act through empty space without contact are known as action at a distance force. An example of this force is gravity.


Problems often arise that make it necessary to deal with one or more forces acting on a body. To solve problems involving forces, a means of representing forces must be found. True wind speed at sea involves two different forces and is obtained through the use of the true wind com-puter.

Ground speed and course of aircraft are computed by adding the vector representing air-craft heading and true air speed to the vector representing the wind direction and speed. In com-putation of the effective fallout wind and other radiological fallout problems, the addition of forces is used. From these examples, it is evident that the addition and subtraction of forces has many applications in meteorology.

A force is completely described when its magnitude, direction, and point of application are given. A vector is a line that represents both magnitude and direction; therefore, it maybe used to describe a force. The length of the line represents the magnitude of the force. The direc-tion of the line represents the direction in which the force is being applied. The starting point of the line represents the point of application of the

Figure 2-1-1.—Example of a vector.

force. (See fig. 2-1-1.) To represent a force of 10 pounds or 10 knots of wind acting toward due east on point A, draw a line 10 units long, start-ing at point A and extending in a direction of 090°.

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