Everything around us is in motion. Even a body supposedly at rest on the surface of Earth is in motion because the body is actually moving with the rotation of Earth; Earth, in turn, is turning in its orbit around the Sun. Therefore, the terms rest and motion are relative terms. The change in position of any portion of matter is motion. The atmosphere is a gas and is subject to much motion. Temperature, pressure, and den-sity act to produce the motions of the atmosphere. These motions are subject to well-defined physical laws. An explanation of Newtons laws of mo-tion can help you to understand some of the reasons why the atmosphere moves as it does.

Sir Isaac Newton, a foremost English physicist, formulated three important laws relative to motion. His first law, the law of inertia, states, "every body continues in its state of rest or uniform motion in a straight line unless it is com-pelled to change by applied forces." Although the atmosphere is a mixture of gases and has physical properties peculiar to gases, it still behaves in many respects as a body when considered in the terms of Newtons law. There would be no move-ment of great quantities of air unless there were forces to cause that movement. For instance, air moves from one area to another because there is a force (or forces) great enough to change its direc-tion or to overcome its tendency to remain at rest.

Newtons second law of motion, force, and acceleration states, "the change of motion of a body is proportional to the applied force and takes place in the direction of the straight line in which that force is applied." In respect to the at-mosphere, this means that a change of motion in

the atmosphere is determined by the force acting upon it, and that change takes place in the direc-tion of that applied force.

From Newtons second law of motion the following conclusions can be determined:

1. If different forces are acting upon the same mass, different accelerations are produced that are proportional to the forces.

2. For different masses to acquire equal acceleration by different forces, the forces must be proportional to the masses.

3. Equal forces acting upon different masses produce different accelerations that are propor-tional to the masses.

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