OVERVIEW Describe the laws of motion and determine how motion is affected by external forces.

OUTLINE

Any general discussion of the principles of physics must contain some consideration of the way in which mass, force, and motion are related. In physics, the laws of motion state that an object at rest never starts to move by itself; a push or a pull must be exerted on it by some other object. This applies to weather also. Weather has many complex motions, both in the vertical and horizontal planes. To fully understand how and why weather moves, you must have a basic knowledge of motion and those external forces that affect motion.

Learning Objective: Describe the laws of motion and determine how motion is af-fected by external forces.

In dealing with motion several terms should be defined before you venture into the study of motion. These terms are inertia, speed, direction, velocity, and acceleration.

Inertia 

An object at rest never moves unless some-thing or someone moves it. This is a property of all forms of matter (solid, liquid, or gas). Inertia, therefore, is the property of matter to resist any change in its state of rest or motion.

Speed is the rate at which something moves in a given amount of time. In meteorology, speed is the term that is used when only the rate of movement is meant. If the rate of movement of a hurricane is 15 knots, we say its speed is 15 knots per hour.

Direction is the line along which something moves or lies. In meteorology, we speak of direc-tion as toward or the direction from which an object is moving. For example, northerly winds are winds COMING FROM the north.

Velocity describes both the rate at which a body moves and the direction in which it is traveling. If the hurricane, with its speed of 15 knots per hour, is described as moving westward, it now has velocityboth a rate and direction of movement.

This term applies to a rate of change of the speed and/or the velocity of matter with time. If our hurricane, which is presently moving at 15 knots, is moving at 18 knots 1 hour from now and 21 knots 2 hours from now, it is said to be ac-celerating at a rate of 3 knots per hour.

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