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Theories

Models

Laws and Principles

Concept

CONCEPTS, MODELS AND THEORIES

In physics we deal with concepts, laws, principles, models, and theories. Let us briefly consider the meaning of each of these terms.

A concept is an idea or a physical quantity that is used to analyze natural phenom­ena. For example, the abstract idea of space is a concept and so is the measurable physical quantity, length. In physics we use concepts such as mass, length, time, acceleration, force, energy, temperature, and electric charge. One can define a physical quantity by the procedure used to measure it. For example, temperature can be defined as the reading on a "standard" thermometer, or electric charge by the force that electrified bodies exert on each other.

A physicist tries to establish mathematical relationships, called laws, between physical quantities through experimentation or theoretical analysis. Mathematics is the natural language of physics because it allows us to state the relationships concisely.

Whereas a law may be restricted to a limited area of physics, a principle is a very general statement about how nature operates. It spans the whole subject and is part of its foundation. Consider a boat floating steadily down a river. The principle of relativity states that the laws of physics deduced by people on the boat must be the same as those discovered by people on land. It makes no reference to specific laws, but does force us to ensure that the laws we formulate do not violate this principle. As we will see, this seemingly innocuous statement has profound consequences.

A model may merely replace the real thing to simplify the analysis. For example, in some problems one might treat the earth and the moon as if they were point particles. A model is often a mental picture of the structure or workings of a system

There are also purely mathematical models whose mathematical properties reflect those of the real system. In some cases we begin to suspect that there is more to the model than pure mathematics. That is, perhaps the mathematical entities represent actual physical quantities. Quarks, for example, were first pro­posed as part of a mathematical model of elementary particles. There is now so much supportive evidence that we regard them as "real." But the reality of quarks cannot be guaranteed since we cannot actually look inside a nucleus. It is a quark model that successfully accounts for a range of phenomena.

A theory uses a combination of principles, a model, and initial assumptions (called postulates) to deduce specific consequences or laws. By organizing data from different areas, or by tying together concepts mathematically, a theory reveals an underlying unity in diverse phenomena. For example, Newton's theory of gravita­tion explained why an apple falls to the earth, the motion of the planets about the sun, why the tides occur, and even the shape of the earth. His theory showed that the same laws of physics apply to objects on earth as to the celestial bodies.

 

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IV Синтетические иммуноактивные средства | Derived Units
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