# Models, Theories, and Laws

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Douglas C. Giancoli – Physics for Scientists and Engineers with Modern Physics

When scientists are trying to understand a particular set of phenomena, they often make use of a . A model, in a scientist’s sense, is a kind of analogy or mental image of the phenomena in terms of something we are familiar with. One example is the wave model of light. We cannot see waves of light as we can with water waves, But it is valuable to think of light as made up of waves because experiments indicate that light behaves in many respects as water waves do.

The purpose of a model is to give us an approximate mental or visual picture— something to hold on to—when we cannot see what actually is happening. Models often give us a deeper understanding: the analogy to a known system (for instance, water waves in the above example) can suggest new experiments to perform and can provide ideas about what other related phenomena might occur.

Usually a model is relatively simple and provides a structural similarity to the phenomena being studied. A is broader, more detailed, and can give quantitatively testable predictions, often with great precision.

It is important. however, not to confuse a model or a theory with the real system or the phenomena themselves.

Scientists give the title to certain concise but general statements about how nature behaves (that energy is conserved, for example). Sometimes the statement takes the form of a relationship or equation between quantities (such as Newton’s second law, F = ma).

To be called a law, a statement must be found experimentally valid over a wide range of observed phenomena. For less general statements, the term is often used (such as Archimedes’ principle).

Scientific laws are different from political laws in that the latter are : they tell us how we ought to behave. Scientific laws are : they do not say how nature should behave, but rather are meant to describe how nature does behave. As with theories, laws cannot be tested in the infinite variety of cases possible. So we cannot be sure that any law is absolutely true. We use the term “law” when its validity has been tested over a wide range of cases, and when any limitations and the range of validity are clearly understood.

Scientists normally do their research as if the accepted laws and theories were true. But they are obliged to keep an in case new information should alter the validity of any given law or theory.