Matter and interactions

Physics is the branch of science which seeks to understand the properties and behaviors of the world around us at all levels of scale.  Physics should answer the following questions.

What are the constituents that make up the world around us and how do they interact?

As physicists, we observe, we experiment, and we build conceptual models.  We build models on many different levels.  We hope to develop theories, grand models that embrace a huge range of phenomena.  Our theories cannot be proved absolutely.  If our confidence in a theory is great, then its key features are referred to as laws.

Models are made by people for people.  One must be careful not to confuse the model with the reality.  A model is just a simple, manageable representation.  Models can change as our knowledge changes, but the underlying reality presumably does not change.  The medium we use to create these models is usually mathematics.  Different physical models have different ranges of applicability, they have been verified to work on different scales.  The science of Physics is often divided into Classical Physics and Modern Physics.

Classical Physics in a model of the macroscopic world around us.  All the laws of classical physics were known by the end of the 19th century.  The known properties of matter at the end of the 19th century were mass and charge.  The smallest constituents were atoms.  The known interactions were gravity, modeled by Newton's law of gravitation, electromagnetic interactions, modeled by Maxwell's equations, and contact force arising from the requirement that "atoms need their space".  Consequences of the interactions were described by Newton's laws of motion, which predict how matter behaves when acted on by forces.  Statistical physics and thermodynamics were developed for describing systems with a large number of degrees of freedom.

Classical physics works well describing and predicting almost all everyday phenomena.  In this class we will study Classical Physics.

The Vocabulary of Physics

Most disciplines have their own special vocabulary.  The vocabulary of physics includes words whose meaning in everyday language may depend on the context in which they are used.  In physics, these words have only one precisely defined, context-independent meaning. 

Take, for example, the word "force". 

Only the first of these three sentences uses the word "force" to refer to the concept it describes in physics.

Words such as position, velocity, acceleration, energy, power, etc, all have context-dependent meaning in everyday language.  It is important that when communicating about physics, you always use the precise, context-independent definition of these words.  Words whose definitions you should keep in mind appear in bold red font in the Web material.


The result of every measurement has two parts, a number and a unit.  The number is the answer to "How many?" and the unit is the answer to "Of what?".  Units are standard quantities such as a second, a meter, a mile.  The most widely used units today are those of the international system, abbreviated SI (Systeme International d'Unites).  Examples of SI units are the meter (m) for length, the second (s) for time, and the kilogram (kg) for mass.  The unit of an observable carries information about the dimension of the observable.  If a quantity is measured in meters, than its dimension is length (L), if it is measured in kilogram, than its dimension is mass (M).

SI base units:

The 20 SI prefixes used to form decimal multiples and submultiples of SI units are given in Table below.

SI prefixes

Links:  Reference (SI units)