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 question:

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.

Conceptual models are extensively used in physics.  The more features can be understood in terms of a model, the better it is.  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 mathematics.  A theory or law invariably is specified in mathematical form.

We have models that work well on different scales.

Examples of 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 19^th century.  Classical physics works well describing and predicting almost all everyday phenomena.  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.

• Modern Physics is the physics of the 20^th century.  The main building blocks, the theory of relativity and quantum mechanics, were developed early in that century.
  Two different types of problems with classical physics became obvious at the end of the 19th century.  One problem was an internal inconsistency.  The other type of problem arose from measurements that could not be understood using classical physics.

Quantum mechanics is a model of the microscopic world.  This class will introduce you to quantum mechanics and some of its applications.

Our intuition has been build up in a way that ignores quantum mechanical behavior.  But if we only observe macroscopic objects and our experiments involve only macroscopic objects, then the models we make describing and predicting their behavior, even if they work very well, may not describe the behavior of microscopic objects accurately.  We can hope that they do, but we have to do experiments on the microscopic scale to test them.  Many physical properties of macroscopic systems are not continuous.  It is now common knowledge that matter is made of atoms and each atom consists of a nucleus surrounded by electrons.  In classical physics, we ignore this granularity and treat matter as being continuous.  What we observe and what the laws of classical physics describe are the average physical properties.  All our measurements on macroscopic objects measure the average the behavior of a huge number of microscopic objects.  Many of the peculiarities of the microscopic behavior average out.

But even if the laws of classical physics are violated in the microscopic world, there are certain principles that we expect not to be violated, because that would have consequences in the macroscopic world.  These principles are often expressed in terms of conservation laws.

Conservation laws:

• Conservation of Energy

• Conservation of Momentum

• Conservation of Angular Momentum

There are various ways to put the laws of quantum mechanics into mathematical form.  One such form is a wave equation.  To become familiar with the mathematical description of wave, we will spend some time reviewing the properties of mechanical waves.