Physics

The study of energy is known as physics. That is the most general definition of physics you can think of, which gives one some idea of the vastness of topics coming under physics. Everything concerning energy in some form or the other is covered by physics.

Branches of Classical Physics

Physics as we knew it till the end of the nineteenth century is known now as Classical physics. It is broadly classified into the following branches:

Mechanics
Accoustics
Optics
Thermodynamics
Electromagnetism

We shall now look into them one by one.

Modern Physics

The twentieth century ushered in the two new broad topics of a) relativity and b) quantum physics. Unification of these two theories and the sub topic of particle physics are the subjects of twenty-first century physics.

Mechanics

Etymologically, the study of machines is called Mechanics. It broadly covers kinematics, mechanical forces, work, power, and energy, and various properties of matter.

Kinematics is the study of (relative) motion - displacement, velocity, acceleration etc. The two relations at the heart of kinematics are: $v=\frac{dx}{dt}$ , and $a=\frac{d^2x}{dt^2}$ where $x$ is displacement, $v$ is velocity, $a$ is acceleration, and $t$ is time. The simplest topic under kinematics is uniform rectilinear motion. Other simple topics are projectile motion, uniform circular motion, simple harmonic motion etc.

Then we move on to the study of forces, which is almost fully summarized by the three famous laws of motion due to Newton:

A body continues to be in its state of uniform rectilinear motion until it is disturbed by an external force. This property is known as inertia.
The rate of change of momentum of a body with respect to time is directly proportional to the force acting on it.
Every action as an equal and opposite reaction.

Here we come across two completely new quantities: mass and force. Mass is the one of the two most basic intrinsic properties of a body. It is a measure of its inertia. Momentum is defined as the product of the mass and velocity of a body. Force is something that changes or tends to change the momentum of a body.

Mechanical work is defined by the relation $W =\int^{x_f}_{x_i} F\,dx$ where $W$ is work done, $F$ is force, $x$ is displacement, and subscripts $i$ and $f$ denote the initial and final states respectively. Similarly, mechanical power is defined as $P =\int^{v_f}_{v_i} F\,dv$ where $P$ is power delivered and $v$ is velocity. Energy is the other basic intrinsic property of a body. Mechanical energy is simply the capacity of a body to do mechanical work.

Among the various properties of matter are elasticity, surface tension, viscosity etc. But the most important one is gravity. Gravity is indeed considered one of the most mysterious things not only in physics but in science as a whole. I'll tell you more about gravity when we come to modern physics.

Accoustics

Accoustics is etymologically the study of sound. Sound waves are mechanical waves - they travel by actual vibrations in some material medium. Accoustics concerns itself with mechanical waves in general. Phenomena like forced vibrations, resonance, damped vibrations and Doppler effect come under this branch of physics (we shall meet the last of these again in wave optics).

Optics

Etymologically, optics is the study of vision. Light waves are electromagnetic waves - they consist of mutually perpendicular electric fields and magnetic fields, and can travel through vacuum. Optics is the study of electromagnetic waves in general. So it covers all waves in the electromagnetic spectrum given below:

Gamma rays
X - Rays
Ultraviolet Rays
Visible light
Infrared Rays
Microwaves
Radiowaves

Notice how I switch between waves and rays in the table. Actually one of the most controversial questions in optics is whether light is an wave or a ray. Accordingly there are two branches of optics, but only ray optics belongs to classical physics. We shall meet wave optics in modern physics. In ray optics we have things like reflection and refraction, and the dispersion of white light into its constituent colours.

Thermodynamics

It does not have a direct etymological meaning, but I suppose you could call it the study of heat transfer. Anything in physics related to heat comes under thermodynamics - you have nothing called thermostatics (though you have something called a thermostat.)

I'll come back to this later.

Electromagnetism

It is the combined study of electricity and magnetism, and the most important addition to classical physics after Newton's work.

I'll come back to this later as well.

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