# Difference between revisions of "Physics"

The study of energy is known as physics. Everything concerning energy in some form or the other is covered by physics.

## Branches of Classical Physics

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

## Modern Physics

Two new areas of physics, relativity and quantum physics, were discovered in the 20th century. Unification of these two areas and particle physics is the chief focus of physics in the 21st century.

## Mechanics

Main article: Mechanics

Mechanics is the study of movement. Kinematics, mechanical forces, work, power, energy, and matter are all part of mechanics.

Kinematics is the study of (relative) motion - displacement, velocity, acceleration etc. The two relations at the heart of kinematics are: $\delta x'(t)=v(t)$ and $v'(t)=a(t)$ where $\delta x(t)$ is displacement at time $t$, $v$ is velocity, $a$ is acceleration, and $t$ is time. Uniform rectilinear motion, projectile motion, uniform circular motion, and simple harmonic motion are some of the types of problems studied in kinematics.

The rules of physics are almost fully summarized by the three famous laws of motion formulated by Isaac 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.

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, or, informally, "a push or pull".

Mechanical work is defined by the relation $W =\oint^{x_f}_{x_i} F\,dx$ 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 $P =\oint^{v_f}_{v_i} F\,dv$ 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, and viscosity. 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.

## Acoustics

Main article: Acoustics

Acoustics is etymologically the study of sound. Sound waves are mechanical waves - they travel by actual vibrations in some material medium. Acoustics concerns itself with mechanical waves in general. Phenomena such as forced vibrations, resonance, damped vibrations and Doppler effect come under this branch of physics.

## Optics

Main article: 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 a vacuum. Optics is the study of electromagnetic waves in general. So it covers all waves in the electromagnetic spectrum given below:

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. Wave optics are a topic of modern physics. In ray optics covers topics such as reflection and refraction, and the dispersion of white light into its constituent colors.

## Thermodynamics

Main article: Thermodynamics

Thermodynamics does not have a direct etymological meaning, but it could be named the study of heat transfer. Anything in physics related to heat is classified as thermodynamics. There are three laws of thermodynamics:

• The First Law of Thermodynamics is a form of conservation of energy: The change in internal energy of a system is equal to the sum of the energy transferred to the system by heat and the work done on the system.
• The Second Law of Thermodynamics states that the efficiency of heat engines must always be < 1.
• The Third Law of Thermodynamics states that the temperature of a system cannot reach absolute zero (0 K); as the system approaches absolute zero, entropy approaches a constant.

## Electromagnetism

Main article: Electromagnetism

Electromagnetism is the combined study of electricity and magnetism, and the most important addition to classical physics after Newton's work. The concept of electromagnetism has wide applications in everyday devices such as computers, televisions, linear particle accelerators, and more.