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## 1. Science

Science is a systematic attempt to understand natural phenomena in as much detail and depth as possible and use the knowledge, so gained to predict, modify and control phenomena.

Science is exploring, experimenting and predicting from what we see around us.

## 2. Physics

Physics is a branch of science. The word Physics comes- from a Greek Word Physic meaning nature and in Sanskrit, it is called Bhautiki. Thus, Physics refers to the study of the physical world, i.e. the study of the basic laws of nature and their manifestation in different natural phenomena.

## 3. Scope and Excitement of Physics

To define the scope and excitement of physics, physics is categorized into two groups, on the basis of the magnitude of physical quantities involved in it, i.e. macroscopic and microscopic groups of Physics.

### (i) Macroscopic Group of Physics

It deals with the subjects included in classical physics. It consists of phenomena at the laboratory, terrestrial and astronomical scales.

Classical physics can be given as

(a) Mechanics

It deals with the study of the motion of particles, rigid and deformable bodies and general system of particles. It is based on the law of gravitation and Newton’s laws of motion.

(b) Electrodynamics

It deals with the study of electric and magnetic phenomena associated with charged and magnetic bodies. It is based on the laws given by Coulomb, Oersted, Ampere, and Faraday.

(c) Optics

It deals with the study of phenomena related to light. Working of the human eye, telescope, microscope, etc. are studied in this topic.

(d) Thermodynamics

It deals with the study of the system in macroscopic equilibrium considering changes in internal energy, temperature, entropy, etc.

### (ii) Microscopic Group of Physics

It deals with the study of constitution and structure of matter at every minute scale of length, i.e. at the scale of atoms and nuclei or even smaller than these.

This group of physics can be studied under the subject quantum physics.

## 4. Fundamental Forces in Nature

There are four fundamental forces in nature

### (i) Gravitational Force

The force of mutual attraction between any two objects because of their masses is called gravitational force. This force was discovered by Isaac Newton.

### (ii) Electromagnetic Force

The force associated with charged particles is called electromagnetic force. This force was first discovered for charges at rest, i.e. for like charges, force is attractive and for unlike charges, force is repulsive.

### (iii) Strong Nuclear Force

The nucleus consists of protons and neutrons. Protons being positively charged could repel each other and collapse the nucleus. Gravitational force is negligible comparing to electric force to overcome the repulsion of protons inside the nucleus. Thus, a new force came into existence known as strong nuclear force.

### (iv) Weak Nuclear Force

This force appears only between elementary particles involved in nuclear processes of radioactivity like p-decay of a nucleus, etc. In p-decay, the nucleus emits an electron and an uncharged particle called neutrino.

Name

Relative strengths

Range

Operates among

Gravitational force

$${10^{ – 39}}$$

Infinite

All objects in the universe

Weak nuclear force

$${10^{ – 13}}$$

Very short, sub nuclear size,$$($$~$${10^{ – 16}}{\rm{m}})$$

Some elementary particles, particularly electron and neutrino

Electromagnetic force

$${10^{ – 2}}$$

Infinite

Charged particles

Strong force

1

Short, nuclear size$$($$~$${10^{ – 15}}{\rm{m}})$$

Nucleons, heavier elementary particles

## 5. Nature of Physical Laws

One of the facts observed by the physicist is that during a physical phenomenon governed by different forces, several quantities may change with time, but some special physical quantities remain constant with time. They are called conserved quantities of nature and this is called the law of conservation.

There are four laws of conservation in classical physics.

### (i) Law of Conservation of Energy

Law of conservation of energy states that energy can neither be created nor destroyed but it can be changed from one form to another, i.e. the total sum of all kinds of energy in this universe remains same, e.g. When an object is falling freely under gravity and released from rest, the initial potential energy is completely converted into the kinetic energy and just before it hits the ground, potential energy of the body is completely converted into kinetic energy.

Thus, both kinetic energy and potential energy of the object change with time, but their sum remains constant.

### (ii) Law of Conservation of Mass

Earlier it was assumed that mass is indestructible and the law of conservation of mass states that matter can neither be created nor destroyed. But Einstein’s theory of relativity (energy mass relation, $${\rm{E\;}} = {\rm{m}}{{\rm{c}}^2}$$ where m is the mass and c are the speed of light in vacuum) modified it. For example, in an exothermic reaction, the total binding energy of the reacting molecules is less than the total binding energy of the product of molecules, the difference appears as heat energy

.

### (iii) Law of Conservation of Momentum

Momentum is the quantity of motion of a moving body (generally measured as the product of mass and velocity of the body), It is a vector quantity. The momentum of an isolated system is also conserved. Momentum can be classified into two types and law of conservation is valid for both.

(a)Law of Conservation of Linear Momentum

This law states that, if no external force act on a system, then its linear momentum remains constant.

$$\sum {{\rm{F}}_{{\rm{ext}}}} = {\rm{\;}}0,{\rm{\;}}{\bf{p}}{\rm{\;}} = {\rm{\;constant}}$$

e.g. A rifle gives backward kick on firing a bullet, before firing, both bullet and the rifle are at rest and initial momentum of the system is zero. As soon as the bullet is fired, it moves forward with a large velocity. In order to conserve momentum, the rifle moves backward with such a velocity that the final momentum of the system is zero.

(b) Law of Conservation of Angular Momentum

The law of conservation of angular momentum states that, if no external torque acts on a system, then its angular momentum remains constant.

$$\sum {{\rm{F}}_{{\rm{ext}}}}{\rm{\;}} = {\rm{\;}}0,{\rm{\;L\;}} = {\rm{\;constant}}$$

e.g. When a planet approaches the sun while revolving in its elliptical orbit, the moment of inertia of the planet about the sun decreases. To conserve the angular momentum, its angular speed increases.

Note A body rotating about an axis has rotational inertia, called moment of inertia. Also, it is associated with a momentum called angular momentum.

### (iv) Law of Conservation of Charge

Law of conservation of charge states that the net charge of an isolated system remains constant, e.g. Electric charge is conserved during fission of a $${}_{92}^{235}{\rm{U}}$$ nucleus by a neutron.

$${}_0^1{\rm{n\;\;}} + {\rm{\;}}{}_{92}^{235}{\rm{U}} \to {}_{56}^{141}{\rm{Ba}} + {\rm{\;}}{}_{36}^{92}{\rm{Kr}} + {\rm{\;}}3{}_0^1{\rm{n\;\;}} +$$ Energy

Total charge before fission $$=$$ Total charge after fission

$$0{\rm{\;}} + {\rm{\;}}92{\rm{\;}} = {\rm{\;}}56 + {\rm{\;}}36$$

## 6. The universality of Laws of Conservation

(i) Laws of conservation are the most significant laws in science as these laws are universal ranging from microscopic level to macroscopic level.

(ii) It often happens that we cannot solve the full dynamics of a complex problem involving different particles and forces. The conservation laws can still provide useful results in practice.

(iii) Conservation laws are closely related to the symmetries of nature. The symmetries of space, time and other types of symmetries have played an important role in developing the modern theories of fundamental forces.

## 7. Physical Quantities

All the quantities which can be measured directly or indirectly in terms of which laws of physics are described and whose measurement is necessary are called physical quantities.

Physical quantities can be further divided into fundamental and derived quantities.

### (i) Fundamental Quantities

Those physical quantities which are independent of other physical quantities and are not defined in terms of other physical quantities are called fundamental or base quantities, in mechanics, e.g. Mass, length, time, temperature, luminous intensity, electric current, amount of substance.

### (ii) Derived Quantities

Those quantities which can be derived from the fundamental physical quantities are called derived quantities, e.g. Velocity, acceleration, linear momentum, etc.