It is a systematic and organized knowledge about the various natural phenomena that is obtained by careful experimentation, keen observation and accurate reasoning. The knowledge of science can be divided into two broad categories:

Physical Sciences

It deals with the (i) study of natural world that deals with the concepts space, time, matter, energy, radiation, motion etc., (ii) Study of every substance, its structure, its composition and changes in which it takes part. These sciences include Physics, Chemistry, Geology, Geography, Astronomy etc.

Biological Sciences

It deals with the behavior of living things. These sciences include Botany, Zoology, Physiology, Anthropology, Forensic science etc.

Scientific attitude & Scientific method

The Scientific attitude requires a flexible, open-minded approach towards solving problems. Other important points of view should not be neglected without any reason. To solve a problem, the steps are following:

The Scientific attitude requires a flexible, open-minded approach towards solving problems. Other important points of view should not be neglected without any reason. To solve a problem, the steps are following:

1. Suggest a solution for a problem

2. Try the solution

3. If it works satisfactorily, it is adopted, otherwise it is replaced by a better solution.

The step by step approach used by a scientist in studying natural phenomena and establishing laws which govern these phenomena is called scientific method. This method consists of the following steps:

1. Taking a large number of systematic observations by doing controlled experiments.

2. Studying these observations and making qualitative and quantitative reasoning.

3. Suggesting mathematical models to explain for the observed behavior.

4. Predicting new phenomena on the basis of suggested model.

5. Modifying the theory in the light of the fresh evidences.

What is Physics?

Physics is the branch of science that deals with the study of basis laws of nature and their manifestation in various natural phenomena. It is concerned with the interaction of matter with matter or energy. It deals with the various features of the natural world such as space, time, matter, motion, energy, radiation, etc.

The word physics originates from a Greek word which means nature. This word was introduced by ancient scientist Aristotle in the year 350 B.C.

Domains of Physics

Physics has two main domains:

Microscopic Domain

Classical physics deals with macroscopic phenomena at the laboratory, terrestrial and astronomical scales.

Microscopic Domain

Quantum physics deals with microscopic phenomena at the minute scales of atoms, molecules and nuclei.

Branches of Physics

Mechanics

It deals with the equilibrium or motion of material bodies at low speeds. It is based on laws of gravitation.

Optics

It deals with the nature and propagation of light. It deals with the formation of images by mirrors and lenses, colors in thin films, etc.

Thermodynamics

It deals with a macroscopic system in equilibrium and is concerned with the changes in internal energy, temperature, entropy, etc., of the system through external work and heat.

Electrodynamics

It deals with electric and magnetic phenomena associated with charged and magnetic bodies. It is based on laws given by Coulomb, Oersted, Ampere and Faraday. These laws were later on unified by Maxwell.

Quantum Mechanics

It deals with the mechanical behavior of sub-microscopic particles like atoms and nuclei and their interaction with projectiles like electrons, photons and other elementary particles.

Relativity

It deals with the motion of the particles having speeds comparable to the speed of light. It is theory of invariance in nature

Fundamental Forces in Nature

There are four fundamental forces:

Gravitational Force

It is the force of mutual attraction between two bodies by virtue of their masses. According to Newton’s law of gravitation, the gravitational attraction between two bodies of masses m_1 & m_2 and separated by distance r is given by

F = G\frac{{{m_1}{m_2}}}{{{r^2}}}

where G is the universal gravitational constant.

Properties of Gravitational Force

1. It is a universal attractive force.

2. It is directly proportional to the product of the masses of the two bodies.

3. It is inversely proportional to the square of the distance between two bodies i.e. it obeys inverse square law.

4. It is a long range force and does not need any intervening medium for its operation.

5. Gravitational force between two bodies does not depend upon the presence of other bodies.

6. It is the weakest force known in nature.

7. It is a central force i.e. it acts along the line joining the centers of the two bodies.

8. It is a conservative force i.e. work done in moving a body against the gravitational force is path independent.

9. Gravitational force between two bodies is thought to be caused by an exchange of a particle called graviton.

Examples of Gravitational Force

(i) All bodies fall because of the gravitational force of attraction exerted on them by the earth.

(ii) Gravitational force governs the motion of the moon around the earth the motion of the planets around the sun.

Electromagnetic Force

The force acting between two electric charges at rest is called electrostatic force. According to Coulomb’s law, the magnitude of the electrostatic force F between two point charges q_1 and q_2 separated by distance r in vacuum is given by

F = \frac{1}{{4\pi {\varepsilon _0}}}\frac{{{q_1}{q_2}}}{{{r^2}}}

where \varepsilon _0 is the permittivity of vaccum. The force acting between two magnetic poles is called magnetic force. In fact, electrostatic and magnetic forces are closely inter-related and are considered as the two facets of a general force known as electromagnetic force.

Important properties of electromagnetic force

1. Electromagnetic force may be attractive or repulsive. Like charges repel each other and unlike charges attract each other.

2. It is inversely proportional to the square of the distance between two bodies i.e. it obeys inverse square law.

3. It is a long range force and does not require any intervening medium for its operation.

4. It is a central and conservative force.

6. It is 10^{36} times stronger than the gravitational force.

7. It is caused by the exchange of photons between two charged particles.

Examples of electromagnetic force

When a spring is compressed/elongated, it exerts a force of elasticity due to the net repulsion / attraction between its neighboring atoms. This net repulsion or attraction is the sum of the electrostatic forces between the electrons and nuclei of the atoms. 

Strong Nuclear Force

It is the force which binds together the protons and neutrons in a nucleus.

Important properties of strong nuclear force

1. It is the strongest interaction known in nature, which is about 100 times stronger than the electromagnetic force and about 10^{38} times stronger than the gravitational force.

2. It is a short range force that operates only over the size of the nucleus ( 10^{-15} m ).

3. It is basically an attractive force, but becomes repulsive when the distance between the nucleons becomes less than 0.5 fermi (1 fermi = 10^{-15} m ).

4. It varies inversely with some higher power (>2) of distance.

5. It is a non-central and non-conservative force.

6. It has charge independent character i.e. nuclear forces between proton-proton, proton- neutron and neutron-neutron are almost equally strong.

7. It is caused by the exchange of particles, called π-mesons.

Examples of strong nuclear force

(i) Nuclear forces bind together the protons and neutrons in the nuclei. So they are responsible for stability of nuclei and atoms.

(ii) Radioactivity occurs in heavier nuclei because of insufficient nuclear force between their protons and neutrons.

Weak Nuclear Force

It is the force that appears only between elementary particles involved in a nuclear process.

Important properties of weak nuclear force

1. The weak nuclear force is 10^{25} times stronger than the gravitational force, but much weaker than strong nuclear and electromagnetic forces.

2. It operates only through a range of nuclear size (~ 10^{-15} m ).

4. The messenger particles that transmit the weak nuclear force between elementary particles are the massive vector bosons ( W^±, Z ).

Examples of weal nuclear force

(i) Any process involving neutrino and antineutrino is governed by weak nuclear force.

Conservation Laws

In any physical process involving the different forces, some physical quantities remain unchanged with time. Such quantities are called conserved quantities. The laws which govern the conservation of these quantities are called conservation laws. In classical physics, we usually deal with the following four conservation laws:

Law of conservation of energy

Energy can neither be created nor destroyed but it can be changed from one form to another. Hence, total energy of an isolated system remains constant.

Examples

(i) When a body falls freely, under gravity, its potential energy gradually changes into kinetic energy. But its total mechanical energy (kinetic energy + potential energy) remains constant at any point of its motion.

(ii) During the oscillation of a simple pendulum, the energy of the bob changes gradually from kinetic to potential as it moves from mean position to either of the extreme positions. The energy changes from potential to kinetic as the bob moves from either of extreme positions to the mean position. At all points of its motion, total energy of the bob remains constant.

Law of conservation of linear momentum

No external force acts on a system, then its linear momentum remains constant.

Examples

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

(ii) Suppose a radioactive nucleus, initially at rest, decays spontaneously into fragments. To conserve linear momentum, the heavier and lighter fragments will fly in opposite directions, with the former having a proportionately smaller speed than the latter.

Law of conservation of angular momentum

No external torque acts on a system, then its angular momentum remains constant.

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

Angular momentum (L) = Moment of inertia (I) × Angular speed (ω)

Examples

(i) While revolving in its elliptical orbit, when a planet approaches the sun, its moment of inertia about the sun decreases. To conserve the angular momentum, its angular speed increases.

(ii) In a Tornado as the air rushes towards the center, its moment of inertia decreases. To conserve the angular momentum, the angular speed of the air increases.

Law of conservation of angular momentum

The total charge of an isolated system remains constant. This implies that the electric charges can neither be created nor destroyed, only they can be transferred from one body to another.

Examples

(i) When a glass rod is rubbed with silk cloth, both develop charges. It is observed that the positive charge developed on the glass rod has the same magnitude as the negative charge developed on silk cloth. So total charge after rubbing is zero as before rubbing i.e., electric charge is conserved.

Relation Between Conservation Laws and Symmetries of Nature

Symmetries of time leads to law of conservation of energy

If we perform an experiment at a certain place today and repeat the same experiment after one year at the same place, we obtain exactly the same results. This symmetry of nature with respect to translation or displacement of time is called homogeneity of time and it leads to the law of conservation of energy.

Symmetries of space leads to law of conservation of linear momentum

Laws of nature take the same form everywhere in the universe i.e., there is no preferred location in the universe. This symmetry of the laws of nature with respect to translation in space is called homogeneity of space and gives rise to the law of conservation of linear momentum.

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