Science | Class 11th Notes | Physics | Unit- 1 Physical World And Measurements Chapter-1 Physical World


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Science for class – XI

Unit -1: Physical world and measurements

Although we have been studying the physics from a long time, still there is not everything we have been able to clear out in our previous classes. We need to revisit the world of physics with a new dimension and explore the basics of physics and physical study again so that nothing remains uncovered and no single basic concept of physics is left. In unit Ist i.e. Physical world and measurements, we look at all the basic knowledge of physics so that armed with this knowledge we can move onto further deep study of physics in further units.

Chapter-1: Physical world

1.1 Introduction

We start this long journey of physics and physical world with a basic definition of physics and discuss about various branches of physics. We would discuss about some of the most inspiring physical discoveries and how they were made, forces operating in nature and some basic physical laws. Please note that the word physical means “related to physics”, so physical laws are laws related to physics, physical theories are theories related to physics and so on.

1.2 What is physics?

Physics- Physics is the study of nature. It is the branch of science dealing with the study of nature and natural phenomena.

Scientific theory- A scientific theory is a set-up that helps to explain a natural phenomenon or the behaviour of a natural system on the basis of the established laws of nature.

For example:- The theory of solar system, where sun occupies the central position is known as Copernican theory of solar system.

1.3 Physical theories and their branches

Today’s physics can be described and understood with the help of the following five theories:-

  1. Mechanics (or Newtonian mechanics) – The theory of motion of material objects at low speeds.
  2. Thermodynamics – The theory of heat, temperature and the behaviour of a system of a large number of particles.
  3. Electromagnetism – The theory of electricity, magnetism and electromagnetic radiation.
  4. Relativity- The theory of invariance in nature and the theory covering the motion of high-speed moving particles.
  5. Quantum mechanics – The theory of mechanical behaviour of sub-microscopic particles.

Note: Earlier, methods of measurements in in physics were of subjective nature i.e. these depended upon human senses of touch, hearing, sight etc.

  • Methods of experiment in physics:
  1. Subjective methods – Methods of experimenting with the help of simple senses like hearing, seeing, touching, etc.
  2. Objective methods – Methods of experimenting with the help of scientific apparatus. It is used to reduce inaccuracies in subjective methods.

Why is physics called an ‘exact science’ ?

Because of the precision and accuracy in the measurement of physical quantities, physics is called an ‘exact science’ or the ‘science of measurements’.

1.4 How discoveries in physics are made ?

Physics is all about the nature and its phenomena. People observe nature and guess about how certain natural phenomena may happen. Experiments made by many scientists help to find out the reasons behind many natural phenomena. Scientist often find out big discoveries and new theories are made by them in various field of sciences. Some examples are:

Archimedes’ laws of floatation:- Archimedes was asked by a king to tell the purity of the gold in his crown without melting it. He got a clue to solve this problem while bathing in his tub and came out into the street shouting “Eureka! Eureka!” (“I have found it! I have found it!”). Thus he formulated the laws of floatation.

He found that a small drop of liquid is always spherical in shape. It never assumes            cuboidal or any other geometric shape. Due to surface tension, liquids try to possess minimum surface area. It is because, for a given volume, the sphere has minimum surface area.

Electromagnetic induction:- When Faraday moved a magnet near a coil, a galvanometer connected to the coil showed deflection indicating the flow of current through it. This experiment led to Faraday’s theory of Electromagnetic induction.

Its study led to the design of electric generators, motors, etc.

Note:- The cause of forces like gravitational force, magnetic force, etc. is due to the exchange of particles between the two bodies, charges or magnetic poles. So, such forces are termed as Exchange Forces.

Rutherford’s experiment:- Rutherford’s experiment of scattering of α – particles by the gold foil led to the discovery of atomic nucleus. This experiment is also stated in Class 9th Chapter 4.

All the above stated experiments can be explained on the basis of a physical law and had led to important discoveries.

1.5 – Range of length, mass and time intervals in physics

Physical quantities like length, mass and time intervals (or simply time) vary over a wide range. This range can be observed as follows:

  1. Length: 10 -15 m (size of nucleus) to 1025 m (size of universe).
  2. Mass: 10 -30 kg (mass of an electron) to 1055 kg (mass of the universe).
  • Time: 10-22 s (time taken by an electromagnetic radiation to cross a nuclear distance) to 1018 s (life of the sun).

We are able to make such wide ranging measurements with a few methods because:

  1. A quantitative study of the observations in nature tells that these can be explained and understood in terms of few laws.
  2. Though length, mass and time vary over a wide range, yet there is a fairly small number of principles which can be applied to measure them.
  3. The physical phenomena can be easily understood by separating more important features of a physical phenomenon from less important ones and the hidden complexities become clearer.

1.6 – Physics in relation to science

  1. Relation to maths – Physical theories make use of various mathematical concepts which help in the development of theoretical physics.
  2. Relation to chemistry – Study of structure of the atom, radioactivity, etc. have helped in rearrangement of elements in periodic table, to detect even traces of substances in a sample, to know the nature of valency, etc.
  3. Relation to biology – Optical microscope is helpful in the study of biology. Electron microscope has made it possible to see even the structure of cells. X-ray and neutron diffraction have helped in understanding the structure of nucleic acids. Radio-isotopes are used in radio therapy for curing skin diseases.
  4. Relation to astronomy – Modern telescopes help in the study of space and heavenly bodies. Quasars, pulsars have enabled the scientists to see into the farthest limits of the space.

1.7 – Forces in nature

There are four kinds of forces operative in nature:

  1. Gravitational force
  2. Weak force
  • Electromagnetic force
  1. Nuclear (strong) force

We would first look at each of these forces in detail and then discuss their properties.

  1. Gravitational force – The force of attraction between two objects due to their masses is called gravitational force. The equation for gravitational force is, /r2

F = G M1 M2

Where G (= 6.67 × 10 -11 N m2 kg-2) and is called Universal constant of gravitation.

2. Weak force – The force working between two leptons, a lepton and a meson or a lepton and a baryon. Leptons are class of elementary particles including electrons, muons, neutrinos and their antiparticles.

ß – decay- The ß – decay is an example of weak force interaction. In this process, a neutron inside a nucleus changes into a proton by emitting an electron and an uncharged particle, called antineutrino.

3. Electromagnetic force – The force of attraction or repulsion working between two electric charges in motion.

Also, if the charges are not moving, then the force is called the electrostatic force. The electrostatic force is the force between two static (non-moving) electric charges.

4. Nuclear force – The forces operating inside a nucleus between protons and neutrons. In general, the forces responsible for interaction between mesons, baryons and between mesons and baryons. Thus, it is due to interaction between nucleons (baryons) and π – mesons.


  1. Gravitational force:


  1. It obeys the inverse square law.
  2. It is always attractive in nature.
  3. It is a long range force i.e. it extends up to infinity.
  4. Its field particle is graviton.
  5. It is the weakest force operating in nature.
  6. It is a central and conservative force.


  1. Weak force:


  1. Weak forces are short range forces.
  2. The weak forces are about 1025 times stronger than the gravitational forces.
  3. In a weak interaction, neutrino acts as the field particle.


  1. Electromagnetic force:


  1. It obeys the inverse square law.
  2. It may be attractive or repulsive in nature.
  3. It is also a long range force.
  4. Its field particle is photon.
  5. It is about 1036 times stronger than gravitational force.
  6. It is also a central as well as conservative force.


  1. Nuclear force:-


  1. It varies inversely with some higher power of distance.
  2. It is basically an attractive force.
  3. It is a short range force and is operative only over the size of the nucleus.
  4. Its field particle is π – meson.
  5. It is the strongest force operating in nature, about 1038 times strongest than gravitational force.
  6. It is a non-central force.

1.8 – Physical laws of conservation

Finally, in this section, we will study about some conservative laws that are operative in nature. You may already have heard about these in previous classes:-

  1. Law of conservation of linear momentum: It states that if no external force acts on a system, the total linear momentum remains conserved. In absence of external force,

p1 + p2 = constant

Where p1 and p2 are linear momenta of the two bodies at any instant.

  • It is obtained from Newton’s third law of motion.
  • Follows the principle of homogeneity of space i.e. space possesses same properties at all the points.


  1. When two billiard balls strike, they move in opposite directions.
  2. The recoil when a bullet is fired from a gun.
  3. Motion of the rockets.


  1. Law of conservation of energy: It states that energy can neither be created nor destroyed, but can change its form from one to another.
  • It follows the work-energy theorem but can also be explained using principle of homogeneity of flow of timee. time flows uniformly.
  • In mechanics, mass is considered fundamental to matter and matter acquires energy by virtue of its motion or configuration.
  • Einstein’s mass-energy equivalent relation is E = m c2. This has led to law of conservation of mass and energy that unites both laws of conservation of mass and conservation of energy.
  • Release of energy in nuclear fission and fusion is in accordance with this unified law.


  1. The mechanical energy of a freely falling body remains constant. (Mechanical energy = Kinetic energy + Potential energy)
  2. On vibration, the mechanical energy of a simple pendulum remains constant while it swings between two extreme points.


  1. Law of conservation of angular momentum : It states that if no external torque acts on a system, the total angular momentum of the system remains conserved.
  • It follows Newton’s third law of rotatory motion. Also, it can be obtained from principle of isotropy of space e. space possesses same properties in all directions.


  1. Velocity of a planet orbiting the sun in an elliptical orbit increases when it is closer to the sun and decreases when it is far from the sun.
  2. The alarming high-speeds of inner layers of whirl-winds.
  • A diver jumping from the spring board exhibits summersaults in air.


The other conservative laws are law of conservation of charge, spin, lepton number, baryon number, parity etc.


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