# Physics Basics and Key Forces Notes and MCQs for NEET & JEE

Customized study notes on Physical World and Measurement, especially Physics Basics and Key Forces. These notes are based on NCERT Physics Class XI.

## What is Physics?

Ever since humans arrived on Earth, they have been curious about the natural world around them. They strove to observe natural phenomena carefully, understand it, and develop tools to make their life better. These efforts eventually led to the development of modern science and technology.

Science is defined as a systematic attempt to understand the natural phenomena around us in a profound and detailed manner. Physics is one of the disciplines of natural sciences which deals with the study of the basic laws of nature and their manifestation in various natural phenomena.

Physicists attempt to explain several diverse natural phenomena in terms of certain scientific laws and concepts, such as Newton’s law of gravitation and Maxwell’s equations in electromagnetism.

Reductionism is a method used to derive the properties of larger and more complicated systems by studying the interactions and properties of their smaller, simpler constituent parts. This is a method that is at the very heart of physics.

## Scope and excitement of physics

There are two main domains of interest in physics – macroscopic and microscopic. Classical physics is mostly concerned with macroscopic phenomena and includes subjects like mechanics, thermodynamics, optics, and electrodynamics.

However, classical physics is insufficient to handle the microscopic domain of physics. As of now, quantum theory is accepted as the right framework for explaining microscopic phenomena.

The emerging domain of mesoscopic physics, which is intermediate between the macroscopic and the microscopic, deals with a few tens or hundreds of atoms.

The scope of physics, as you can clearly see, is unbelievably wide.

For example, the scale of length deals with everything from entities as small as protons and electrons (10-14 m or less) to massive galaxies (1026 m) and even the entire universe, whose extent is of the order of 1040 or more.

Similarly, the scale of mass in physics ranges from 10-30 kg (the mass of an electron) to 1055 kg (the mass of the observable universe).

The time scale ranges from around 10-22 seconds to 1018 seconds, which can be obtained by dividing the length scales by the speed of light.

## Physics, technology, and society

A strong connection has always existed between physics, technology, and society. The technological advancements made with the help of physics have made great changes in society throughout history.

Likewise, technology has occasionally given rise to new physics as well. For example, the field of thermodynamics emerged as a result of scientists studying and trying to improve the working of heat engines.

Have you ever wondered how, over the centuries, mankind progressed from the simple wheel to the space shuttle? Many of the facilities that make our daily life so easy and comfortable today, from the internet to mobile phones to microwave ovens, have become possible because of physics.

Physics holds great importance in the field of medicine as well. It is the most fundamental of all sciences and has contributed to many current medical technologies and practices, such as X-Rays, MRI, Doppler ultrasound, electrocardiography (ECG), endoscopy, and much more.

The development of biotechnology and material technology has largely depended on microscopic instruments, such as the electron microscope (EM) and the atomic force microscope (AFM).

Physics has also been actively contributing to the development of alternative energy resources like solar energy, geothermal energy, and hydroelectricity.

Despite all the progress that mankind has made, many questions remain unanswered in physics. For example, are matter and energy two different aspects of the same entity? Can different forces in nature be unified?

Reduction and unification are two important thrusts in physics that physicists have been trying to solve.

## Fundamental forces in nature

Main article: Fundamental forces

Intuitively, we are all familiar with the concept of force due to our experiences in everyday life. There are many kinds of forces seen in the macroscopic domain, such as gravitational force, friction, viscous force, elastic force, and much more. Similarly, in the microscopic domain, such as electric force, magnetic force, nuclear force, and so on.

In the twentieth century, it was discovered that many of these forces arise from only a small number of fundamental forces in nature.

For example, the net attraction or repulsion between neighboring atoms in a spring when it is compressed or elongated gives rise to the elastic spring force. The aforementioned attraction or repulsion takes place because of the unbalanced sum of electric forces between the charged constituents of the atom.

As you can see, this implies that the laws for derived forces are dependent on the laws of fundamental forces in nature.

As of now, we know of four fundamental forces in nature, which have been discussed in detail below.

### Gravitational force

This is a universal force of mutual attraction between any two objects by virtue of their masses.

Each and every object experiences this force due to every other object in the universe. For example, all objects on earth experience the force of gravity due to the earth.

Gravity is a long-range force and does not need any intervening medium. It is the weakest force in nature compared to other fundamental forces.

Gravity also plays an important role in the motion of satellites around the earth, the motion of the planets around the sun, and the formation of and evolution of galaxies, stars, and galactic clusters.

### Electromagnetic force

Electromagnetic force is defined as the force acting between charged particles.

When charged particles are at rest, the force between them is known as static electric force and is given by Coulomb’s law (attractive for unlike charges and repulsive for like charges).

Charges in motion produce magnetic effects, and the magnetic field gives rise to a force on the moving charge.

Since the combined effect of the electric and magnetic fields is generally not separable, the combined effect of this force is called electromagnetic force.

Similar to the gravitational force, the electromagnetic force is a long-range force and does not need any intervening medium.

Electromagnetic force is much stronger than the gravitational force. For example, the electric force between two stationary protons is 1036 times the gravitational force between them, for any fixed distance. Thus, it dominates all phenomena at atomic and molecular scales.

### Strong nuclear force

You already know that an atom’s nucleus is comprised of protons and neutrons. Protons are positively charged particles, whereas neutrons don’t have any charge.

You can clearly foresee that in the absence of a strong attractive force, the nucleus will be highly unstable because of the electric repulsion between its protons.

We’ve already discussed that gravity is negligible compared to the electric force; thus, the attractive force in question cannot be gravitational.

Strong nuclear force is a third different fundamental force that binds protons and neutrons in an atom’s nucleus.

It is around 100 times stronger than the electromagnetic force and, being restricted to the nucleus, is a short-range (10-15 m) force. It is the strongest of all fundamental forces

It is independent of charge and acts equally between two protons, two neutrons, and a neutron and a proton.

Electrons, being outside the nucleus, do not experience this force.

It is now known that protons and neutrons are comprised of even more elementary constituents known as quarks.

### Weak nuclear force

The weak nuclear force is only seen in a handful of nuclear processes like the β-decay of a radioactive nucleus. In this phenomenon, the nucleus emits an electron and an uncharged particle known as the neutrino.

The interaction of a neutrino with other particles gives rise to a weak nuclear force.

The weak nuclear force is stronger than the gravitational force but much weaker than the electromagnetic and strong nuclear forces.

Its range is extremely small, of the order of 10-16 m.

## Towards unification of forces

Unification is one of the basic quests in physics. For several years, physicists have been wondering whether all the fundamental forces we’ve discussed earlier arise from a single force.

Eminent scientists like Newton, Maxwell, Einstein, Faraday, and Oersted have contributed to this venture.

## Nature of physical laws

Whenever we are dealing with a physical phenomenon governed by various forces, it is observed that many physical quantities tend to change with time, while certain special quantities remain constant.

Physical quantities which don’t change with time are known as conserved quantities. Let us now discuss the fundamental laws of conservation in physics.

### Law of conservation of energy

Energy can neither be created nor be destroyed; it can only be converted from one form to another.

The total amount of energy in the universe remains constant.

### Law of conservation of charge

The algebraic sum of charges always remains constant during any process taking place in an electrically isolated system.

### Law of conservation of linear momentum

As long as there is no resultant external force on a system, the total linear momentum of the system remains constant.

### Law of conservation of angular momentum

As long as there is no resultant external torque acting on a system, the total angular momentum of the system remains constant.

These laws of conservation have a deep connection with symmetries of nature, which are explored in detail in more advanced courses in physics.

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