### The Four Fundamental Forces in Nature

In the macroscopic world, we observe several kinds of forces : muscular force, contact forces of support and friction, forces exerted by springs and strings, viscous forces, electric forces, magnetic forces, etc.

All these forces between macroscopic objects arise from two fundamental forces:
• Gravitational force
• Electromagnetic force
In the microscopic world, in addition to the above two forces, two more basic forces are required to account for the various atomic and nuclear processes. These are
• Strong nuclear force
• Weak nuclear force

The ratio of the strength of the four fundamental forces in nature is

Fg:Fw:Fe:Fs = 1 : 10^25 : 10^36 : 10^38

## 1. The Gravitational Force

It is the force of mutual attraction between two bodies by virtue of their masses. It is a universal force. Every body attracts every other body of the universe with this force. According to Newton's law of gravitation the gravitational attraction between two bodies of masses m1 and m2 are separated by a distance r is given by

where G is the universal gravitational constant.

### Important Properties of Gravitational Force

• It is a universal attractive force.
• It is directly proportional to the product of the masses of the two bodies.
• It obeys inverse square law.
• It is a long range force and does not need any intervening medium for its operation.
• It is the weakest force known in nature.
• It is central force (i.e., it acts along the line of joining the centre of two bodies).
• It is a conservative force (i.e.,work done in moving a body against the gravitational force is path independent).
• Gravitational force between two bodies is thought to be caused by an exchange of a particle called graviton.

#### Examples of Gravitational Force

• Gravitational force governs the motion of the moon and the artificial satellites around the earth; and the motion of the planets around the sun.
• Gravitation plays a key role in the formation and evolution of stars, galaxies and galatic clusters.

## 2. The Electromagnetic Force

The force acting between two electric charges at rest is called electrostatic force. According to the coulomb's law, the magnitude of the electrostatic force F between two point charges q1 and q2 separated by a distance r in vacuum is given by

where k is the permittivity of vacuum. The force acting between two magnetic poles is called magnetic force. In fact, electrostatic and magnetic forces are closely interrelated.

For example: A moving charge produces a magnetic field. Also, a magnetic field exerts a force on a moving charge. This force depends both on the magnitude and direction of the velocity of the electric charge. Thus the electrostatic and magnetic forces are inseparable and are considered as two facets of a general force known as electromagnetic force.

### Important Properties of Electromagnetic Force

• Electromagnetic force may be attractive or repulsive. Like charges repel each other and unlike charges attract each other.
• It obeys inverse square law.
• It is a long range force and does not require any intervening medium for its operation.
• It is central force.
• It is a conservative force.
• It is 10^36 times stronger than the gravitational force.
• It is caused by exchange of photons (γ) between two charged particles.

### Examples of Electromagnetic Force

• When a spring is compressed, it exerts a force of elasticity due to the net repulsion/ attraction between its neighbouring atoms. This net repulsion or attraction in the sum of the electrostatic forces between the electrons and nuclei of the atoms.
• The Van der Walls' forces between two neutral molecules of a gas is net sum of the electrostatic forces between the electrons and nuclei of the two molecules.

## 3. The Strong Nuclear Force

The strong attractive force which binds together the protons and neutrons in a nucleus is called strong nuclear force. This force cannot be electrostatic force because positively charged protons strongly repel each other at such small separation of the order of 10^- 15 m.

Also the gravitational attraction between the two protons being much weaker, cannot overcome this electrostatic repulsion. So a new attractive force must be acting between the nucleons (protons and neutrons). This strong nuclear force is the strongest of all the fundamental forces, about 100 times stronger than the electromagnetic force.

### Important Properties of Strong Nuclear Force

• It is the strongest interaction known in nature, which is about 10^38 times stronger than the gravitational force.
• It is a short range force that operates only over the size of the nucleus.
• It is basically an attractive force, but becomes repulsive when the distance between the nucleons becomes less than 0.5 fermi.
• It varies inversely with some higher power (>2) of distance.
• It is a non-central and non-conservative force.
• It is a charge independent character i.e., nuclear forces between proton-proton, proton-neutron, neutron-neutron are almost equally strong.
• It is caused by the exchange of particles, called II-mesons.

### Examples of Nuclear Force

• Nuclear forces bind together the protons and neutrons in the nuclei. So they are responsible for stability of nuclei and hence of the atoms and ultimately of all matter that exists in the universe.
• Radioactivity occurs in heavier nuclei because of insufficient nuclear force between their protons and neutrons.
• The concept of nuclear force is useful in obtaining nuclear energy via the processes of  nuclear fission and fusion.

## 4. The Weak Nuclear Force

It is the force that appears only between the elementary particles involved in a nuclear process such as the beta decay of a nucleus. In a beta decay, the nucleus emits an electron and an uncharged particle called neutrino. The electron and neutrino interact with each other through the weak nuclear force. The weak nuclear force is much stronger than the gravitational force, but much weaker than the strong nuclear and electromagnetic forces. This is obvious from the fact that the decay of an elementary particle caused by weak nuclear force (e.g., the decay of a pion to a muon and a neutrino) is much slower than the decays caused by strong nuclear or electromagnetic forces.

### Important Properties of Weak Nuclear Force

• Any process involving neutrino and anti-neutrino is governed by weak nuclear force because these particles can experience only weak interaction and not the strong nuclear interaction.
• Weak nuclear force is 10^20 times stronger than the gravitational interaction.
• It operates only through a range of nuclear size.
• The messenger particles that transmit the weak force between elementary particles are the massive vector bosons (W+-,Z).

That's all about four fundamental forces in nature!!