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:
1. Gravitational force
2. 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
1. Strong nuclear force
2. 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
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 and separated by distance r is given by
F = G m1m2/r2
where G is the universal gravitational constant. Important 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 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 centres 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:
1. All bodies fall because of the gravitational force of attraction exerted on them by the earth.
2. Gravitational force governs the motion of the moon and the artificial satellites around the earth; and the motion of the planets around the sun.
3. Gravitation plays a key role in the formation and evolution of stars, galaxies and galactic clusters.
THE 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 q1 and q2 separated by distance r in vacuum is given by
F= q1q2/4πε0 r2
where ε0 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 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 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 force.
5. It is a conservative force.
6. It is 1036 times stronger than the gravitational force.
7. It is caused by the exchange of photons (γ) between two charged particles.
Examples of electromagnetic force:
1. When a spring is compressed/elongated, it exerts a force of elasticity due to the net repulsion / attraction between its neighbouring atoms. This net repulsion or attraction is the sum of the electrostatic forces between the electrons and nuclei of the atoms.
2. The Van der Walls’ force between two neutral molecules of a gas is net sum of the electrostatic forces between the electrons and nuclei of the two molecules.
ELECTROMAGNETIC NATURE OF SOME MACROSCOPIC FORCES
The macroscopic forces of our daily life such as tension, friction, contact force, spring force, etc., are derived from the fundamental electrostatic force only. All matter consists of charged particles like electrons and protons. The strong electromagnetic force between these particles is responsible for the structure of atoms and molecules, rate of chemical reactions, and the mechanical, thermal and electrical properties of materials.
(i) Contact force between two bodies:
When we place two bodies in contact with each other, their atoms come close to each other at the surface of contact. Large electromagnetic forces begin to act between the charged constituents of these atoms.
Generally, these forces act normal to the surface of contact and are of pushing or repelling nature. For example, a book lying on a table pushes it downwards while the table pushes the book upwards.
(ii) Force of friction:
Sometimes, the electromagnetic contact force between two bodies may have a component acting parallel to the surface of contact. This component is called friction. When bodies are placed with their smooth surfaces in contact, they provide only a small parallel component of contact force and hence friction between them is negligibly small. For example, when we climb up a tree, its rough trunk provides sufficient frictional force parallel to the surface of the tree which helps us cling on to the trunk. It is difficult to climb up a smooth metallic lamp post because it does not provide enough parallel frictional force.
(iii) Elastic force in a spring:
When a spring is elongated (or compressed), it exerts a force of elasticity which arises due to the net attraction (or repulsion) between the neighbouring atoms of the spring. This net attraction (or repulsion) is the (imbalanced) sum of the electrostatic forces between the electrons and nuclei of these atoms.
(iv) Van der Walls’ force. The Van der Walls’ force between two neutral molecules of a gas is not a fundamental force but a derived force. It is the net residual force obtained by summing up the fundamental electrostatic forces between the various electrons and nuclei of the two molecules.
(v) Force in a taut string/rope:
Consider a block tied to the lower end of a string suspended from a rigid support. The string is in a state of tension. The electrons and protons of the lower end of the string exert electrostatic forces on the electrons and protons of the block. The resultant of these forces balances the weight of the block. Generally, a string under tension exerts an electromagnetic force of pulling nature on the two bodies attached to its two ends.
Electromagnetic force is much stronger than the gravitational force
Electromagnetic force is much stronger than the gravitational force When we hold a book in our hand, the earth exerts a very large gravitational force on the book due to its huge mass. This force is balanced by the normal force exerted by our hand. But the latter force is the net electromagnetic force between the charged constituents of our hand and the book at the surface of contact. Clearly, electromagnetic force is much stronger than the gravitational force.