## Tuesday, October 03, 2017

Chapter – 1
Physical World
Physics: Physics is the branch of science which deals with nature and natural phenomenon.
Fundamental forces: According to Newton the force of attraction between every pair of particles is directly proportional to the product of the masses of the particle and inversely proportional to the square of the distance between them.
Let A and B are two particle of masses and respectively. Let be the distance between these particles. Newton’s law of gravitation From (1) and (2) we get Where is known as universal gravitational constant
Note: The force of attraction between earth and any other body of the Universe is known as force of gravity or gravity. So gravity is the special case of gravitational force.

Characteristics of gravitational force:
1. Gravitational force between two bodies form an action and reaction pair i.e. forces are equal in magnitude but opposite in direction.
2. Gravitational force is a central force i.e. it acts along the line joining the centres of two interacting bodies.
3. Gravitational force between two bodies is independent the nature of the medium.
4. Gravitational force is conservative force i.e. the work done by this force on a body is independent on the path followed by the body and depends only on the initial and final position of the body.
5. This force is always attractive.
Electromagnetic force:

1. Electromagnetic: The force of attraction between and separated by a distance in is directly proportional to the magnitude of charges and and inversely proportional to square of the distance between them (electrostatic force) Characteristic of Electromagnetic force:
1. It is both attractive as well as repulsive force.
2. It is a long rang force just like gravitational force.
3. It is a central force.
4. It obeys inverse square law.
5. It is a conservative force.
6. Electromagnetic force between two charged particle is due to the exchange of proton between them.
Electromagnetic force: Elective and magnetic effect due to the charges are inseparable, so the force between charges is known as electromagnetic force.
E.g.: The motion of an electron around the nucleus of an atom is due to the electro-magnetic force.
Strong Nuclear force: The force holding the proton and neutron inside a nucleus is known as strong nuclear force.
Characteristics of Strong Nuclear force:
1. It is attractive in nature.
2. It acts over a very short distance i.e. it is a short range force.
3. It is a non-central force.
4. It does not obey inverse square law.
5. It is hundred times stronger than Electromagnetic force and times stronger than gravitational force.
Example of Strong Nuclear force: The proton and the neutron are boiling blocks of a nucleus of an atom. A nucleus enhance atom can wish if there is a strong nuclear force between the protons and the neutrons.

Note: Quarks – Protons and neutrons are made up of more elementary particles called, quarks. Quarks are suppose to possess fractional electric charge.
Weak Nuclear force: The forces between the elementary particle emitted during radio-active decay of a radio-active substance is known as weak nuclear force

E.g.: In B decay, electron and a neutrino are emitted. The force between an electron and a neutrino is known as Weak Nuclear force.
Relative strength of various forces:
 S.N. Name of the force Relative magnitude 1 Strong Nuclear force 1 2 Electromagnetic force 3 Weak nuclear force 4 Gravitational force Laws of conservation of charge: According to this law the total charge of an isolated system remain constant.

Or
According to this law charge can neither be created nor be destroyed but it can be transferred from one body to another, i.e. total charge of a system under this force remains the same.
Laws of conservation of mass energy: According to Einstein, mass and energy are inter convictable quantities i.e. mass of a system can be converted into energy and energy of a system can be converted into mass. But, the sum of mass and energy of system remains constant.
The energy , equivalent to mass is given by the relation. Where, = meaning mass and velocity of light in vacuum.
Law of conservation of energy: According to this law energy can neither be created nor can be destroyed but it can be changed from one form to another form.
E.g.: Total energy of a vibrating implies pendulum also remains the same at any instant.
Laws of conservation of linear momentum: Linear momentum of a body is the product of its mass and velocity i.e. . According to the law of conservation of linear momentum the total momentum of an isolated system remains conserve.
Laws of conservation of angular momentum: Angular momentum of a system or a particle about and axis of rotation is defined as the product of linear momentum of the system or the particle and its perpendicular distance from the axis of rotation i.e. of the particles into perpendicular distance of the particle from the axis of rotation.
According to the laws of conservation of angular momentum, the total angular momentum of a system remains conserve if no external torque at acts on it.
Q. Who discovered neutron?