** Motion:- **When a body changes its position with respect to other bodies, then body is called as to be in motion.

The entire subject of rigid body mechanics is based on three fundamental law of motion given by an American scientist Newton.

**01. Newton’s first law of motion:- **According to Newton’s first law, every body continues in its state of rest or of uniform motion in a straight line, unless it is compelled by some external force to change the state.

Newton’s first law can be divided into two parts

** Inertia: –** This inherent properties of the bodies that they do not change their state unless acted upon by an external force, is called inertia. It is because of inertia that a body cannot change its state of rest or of uniform motion by itself.

** Force**: – force may be called as an agent that causes or tends to cause acceleration. Therefore, Force is a pull or push which generates or tends to generate motion in a body at rest and stops or tends to stop a body which is in motion, increases or decreases the magnitude of the velocity of the moving body or change the direction of motion of the moving body.

The unit of force is Newton.

**1-Newton: **It is magnitude of force, which develops an acceleration of 1 m/s² in 1 kg mass of the body.

**02. Newton’s second law of motion: – **According to Newton’s second law, the time of change of momentum of a body is directly proportional to the external force applied on it and the change in momentum takes place in the direction of force.

Therefore, if a body of mass ‘m’ moving with a velocity ‘v’ has got a momentum‘mv’,

then , F ∝ d(mv)/dt

Or. F = k × d(mv)/dt = m × dv/dt = ma

(Here, k = 1, constant of proportionality and dv/dt = a, acceleration produced in the direction of the body)

** Mass:** The properties of matter by which the action of one body can be compared with that of another is defined as mass.

* So, m *= ₰ × v

Where, ₰ = Density of body and v = Volume of the body

(Weight of a body is the force with which the body is attracted towards the center of the earth.)

** Momentum:** It is defined as the total quantity of motion contained in a body and is measured as the product of the mass and its velocity.

Momentum = Mass × Velocity = m × v

**03. Newton’s third law of motion: –** It states that to every action, there is an equal and opposite reaction. I.e. The force of action and reaction between interacting bodies are equal in magnitude, opposite in direction and have the same line of action. Therefore,

In general, if a body ‘A’ exerts force ‘FAB‘(which may be gravitational, electrical or magnetic etc.) on a body B, then the body ‘B’ will exert a force ‘FBA‘ on body ‘A’,

such that, FAB = – FBA

**Line of action:** The straight line along which a force is directed is called the line of action of the force.

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