FORCE & MOTION

"Force is an agent which changes or tends to change the state  of rest or of uniform motion of a body."
In the light of Newton's 2nd law of motion Force may be defined as :
"Force acting on a body is equal to the product of the mass and acceleration produced in the body."
i.e.
F = ma
Force can accelerate or decelerate a body.  Force is a vector quantity.
UNITS OF FORCE
(i) NEWTON (N) in S.I system    (ii) DYNE in C.G.S system    (iii) POUND (Lb) in BRITISH ENGINEERING SYSTEM (F.P.S)
NEWTON
Newton is the unit of force and can be defined as:
"The amount of force that produces an acceleration of 1 m/s2 in a body of mass 1-kg is equal to 1 NEWTON."
1 N = 1 kg x 1m/s2
[ N = kg m/s2]
NEWTON 'S FIRST LAW OF MOTION
STATEMENT:    Newton 's first law of motion states that:
"Every body remains at rest or continues to move with uniform velocity in straight line unless an unbalanced force acts upon it".
EXPLANATION
First law of motion consists of two parts:
PART NO 1:
The first part states that a body at rest remains at rest unless an unbalanced force acts upon it.    This part is in accordance with our common experience for example, a book lying on a table remains at    rest unless it is lifted or pushed by an external force.
PART NO 2 :
Second part states that a body in motion remains in motion with uniform velocity unless an unbalance    force acts upon it. This part is not self-evident because a ball pushed once does not continue its motion    forever. A little consideration however, shows that there is an opposing force like ground friction and air    friction acting in this case. These frictional forces are responsible to stop the ball. If we eliminate these    opposing forces, a body in motion will continue its motion forever.
INERTIA
"Tendency of a body by virtue of which the body at rest or moving with uniform velocity retains its state is called INERTIA."
OR
"Property of a body by which a body resists a force, applied to it to change its state of rest or of uniform velocity is called INERTIA."
INERTIA of a body is directly related to its mass. Heavy bodies have greater inertia while lighter bodies    have little inertia.
LAW OF INERTIA AND THE FIRST LAW OF MOTION
Every body in the universe opposes the force which tends to change its state of rest or of uniform    motion. This property INERTIA is a direct consequence of FIRST LAW OF MOTION. As heavy bodies due    to greater INERTIA requires forces of large magnitude and bodies of small masses require small forces.    By the above explanation of INERTIA we conclude that the state of rest or motion does not change by its    self unless an external force acts upon it, which is according to the FIRST LAW OF MOTION.    Thus the FIRST LAW OF MOTION is also called LAW OF INERTIA.