PHYSICAL QUANTITIES IN LINEAR MOTION:

1.INERTIA :

LOOK AT A ROCK, DOES IT MOVE WHEN WIND BLOWS?
LOOK AT A BODY AT REST, UNLESS YOU PUSH OR PULL, IT CANNOT MOVE AT ALL.
LOOK AT THE SMOKE COMING OUT FROM A JET PLANE FOR  SOME SECONDS. YOU CAN SEE THE SMOKE IS MOVING SWIFTLY WITH A CONSTANT(SAME) SPEED.

ALSO LOOK AT THE FLOW OF A RIVER, EVEN IT MAY APPEAR SOMETIMES FLOWING  IN A CONSTANT SPEED.

ANY OBJECT WHICH IS AT REST OR ANY BODY WHICH IS MOVING AT A CONSTANT SPEED IS SAID TO POSSESS A PHYSICAL QUANTITY NAMED INERTIA.

INERTIA IS THE INABILITY OF A BODY ,TO CHANGE BY ITSELF ITS STATE OF REST               
 (OR)                                                         
INABILITY OF A BODY (WHICH IS  IN A LINEAR MOTION) TO CHANGE BY ITSELF ITS STATE OF CONSTANT (SPEED)MOTION.

INERTIA IS DIRECTLY PROPORTIONAL TO MASS .
i.e. IF THE MASS OF A BODY INCREASES , THEN INERTIA WILL ALSO INCREASE.
FOR EXAMPLE, A PAPER CANNOT STAY AT REST EVEN FOR A SMALL GUST (FLOW OF AIR), BECAUSE IT IS A LIGHTER OBJECT.i.e.ITS MASS  IS LESS.
BUT  A ROCK CANNOT FLY OR MOVE ,WHEN THERE IS A HEAVY FLOW OF AIR. BECAUSE ROCK'S MASS IS MORE THAN THAT OF A PAPER.
A BOOK ON THE TABLE WILL BE AT REST IF IT IS NOT GIVEN ANY EXTERNAL AGENCY CALLED FORCE.
FROM THE ABOVE EXAMPLES ,ROCK AND THE BOOK ARE SAID TO BE  IN INERTIA.