Inertia: An object remains at rest or moves in a straight line with uniform speed if no external force acts on it. Here force means unbalanced force.
This property of an object due to which it resists change in its state of rest or motion is called inertia. If the mass of the object is greater the inertia of object will also be greater.
Two objects of equal mass (one of which is moving and other is stationary) have the same inertia because inertia depends only on mass. It does not depend on velocity and size of object. It has no unit or dimension
Example: (i) we place a smooth card on a glass and place a coin on it. If we hit the card with our finger, the card slides forward but the coin remains at its place due to inertia, hence it falls into glass.
(ii) The principle of inertia is take advantage of unloading grains from sea ships and loading them into goods trains. The grain falls from the ship on a wide belt, which rotates rapidly with the help of rollers. At the other end, the grain runs forward rapidly on the same surface and falls into the compartment of goods train.