Principle of Inertia

Isaac Newton's First Law of Motion states that: Every object continues in its state of rest or of uniform motion unless it is acted upon by an external force.

Which means that, if something is moving, then it will keep on moving until something else stops it (Usually friction against air particles).

Or if something is stationary, then it will not move until something else moves it (Wind, or a itchy foot)

Inertia as defined by Wiki: Inertia is a non-quantifiable property of matter by which it remains at rest or in uniform motion in the same straight line unless acted upon by some external force.

Or defined by textbook as: Property of the mass of a body which resists change from its state of rest or motion.

The larger the mass of the object, the larger its inertia.

Refer to phyT/B pg31 and 32.