It is the first law of Newton.The "Law of Inertia". It states that: "The object at rest will remain at rest or the object in motion will remain in motion unless it is acted upon by unbalanced force.

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It is the first law of motion or the Law of Inertia.
"An object at rest remains at rest, and a body in motion remains in motion with a constant speed, unless acted upon by an unbalanced force."
⇒ The bigger the mass of an object, the harder it moves.
   ⇒ Mass is the measure of Inertia.
   ⇒ Resist stopping or changing.
          · Train that suddenly stops.
          · Pulling a tablecloth from under the dishes.
          · The example earlier