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