Newton's first law of motion is also known as the:

Sir Isaac Newton stated his three laws of motion in his book "Philosophiae Naturalis Principia Mathematica" ("Principia"), published in 1687.

Newton's first law of motion states that an object at rest stays at rest, and an object in motion continues to move with the same speed in a straight line, unless it is acted upon by an external unbalanced force.

This law is called the law of inertia. Inertia is the natural tendency of an object to resist any change in its state of rest or of uniform motion. The idea was first developed by Galileo Galilei, who showed through experiments with balls rolling on inclined planes that an object would keep moving forever if there were no friction.

The inertia of an object depends on its mass: the greater the mass, the greater the inertia. A loaded truck is harder to start or stop than a bicycle because it has more inertia.

Inertia is of three types:

Inertia of rest: a body at rest tends to remain at rest. For example, when a bus starts suddenly, passengers fall backward; dust falls out of a carpet when it is beaten; a coin on a card falls into the glass when the card is flicked.

Inertia of motion: a body in motion tends to stay in motion. For example, when a moving bus stops suddenly, passengers fall forward; an athlete runs some distance before a long jump.

Inertia of direction: a body tends to keep moving in the same direction. For example, when a car turns sharply, passengers are pushed outward; mud flies tangentially off a rotating wheel.

Seat belts in cars protect passengers from the effects of inertia during sudden stops.

Newton's second law (F = ma) states that the rate of change of momentum is proportional to the applied force, and the third law states that to every action there is an equal and opposite reaction.

Why not the other options?

• ✗The law of acceleration (F = ma) is the second law.
• ✗The law of action and reaction is the third law.
• ✗The law of gravitation is a separate law of Newton.