What Is the Law of Inertia?

Christina Lappas/CC-BY-2.0

The law of inertia states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. The law of inertia is sometimes referred to as Newton’s first law of motion.

Sir Isaac Newton presented his three laws of motion for the first time in 1686 in “Principia Mathematica Philosophiae Naturalis.” The first law, the law of inertia, states that an object’s velocity will change only if there is an external force making it change.

A good example of this law as seen in every day life is that of a kite flying in the sky. The kite will remain in the air, at its current velocity unless it is changed by the speed or direction of the wind. Another excellent example of Newton’s first law is seen when a package is dropped out of a moving airplane. The package will continue falling at the speed at which the airplane was moving until an external force, in this case, gravity, makes velocity change.

If an object has no velocity, that object remains at rest. Nothing will change that unless an external force moves it. The amount of change that object is subjected to is explained by Newton’s second law of motion which states that a force is equal to the change in momentum.