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Inertia is the property of inertness. Something that has inertia will tend to do little or nothing. In common usage, inertia has a different meaning: something that has inertia will not (or will rarely) change on its own.

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1 Physics definition

Physics definition

In physics, all matter has inertia. An object with inertia will not accelerate without a certain type of impetus, called a force.

Classical background

The conservation of energy in Newton's laws of motion leads to the conservation of momentum. Therefore :

"An object in motion tends to stay in motion, and an object at rest tends to stay at rest unless acted upon by an external force." – Isaac Newton

Relation to Relativity

With relativity, acceleration is relative to an inertial frame. Inertial mass is a measure of inertial frame-independent inertia, and momentum is a measure of inertial frame-relative inertia.