**Inertia** is the property of matter that arises from the conservation of

energy in

Newton's laws of motion, which leads to the conservation of momentum, and thus, as stated by

Isaac Newton in one of his laws of physics, "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."
More precisely, inertia is the property of an object that requires the application of an external

force in order to change the object's

velocity 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.

The underlying reason for this property of matter is unknown.

All Wikipedia text
is available under the
terms of the GNU Free Documentation License