inertia

inertia (ĭnûr`shə), in physics, the resistance of a body to any alteration in its state of motion, i.e., the resistance of a body at rest to being set in motion or of a body in motion to any change of speed or change in direction of motion. Inertia is a property common to all matter. This property was first observed by Galileo and restated by Newton as his first law of motion, sometimes called the law of inertia. Newton's second law of motion states that the external force required to affect the motion of a body is proportional to that acceleration. The constant of proportionality is known as the mass, which is the numerical value of the inertia; the greater the inertia of a body, the less is its acceleration for a given applied force.

---

inertia

Inherent property of a body that makes it oppose any force that would cause a change in its motion. A body at rest and a body in motion both oppose forces that might cause acceleration. The inertia of a body can be measured by its mass, which governs its resistance to the action of a force, or by its moment of inertia about a specified axis, which measures its resistance to the action of a torque about the same axis.