Inertia, the tendency of an object at rest to remain at rest and of an object in motion to remain in motion. Inertia is a property of matter and is described by Sir Isaac Newton's first law of motion: Unless acted upon by an outside force, a body at rest will remain at rest and a body in motion will continue to move in a straight line with uniform speed. A child on a sled, if the rope is suddenly jerked, is left sitting on the snow; or a passenger in an automobile, if the car suddenly stops, pitches forward. These instances illustrate inertia.