inertia
4.2 Newtons First Law of Motion: Inertia Newtons first law of motion states that a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force. This is also known as the law of inertia. Inertia is the tendency of an object to remain at rest or remain in motion. Inertia is related to an objects mass. Mass is the quantity of matter in a substance.