The name of the force that causes objects in motion to stay in motion is called inertia. Inertia is a fundamental principle in the field of physics, specifically articulated in Newton's First Law of Motion. This law states that an object will remain at rest or continue moving in a straight line at constant speed unless acted upon by an external force. Inertia is directly related to an object's mass, meaning the greater the mass, the greater the amount of force required to change its motion.