The natural tendency (as defined in Sir Isaac Newton’s First Law of Motion of 1687) of objects to resist changes in their state of motion. Therefore, a body at rest tends to stay at rest and, once set in motion, a body tends to stay moving at a constant speed in a straight line (or along a geodesic in curved space) unless acted on by an outside force. An example of an inertial force is centrifugal force, which in reality is just due to a body trying to continue in a straight line while constrained to move along a curved path.