Inertia is the tendency of an object to resist changes in its state of motion. This means that an object at rest will stay at rest, and an object in motion will continue moving at a constant speed in a straight line unless acted upon by an external force. This principle is a key concept in Newton's First Law of Motion. Inertia depends on the mass of an object; the greater the mass, the greater the inertia. For example, it is much harder to push a heavy boulder than a light ball because the boulder has more inertia. Understanding inertia helps explain everyday phenomena, such as why passengers lurch forward in a car when it suddenly stops.