Inertia is the tendency of objects that are in motion, to stay in motion. This also applies to objects at rest. The amount of inertia is in proportion with the amount of mass in an object. An elephant is extremely harder to get moving than a mouse. Also, if both an elephant, and a mouse were charging at you, the elephant would definitely be harder to stop. This concept is known as Newton's first law of motion. Objects in motion will stay in motion at the same velocity unless acted on by an outside force. Again, objects will stay at rest, unless an outside force(Being a push or a pull) acts on it.