Inertia
For example, a rubber ball will not start bouncing around unless someone picks it up and throws it. Basically, if an object is not moving, it will not start moving unless something else acts upon it. The same idea can be applied to motion: an object in motion will stay in motion unless some outside, opposing force acts upon it. Inertia is also called Sir Isaac Newton's First Law of Motion. The First Law of Motion says that: