Inertia is what causes an object to maintain its state of motion. According to the Physics Classroom, Newton's first law of motion states that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This means objects tend to keep doing what they are already doing.
Galileo developed the concept of inertia in the 17th century. He is also responsible for the development of the concept of friction. Friction is the force that opposes inertia. In a world without friction, once an object is in motion, it would not stop until another force caused it to stop. A driver could start the engine on a car, accelerate to highway speed, kill the engine and coast to his destination without using any more energy, because the car is in motion. However, without the force of friction, acceleration, steering and braking, a traditional automobile becomes problematic.
According to Newton's law, there is no need for force to keep an object moving. Instead, it is the lack of an opposing force that keeps it moving. When the opposing force of gravity acts on a body, it causes it to stop moving.