Newton's first law states that an external force is required for any change in the object's state of motion. To understand this better, consider a ball at rest. The forces acting on it are the gravitational and the normal force that are equal in magnitude and opposite in direction. Hence, the net force is balanced. If, however, the ball is pushed softly from one side, the forces are unbalanced. There is a net force acting on the object and the ball starts rolling in the direction of force. Once in motion, no net force acts on the ball, but the ball keeps rolling. It would have continued to roll forever if it were not for the air resistance and the frictional forces. These act opposite to the direction of motion and bring the ball to rest. The object's resistance to change in its state of rest or motion is called inertia. Thus, the first law is also known as the law of inertia. Inertia offered by any object is directly proportional to its mass.