Newtons proposed the concept of inertia. According to him an object at rest tends to stay at test and an object in motion tends to continue in motions unless acted up on by an external force. This is the principle of inertia. The tendency of the body to continue to stay as it was is called its inertia. You can demonstrate this principle with a simple experiment. As shown in the video when the hoop is pulled out the pen cap falls in to the bottle. Here the hoop is acted upon but the cap is not. As a result the pen cap falls straight into the bottle and does not come along with the hoop. The pen drops down because of the gravitational force on it downwards. This video demonstrates the concept of Inertia which is Newtons first law of motion.