Newton’s first law is also called “The law of Inertia”, but what is the inertia of a body? If an object is at rest, it tends to remain at rest and if it is in motion, it tends to stay in motion. In other words, the inertia of a body is the resistance of any physical object to any change in its velocity.

The inertia of a body varies with mass: a more massive object has a greater tendency to resist changes in its state of motion, and therefore a higher inertia.

For example, as shown in Fig. 1, when the bus driver brakes suddenly, the lower part of a passenger body, which is in contact with the bus, comes to rest as the bus comes to rest but, the upper part of the passenger body continues to move forward due to the inertia of motion. As a result, we fall in the forward direction.

Figure 1: On the left the bus is moving with uniform motion in a straight line and the passenger has the same motion. On the right, when the bus stops, the part of the body (lower part) in contact with bus stops, but due to inertia the upper part continues to move forward and thus he moves forward.