Newton’s first law is stated that an object at rest tends to stay at rest and an object in motion tends to stay in motion (same speed, direction and maintained) unless acted upon by an unbalanced force. This law is sometimes referred to as the law of inertia.

Inertia is the reluctance of an object has to change its state of motion. All objects resist this change in their state of motion. Some objects have more of a tendency to resist the changes in their state of motion; this is because of mass. Mass is the amount of matter in an object. It is represented using kilograms (kg). Mass is directly proportional to inertia; inertia is solely dependent upon mass. The more mass an object has, the more inertia an object has- the more of a tendency an object has to resist motion and vice versa. Do not confuse mass and weight. Weight is the measure of the gravitational force acting upon an object. Even though mass and weight are proportional, they are not the same thing.

When we say “state of motion” what does that really mean?  It refers to velocity-speed in a given direction. This means that inertia also is the tendency of an object to resist changes in its velocity. So if an object is at rest, it has zero velocity and will remain at zero velocity (unless compelled to change by unbalanced forces). When an object is at rest, we refer to it as being in equilibrium; no net force is acting upon an object and is in a state of balance. But if an object is moving at a constant velocity, but not changing, it is said to have an acceleration of 0m/s/s. Therefore inertia is, as well, the tendency of an object to resist accelerations.