The amount of inertia exhibited by an object depends upon its mass. An object with a smaller mass exhibits a smaller amount of inertia, while an object with a large mass exhibits a large amount of inertia. The two concepts are completely interrelated.
Inertia is the tendency in an object to resist changes in its state of motion, including changes to its speed and direction. Due to inertia, objects display the tendency of moving in a straight line at constant velocity.
Inertia is one of the fundamental principles of physics. Isaac Newton defined the principle of inertia in the first law of his "Philosophiae Naturalis Principia Mathematica," although he did not label it as inertia. Inertia may also be explained as the amount of resistance displayed by an object to change its velocity. This amount is quantified by its mass.
On the Earth's surface, inertia is often difficult to ascertain due to the effects of friction, air resistance and gravity. Mass is not the same as weight, although mass is often calculated by measuring an object's weight. Weight is affected by external forces, such as gravity, while mass is not. For example, a man on the moon would weigh differently than a man on Earth but would have the same mass.