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Mass
Mass is related to matter, which is any physical substance. Mass measures the quantity of matter in an object. Scientists typically measure mass by weight rather than height or width because an object that appears very large, like a blown-up balloon might not be very dense and may therefore have less matter than a physically smaller object.
Inertia
The tendency of a physical object to resist changes in motion is called inertia. This tendency is captured in the well-known aphorism, "A body at rest tends to stay at rest while a body in motion tends to remain in motion." This tendency, of course, does not mean that something that is moving will never stop moving. It simply means that if nothing interferes with that movement, the object will likely remain moving. The law of inertia is Isaac Newton's first law of motion.
Mass Inertia Relationship
Objects with more mass have more inertia. Thus, a very large boulder will be more difficult to move than a very small pebble. Similarly, once the large boulder starts moving, it will be more difficult to stop than the small pebble.
Applications and Examples
The relationship between mass and inertia is frequently demonstrated through a simple magic trick. When a magician pulls a very lightweight tablecloth out from underneath heavier objects, the objects don't move if the trick is performed correctly. This is an example of mass affecting inertia. The table objects' inertia is not affected by the motion of the much lighter tablecloth, and consequently they remain still. Students can also witness examples of inertia by rolling heavy and light balls and measuring how far each goes. If neither ball hits a wall and both are pushed using the same amount of force, the heavier ball will travel further.