Understanding Mass
Fundamental Quantity in Physics Measures the Amount of Matter
An object's mass is a constant inherent property that measures the amount of matter. An object's mass is related to its inertia and gravitational force.
Mass
Physicists use the concept of mass to measure the amount of matter. However, mass is such a fundamental property of matter that it is difficult to define it in an understandable way. Defining something requires using concepts that are more fundamental than whatever is being defined. There are few if any physics concepts more fundamental than mass that can be used to define mass.
Mass is therefore usually defined operationally by comparing an object to the International Standard Kilogram. This standard kilogram is a platinum-iridium bar stored at the International Bureau of Weights and Measures in France. It is defined to have a mass of exactly one kilogram. An object twice as massive as the standard kilogram has a mass of two kilograms, and so forth.
The kilogram is the standard unit for measuring mass. Weight and other forces are measured in Newtons which are kilograms meters per second squared. In the British system, weight and other forces are measured in pounds and masses are measured in slugs.
Mass and Newton's Inertia Law
An object's mass is a measure of its inertia. An object's inertia is its resistance to an acceleration when a force acts on it. Accelerating, or changing the velocity, of a more massive object requires a greater force. Hence a more massive object has more inertia. The mass that measures an object's inertia is called the inertial mass.
Mass and Gravitational Force
Mass also enters in to Newton's law of gravity. The gravitational force, both acting on and exerted by, any object depends on the object's mass. The mass related to the gravitational force is the gravitational mass.
In the early twentieth century, Albert Einstein wondered if the gravitational and inertial mass were really the same thing. Most physicists had assumed that they were, but Einstein thought about the question deeply. His conclusion that the inertial and gravitational mass were indeed the same thing led to his general theory of relativity, which is a theory to explain gravity.
Mass as a Fundamental Property of Matter
An object's mass is a fundamental unchanging property of that object. An object's mass will not change by moving the object to a different location or changing the object's environment in any way. For example, a 70 kg astronaut will have a mass of 70 kg on Earth, in the space shuttle, on Mars, or anyplace else in the universe. The astronaut will however have a different weight at all these locations.
Mass and weight are not the same thing. An object's weight is the gravitational force acting on it. Mass and size are also different things. It is possible for an object to be more massive yet smaller than another object. A kilogram of lead will be smaller in size than a more massive five kilogram bag of feathers.
Mass, measured in kilograms, is a fundamental property of matter that measures an object's inertia and its gravitational force.
Further Reading
Knight, R.D., Physics for Scientists and Engineers, Pearson, 2004.
Hecht, E., Physics: Algebra/Trig, Brooks/Cole, 1997.
