What is Mass? A mass of a physical body is the amount of matter it contains. It’s also a measurement of the body’s inertia, or resistance to acceleration (velocity change) when a net force is applied. The degree of an object’s gravitational pull on other bodies is also measured by its mass.
The basic unit of mass is the kilogram (kg) in the SI. Even though mass is generally established by measuring the object’s weight on a spring scale instead of comparing it directly to known masses on a balancing scale, mass is not the same as weight in physics. Because of the reduced gravity on the Moon, an object would weigh less than it does on Earth, yet it would still have the same mass. This is because weight is a force, whereas mass is the parameter that determines the degree of that force (together with gravity).