Mass and inertia have an inherent relationship with each other, in physics. They are directly proportional. When a body has more mass, it will have more inertia. However, the former is not the weight of an object. This is so because a truck loaded with steel beams can weight many tons on the surface of the Earth, out there in outer space it will be weightless, but it will still have mass and inertia. Mass is also directly related to the pull of gravity; a large and dense planet with a strong mass will exert a lot more gravity than a small one; thus, the more mass, the stronger the pull of gravity.
Definition and concept
Mass is a fundamental universal property that gives a body inertia. It is inherently related to inertia and the amount of matter it contains, giving the body a gravitational field of its own. So, we can say that mass is directly proportional to inertia, and the amount of matter a body contains; the more mass an object has, the more inertia it will embody. We should not confuse mass with weight, for weight is the pull of gravity on the surface of a celestial body. For example, a truck weighing 20 tons on the surface of the earth, out there in space free from the force of gravity, the truck will be weightless, but it will continue to have mass and inertia.
Inertia is the inherent property and universal tendency that a body has to maintain its state of motion, or rest, in which it finds itself. Inertia is directly proportional to mass and to the gravitational field a body has, that is to say, the more mass a body contains, the more inertia it deploys, and the stronger the gravitional field it will exert. For examples, a truck loaded with steel bars will be a lot harder to stop than an empty one, because it has more mass (density) than the empty one; a 3-cm-wide steel ball (we took out of a tractor ball-bearing) has a lot more mass than a big styrofoam sphere which is 3 m in diameter.