Keep in mind that inertia is the ability of an object to resist changes in its motion and direction. Logically, an object of a larger mass will be much harder to move and change direction when compared to that of a smaller mass. If a train was traveling at 30 mph and so was a car, the train would be obviously more difficult to stop even though both vehicles were moving at the same speed. This is because the train has a greater mass. Therefore, larger masses in motion or at rest will require a greater amount of effort to actually move or stop. Also, inertia and mass not-coincidentally share the same units because they are directly related to one another