* Inertia: Inertia is the tendency of an object to resist changes in its motion. The more massive an object, the more inertia it has.

* Mass and Inertia: Mass is a direct measure of inertia. More mass means more inertia.

Since a 5kg object has less inertia than another object, that other object must have a greater mass than 5kg.

To be more specific, the other object must have a mass greater than 5kg.