Inertia is the tendency of an object to resist changes in its motion. The more massive an object is, the harder it is to get it moving, to stop it once it's moving, or to change its direction of motion.

Here's a simple way to think about it:

* Imagine pushing a small, lightweight box: It's easy to get it moving and to stop it.

* Now imagine pushing a large, heavy refrigerator: It's much harder to get moving and to stop it.

The refrigerator has much greater inertia because it has much greater mass.

In summary: Mass and inertia are directly proportional. The greater the mass of an object, the greater its inertia.