Here's the key relationship:

* Greater mass = Greater inertia: 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.

Think of it this way:

* Pushing a shopping cart: It's much easier to push an empty cart than one filled with groceries. The empty cart has less mass and therefore less inertia.

* Stopping a car: A large truck takes longer to stop than a small car at the same speed because the truck has more mass and therefore more inertia.

In physics, we express this relationship with Newton's First Law of Motion:

* An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

This means that to overcome inertia and change an object's motion, you need a greater force if the object has a greater mass.