Here's why:

* Newton's Second Law of Motion: This law states that force (F) is equal to mass (m) times acceleration (a): F = ma.

* Acceleration and Stopping: Acceleration is the rate of change of velocity. To stop an object, you need to decrease its velocity. The greater the acceleration, the faster the object stops.

* Force and Stopping: From Newton's Second Law, we see that to get a larger acceleration (to stop faster), you need either a larger force or a smaller mass.

Example:

Imagine pushing a heavy box and a lighter box at the same speed. It will take much more effort (force) to stop the heavier box because it has more inertia, a resistance to change in motion.

In short: The less massive an object is, the easier it is to stop because it requires less force to change its momentum.