Inertia has a great deal to do with mass. The larger the mass, the more inertia. The less mass, the less inertia. It is much more simple to play golf with a small golf ball, rather than a bowling ball. The much larger bowling ball, is harder to move than the golf ball.
To change motion of an object, inertia must be overpowered by an unbalanced force acting on the object. Like, if a small man were wrestling a sumo. The unbalanced object would be the sumo acting on the small man. The small man may be still and braced, ready for the sumo to attack. But when the sumo lunges and hits the man, the force is suddenly unbalanced, and the inertia is overcome. The small man will then go flying across the room.
The man will then keep moving after he gets hit by the sumo. The inertia keeps him moving, until acted upon by something. He could keep moving forever in fact, if it weren't for friction and gravity. The man will hit the ground, because of gravity, and be stopped by friction. Friction opposes the motion of anything and everything, which is why the man will stop on the floor. If only he could just slide away, and avoid a lot more pain.