A body’s resistance to changes in motion or speed is called inertia.
Inertia is a property of matter described by Newton’s first law of motion. An object at rest tends to remain at rest, and an object in motion tends to continue moving in a straight line at constant speed unless an external force acts on it. Inertia is not a separate force; it is the tendency of an object to resist changes in its velocity.
Mass measures how much inertia an object has. A loaded truck is harder to accelerate, stop or turn than a bicycle because the truck has greater mass and therefore greater inertia. “Resistance to changes in speed” is a useful shorthand, but inertia also resists changes in direction. Seat belts demonstrate this principle: when a car stops suddenly, passengers’ bodies tend to keep moving forward.