Defining Inertia
Inertia is the resistance of any physical object to any change in its state of motion. This means an object at rest will stay at rest, and an object in motion will stay in motion at a constant velocity, unless acted upon by an external force.
Inertia in Action: An Example
Imagine you are in a car that suddenly stops. Your body continues to lurch forward because of inertia. This is because your body 'wants' to continue moving at the same speed the car was. Seatbelts provide the external force needed to safely stop your body along with the car. An object's inertia is directly related to its mass—the more mass an object has, the more inertia it has.