Inertia is the tendency of an object to resist changes in its motion. This means it applies to both:
* Objects at rest: An object at rest will stay at rest unless acted upon by a force. This is due to its inertia.
* Objects in motion: An object in motion will stay in motion at a constant velocity (same speed and direction) unless acted upon by a force. Again, this is due to its inertia.
Think of it this way:
Inertia is like the "laziness" of an object. It doesn't want to change its state of motion. Whether that state is being still or moving at a constant speed, the object will resist any attempts to change it.
