Newton’s First Law: An object in motion will remain in motion at a constant velocity, unless acted on by an unbalanced force.
What this means: If all of the forces on an object balance (a state called equilibrium) the object will move at a constant speed and direction. (The speed may be zero, but does not have to be.) Therefore, equilibrium = no unbalanced force = constant speed.
Inertia: Inertia is an object’s resistance to a change in speed. Inertia is equivalent to an object’s mass.