Inertia is the tendency of an object to resist changes in its motion. Objects at rest will stay at rest, and objects in motion will stay in motion, unless acted on by an unbalanced force. The greater the mass of an object, the greater its inertia, meaning more force is needed to change its motion. For example, it takes more force to start moving a heavy piano compared to a small toy piano due to the greater mass and inertia of the full-sized piano. If the net force on an object is zero, it will remain at rest or continue moving at a constant velocity.