According to the principle of inertia, what will a moving object do unless a force acts on it?

Inertia is the tendency of an object to resist changes in its state of motion (Newton's first law). An object in motion will continue moving in a straight line at a constant speed unless an unbalanced force acts on it, and an object at rest will stay at rest unless a force moves it. In everyday life, friction and air resistance usually act on moving objects and slow them down, which can make it seem like things stop 'on their own,' but it is actually those forces causing the change. Understanding inertia — objects keep doing what they're doing without a net force — is a key force-and-motion concept.