Text book definition is "In the absence of forces, ("body") at rest will stay at rest, and a body moving at a constant velocity in a straight line continues doing so indefinitely". My thinking. Moon is orbiting earth in a circular path, and not in a straight line. Still that motion follows the first law So shouldn't the definition be "In the absence of forces, ("body") at rest will stay at rest, and a body in (any state of) motion continues doing so indefinitely". Non-linear motion also can be subject to 1st law. So why is the bolded terms used in text book? I am missing something, Please help me get it.