In classical mechanics, an object is considered to be "in motion" when it undergoes a change in its position with respect to a reference point or frame of reference over a period of time. Motion is typically described in terms of displacement, velocity, and acceleration.
Displacement: Displacement refers to the change in position of an object from its initial position to its final position. It is a vector quantity and is defined by both magnitude and direction. For example, if an object moves 5 meters to the right, its displacement is +5 meters in the chosen reference frame.
Velocity: Velocity describes the rate at which an object's position changes with respect to time. It is a vector quantity that includes both speed and direction. The average velocity is calculated as the displacement divided by the time taken. If an object moves 10 meters in 2 seconds to the north, its average velocity is +5 meters per second in the north direction.
Acceleration: Acceleration represents the rate at which an object's velocity changes with respect to time. It is also a vector quantity. Acceleration can result from a change in magnitude or direction of velocity or both. For example, if an object's velocity changes from 5 meters per second to 10 meters per second in 2 seconds, its acceleration is +2.5 meters per second squared (m/s²) in the same direction.
When an object is "in motion" according to classical mechanics, it means that it is changing its position with respect to a chosen frame of reference. The motion can be characterized by its displacement, velocity, and acceleration. Objects in motion will continue to move with the same velocity unless acted upon by an external force, as described by Newton's first law of motion.