Referential, movement, rest and trajectory are basic concepts of Kinematics, part of Physics that studies the movements.
♦ Kinematics concepts:
→ Reference: The frame of reference is the body from which observations of phenomena are made. As an example, we can imagine that passengers inside a car, taking the vehicle as a reference, will be at rest; but taking a fixed point outside the car, all passengers will be in motion.
→ Movement and rest: From the concept of reference, we can understand that movement and rest are relative concepts, because what is in motion for an observer in a given reference can be at rest for another observer and vice versa.
If the Earth is taken as a reference, we can say that buildings, cities, countries, everything is at rest. However, if the Sun is taken as a reference, the Earth and everything on its surface have rotation and translation movements .
→ Trajectory: is the path taken by a body that moves in relation to a referential. This concept is also relative, as two different references can have different views of the same movement. Imagine an aircraft releasing a cargo of supplies mid-flight. The image below shows the trajectories of the payload seen by an observer inside the aircraft and by someone on the ground.
For a passenger on the aircraft, the trajectory of the load is straight; but for someone on the ground, the trajectory is a curve, as the load follows the aircraft.