According to the precepts of physics, everything movement It is the variation of the position of an object over a period of time. Said variation is necessarily relative, since it occurs with respect to a reference point or referential system that accounts for the displacement produced.
It is possible that two or more observers have different appreciations of the moving object according to their relative locations and speeds. That is why two types of movement can be distinguished: relative and absolute.
- Relative motion. Movement that an object presents with respect to a reference point that is in turn mobile. The amount or dimension of the movement will be established according to the also variable location of the observer.
- Absolute movement. Movement that is observed from a fixed reference system.
However, it should be emphasized that there is nothing that is at absolute rest in the universe, so all movements are always relative. Absolute movements are considered to be those observed from reference systems approximately at rest.
Examples of relative motion
- The movement of a moving car relative to another moving car. If they move in opposite directions, from both cars you can see that the other is moving at a speed that is the sum of both speeds, that is, faster than someone would see it on the mainland. If they move in the same direction, both cars see the other moving with a speed equal to the subtraction of the speeds, that is, slower than what someone would see it on land.
- Go down an escalator that goes up. Seen from a staircase going down or up, it is a different movement from that seen from the floor.
- Walking on the deck of a sailing ship. With respect to someone who is on land, the movement is relative since if the boat moves in one direction at a speed and the person in the opposite direction and the same speed, from the ground the person has not moved.
- Flying in an airplane in the opposite direction to the earth’s rotation. From the Earth the movement is relative since the planet is in motion.
- Running alongside a moving car. If we were to reach the same speed as the car, it would appear to be stopped.
- Dropping an object on an ascending plane. The object describes a different motion depending on whether it is observed from inside the plane or from outside.
- A passenger on the subway platform. You can see the train moving through the station, while one on board the train watches the platforms go by as if they were moving.
- Running on a moving conveyor belt. Seen from the floor, if the tape advances, we would be advancing on something that in turn advances, but if it were backward, we could appear to be at rest.
- A boat advances on a river with the current. It will travel faster than if it were against the current. From the boat, however, there will be no major difference.
- A space rocket is moving away from the earth moving away in its orbit. Seen from the ground it is the rocket that moves, seen from the rocket it is the ground that moves away, but seen from the sun both objects move.
Examples of absolute motion
- The Sun seen from Earth. Despite the fact that we are the ones who move around it, the movement of the Sun is practically absolute in our daily lives. From the Earth the Sun moves and the Earth is still.
- The advancement of a motorcyclist. Seen from a pedestrian waiting for the traffic light, it is an absolute movement seen from its reference frame at rest.
- The movement of a bullet. Viewed from a point at rest, the bullet exits the weapon and travels with an absolute motion.
- A home runin baseball. The ball is considered in motion when leaving the stadium, without considering the rotation of the ground at all.
- A falling object. If we watch an object fall while standing still in the vertical direction, its motion will be absolute from our system at rest.
- A space rocket taking off. Seen by those of us left behind on earth, it is the rocket that moves, even though it is part of the orbital displacement of our planet (at least until it leaves it altogether).
- The terrestrial translation. Looking at it from the Sun, the movement is absolute because the Sun is still (if we neglect its movement around the center of the galaxy).
- A car on the highway. Observed from the point of view of the policeman who measures his speed.