Particle motion

Movement is relative

We live in a world that is not static. Things move. Atoms move.

However, there is a critical point when we consider movement. You should be aware that motion depends on your position. That is, when you are reading these pages you are absolutely sure that you, your chair, your computer and the other furniture around you are static. That is true. But it is true for you, not necessarily for other observers. For instance, an alien located on the Sun surface (suppose a very resistance alien) will see you and your house rotating with the Earth and orbiting him at high speed. That is, movement must be always referred to a specific position in the space.

• Motion is not an absolute concept. It is the change of position respect to a certain point in the space named reference system


But, how we know something is moving? Or, what is more or less the same, how can we measure movement? It is very simple: writing down the position of an object at different times. Of course, we must use the same reference system in all our measurements. Position change is named displacement.

Intuitively, we know that all objects do not move in the same way. Some displace more rapidly than others. To be able to compare, we define the term speed that is simply the displacement in a definite period of time. That is, those objects that have a higher displacement in the same time are told to move at a higher speed.


But, are you sure we did not leave something behind?

Yes, we forgot that ‘direction of motion’ is important. For that reason, Physicists define another concept, velocity, that includes not only the magnitude of displacement but also its direction. With velocity, we introduce an important concept in physics: vector quantities. They are values including directions and are usually represented by arrows.

• Velocity is the displacement of an object in a specific direction during certain period of time.

Remember, velocity is a vector. Do not fall down the precipice!

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