Relative motion

Relative motion

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Relative motion, a physical concept that we experience every minute in even trivial circumstances but which at times is complex to explain. It all lies in the fact that each has its own point of view, not only in politics, in culture, in religion and in football terms but also when it comes to measure position and speed with a certain object or person they move.

Relative motion: definition

Relative motion arises from the fundamental principle of Galileo which has long been revealing to us how it is possible to describe the same phenomenon through different reference systems and in relative motion to each other. In fact, when we find ourselves having to explain in precise, scientific terms, how even one point moves, we can't just say A to B but we have to choose a inertial reference system, then fixed, from which to start and then apply i principles of basic dynamics.

We usually think in terms of axes integral to the earth's surface, that is, we reason as if the Earth were still and whoever moves, moves on something stationary. It is an approximation that works in many cases and on which Newton succeeded in formulating the laws of motion correctly.

Everything runs smoothly if we talk about scales of size on a human scale, if instead we go to investigate maximum systems and motions on a large scale, or times that reach years and decades, then the reference system with the Earth as if it were still no longer valid and we are forced to to contemplate in the calculations the fact that our planet is moving continuously.

Relative motion: physics

We have already entered the field of Physics, taking a step further, for fans of these theories, relative motion in this matter and in others that derive from it, we begin to treat thinking we want find how the mechanics is formulated in a system R ' described by 3 space coordinates, one of time and one origin, in motion with respect to the inertial system R. always described with the same number and type of parameters and which we consider "fixed". So R ’seen by R moves.

What is asked in physics is this. Got an object like a ball, if it moves in a certain way looked at by R, how does it move if we start looking at it from R 'which moves in turn?

Points of view, this is the underlying theme of relative motion and many other events that do not center with physics.

Relative motion: examples

Going back to our daily life in which we do not think every minute thinking about reference systems and space-time coordinates, here it is some examples of relative motion. We are at a platform, in the station, our friend Marco has just got on a train for Milan which has started for its destination.

Pass on another train on a different track going in the opposite direction, a train in transit, for example, that does not stop. We see him pass from the platform with a certain speed, Marco who, while the passing train passes, is traveling on his own train in the opposite direction, he will see that train moving much faster than us.

Marco and us we don't have the same reference system for speed measurement and, if we were to calculate it, considering both of us as still, we would obtain two different values.

Two-dimensional relative motion

Speaking of trains with tracks I have already chosen an almost bidirectional example, as if there was only one direction and two transit ways, so we have seen a case where relative motion is two-dimensional. It is not often possible to make this simplification in two dimensions because bodies usually move in a system of 3 coordinates, the notes x, y, z.

Sure, the three-dimensional relative motion has more complex formulas and we always try to report cases in 2D to make it less effort.

Relative and absolute motion

When we talk about relative motion and absolute motion, we compare the reference systems we use by taking as the one with the firm Earth, usually, as reference. Returning to the previous trains, in the company of Marco, the motion of the passing train that we see is the absolute one, the motion that Marco sees and evaluates, from his moving train, is a relative motion.

But it's all relative.

Relative motion: formulas

For those wishing to take a look at the basic formulas used to start studying relative motion in a concrete and scientific way, here they are!

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You may also be interested in:

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  • Gaussian curve and distribution curve
  • Stator: how it works

Video: Relative Motion Gun (July 2022).


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