# Mechanics

Imagine you went to the movies. Soon after, a friend asks you to tell him the movie.

You will probably explain what has happened throughout history, detailing a few more moments, and then tell the end. Well, the kinematics It does something similar with the movement of a body.

 She will describe the movement: how things were at the beginning and throughout the movement, and choosing certain moments to have more insight into this phenomenon.

The word kinematics has its origin in the Greek word Kinemawhich means "movement".

Mechanics is divided into kinematics and dynamics.

## What is movement?

Surely you have a good idea of ​​what movement is, as you live with it in many of your everyday experiences. However, we often know what something is, but we do not know how to define it. Well, what is motion, how to define it?

When you look at an object whose position varies as time goes by, you say that it is moving. On the other hand, if you look at it for a while and it is always in the same place, you will say that it is still or at rest.

Movement is the state of a body, seen by a certain observer whose position varies as time goes by.

## Everything is relative?

Imagine the following situation: You are sitting in a bus that is increasingly approaching an amusement park. Beside you is sitting your father. Is your father on the move?

This seemingly simple question does not have a single answer. Seen by you, does your father's position vary as time goes on? The answer is no, because as time goes by your father's position is always the same: sitting on the bench next to you.

However, seen by a person standing on the sidewalk after the bus passed her, his father drifts further and further away. Therefore, according to the definition of movement given above, your father is standing in relation to you, but in relation to the person on the sidewalk, he is in motion!

Amazing, isn't it? Therefore, the movement is relative, that is, it depends on who observes. A system that has an observer in relation to that has concluded that an object is in motion or not is called referential.

## The shape of the path - the path

Do you remember the story of John and Mary, in which the brothers left little pieces of bread as markers of the way they traveled in order to know how to return home later?

The line we would find connecting these little pieces of bread is called the trajectory.

If all the objects or living things that move around us could leave a trace, we could observe the great diversity of trajectories that we find in our daily lives, some straight and other curves.

However, when moving bodies observe other moving bodies, we have to be careful when it comes to stating the shape of the observed body's trajectory.

Answer: Does the trajectory of a body also depend on a frame of reference?

To answer this question, imagine yourself looking at a plane that passes you and drops a bomb. What is the trajectory of the bomb? And the plane?

You will have got it right if you answered that it depends. In fact, for the plane the bomb falls in a straight line, but for the person watching it does not occur. For her, the bomb falls as well as moves to the right, and the shape of the path is a curve.

Well, for the plane the trajectory is straight. To you on the sidewalk she is curvy.

The shape of the path taken by a body is called a path, and it also depends on the frame of reference!

Notice how some trajectories can be:

elliptic

straight

Circular

parabolic