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Showing content from https://cran.r-project.org/web/packages/sitar/../rmarkdown/../kinematics/vignettes/example.html below:

Basic usage

This is a package for studying movement. So we need a way of coding movement into an object. This is R, so you guessed it! We’ll code movement in the form of a data frame.

If you ask a kid to describe what movement is, probably he will trace a curve in the air with the tips of his fingers. And he will be right, that’s movement indeed! The apparently simple idea of “tracing a curve with the tips of your fingers” can be abstracted to the idea of a position that changes on time. One possible way of representing this is by a list of times and their corresponding positions.

Particularly, our data frames representing movement will have three columns:

• The \(t\) column: representing time
• The \(x\) column: representing the horizontal position at the time \(t\)
• The \(y\) column: representing the vertical position at the time \(t\)

A minimal example could be:

mov <- data.frame(t = c(0, 1, 2, 3, 4), 
                  x = c(0, 1, 2, 3, 4), 
                  y = c(0, 1, 4, 9, 16))

that represents five steps of what seems to be a parabolic movement:

plot(mov$x, mov$y, xlab = "x", ylab = "y")

Notice that the insides of the data frame look like below:

#>   t x  y
#> 1 0 0  0
#> 2 1 1  1
#> 3 2 2  4
#> 4 3 3  9
#> 5 4 4 16

where each row represents one “footprint”, that is, one snapshot of the movement. For instance, we see that at \(t = 2\), the position of the organism was \((2, 4)\).

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