How kineticc equations are made
To solve the quesitons of the kinematic, the main tool we use is the kinematic equation, wich describes the movement that appears in a problem. If there is more than one movement, well, different equations for them all.
But you are not going to receive the equation, YOU have to ASSEMBLE it. It is very easy: first you have to know wich kind of movement appears in the problem, for example: MRU, MRUV or whatever movement is described. Once you know that, you look for the model that adapts to that movement.
For example, let´s suppose we have a MRU, you look for the model and you find it, is is this one:
x = x_{o} + v ( t – t_{o} )
In addition to the math symbols, the kinetic equations have twoconstant components (x_{o}, v and t_{o}) and variables (x and t). Look, so you can distinguish them easily I´ll copy the equation again, this time, in technicolor::
x = x_{o} + v ( t – t_{o} )
The blue ones are the constants, the red ones, the variables. The variables (the red ones) are not to be touched, if you touch them, you may break it and well, it is broken, you can´t use it anymore. If the variables don´t show up you are screwed, there is no kinetic equation.
TO ASSEMBLE A KINETIC EQUATION MEANS TO REPLACE THE CONSTANTS
What you have to do is to find (or sometimes even decide) the value of those constants, the blue ones ( x_{o} , v y t_{o} ).
If your are lucky, you might be given the value of those constants. If no one hands them to you, you may be able to find them or asign them the values you want to asign them. But they are no variables, they are constants! Once they give them to you/you find them/decide them it is donde: they are constants, they do not change, they value is the same until you finish the problem/stand in front of other movement in other class, in other country... How do you want me to put it... THEY DON´T CHANGE.
After you assemble the kinetic equations, the movements they describe have no more secrets for you. All the information of the movement is stored in that equation, you can ask the equation all the questions you want, they will answer, they are pretty docile.
In other words, once you have them... you can use them
NOTE: there are some movements wich describe with more than one equation, but the mechanism keeps being the same as before. For example, you have the oblique shot equation, that posses three equations for every shot:
x = x_{o} + v_{x} ( t – t_{o} )
y = y_{o} + v_{oy} ( t – t_{o} ) + ½ g ( t – t_{o} )²
v_{y} = v_{oy} + g ( t – t_{o} )
When you are writing the equations of an oblique shot, you must be carefull: do not go and shoot someone in the eye.
