Motion with forces
Physics Narrative for 14-16
The next simplest situation
The simplest addition to the natural environment is to to have a single force acting. In practice we do this by adding a single force to the situation. This makes a big difference, because the symmetry is now broken. You cannot arbitrarily choose a point of view without paying attention to this force.
The situation where an object has a single force acting on it is common in physics because we make it so, by adding all the forces representing different interactions to give a single resultant force.
It may be that this resultant force is zero. If so, then you'll have recovered the natural environment – or at least a situation that is not discernibly different from the natural environment.
Adding complexity: more forces acting on the object
Strictly speaking, this is unnecessary, as we can sidestep this analysis as shown. First calculate the resultant force, and then analyse as before. This is strongly recommended, otherwise you find yourself drawn into conversations like this:
Befuddled: The acceleration that would be caused by this driving force, and the acceleration that would be caused by this retarding force.
Better to add the forces to a single resultant force and then find the acceleration for the object. An object has only one acceleration (although this might have components in different directions).
Too many hypotheticals are bad for the digestion.