Forces as Interactions
A force exists because two systems interact. Choose one system at a time and track which force acts on it, where that force comes from, and why the partner force belongs to the other object.
When two objects interact, whose force are we talking about?
Every force statement needs three pieces: the object feeling the force, the interaction partner producing it, and the direction of the force. Saying only “there is a force” leaves out the relationship that created it.
Read this as “the force exerted by A on B.” B is the object the force acts on.
Change the system, not the interaction.
Changing the chosen system does not create a new interaction. It changes which member of the interaction pair belongs on the diagram you are currently building.
Name the interaction
A force is produced by an interaction between systems. Contact is one kind of interaction; gravity can act without contact.
Choose the system
A force vector belongs to the object that experiences that force. Change the chosen object and the relevant vector changes too.
Keep the pair separate
Interaction partners exert equal-magnitude, opposite-direction forces on each other, but those forces act on different objects.
Equal and opposite does not mean “they cancel.”
Cancellation is a statement about the vector sum of forces acting on one chosen system. An interaction pair lives on two different systems, so you cannot add the pair together on one object’s free-body diagram.