Forces are not “things an object has.” In AP Physics 1, a force is an interaction between two objects (or systems) that can change motion and is described with both magnitude and direction.

What a force represents

A force is a model for how one object influences another through an interaction. Every force you write down should answer two questions: “On what object?” and “Exerted by what other object?”

Forces are measured in newtons (N), and the unit can be expressed in base SI units.

Forces as interactions between objects (or systems)

The syllabus emphasis is that forces are vectors describing interactions. That has several practical consequences:

Each force has two “participants”

A single force always involves:

A free-body diagram isolates one object and shows all external forces acting on it as vectors. The labeled arrows (e.g., weight $mg$, normal force $N$, and friction $F_f$) make the “on object/by object” idea concrete by showing which interactions the environment exerts on the object. This kind of diagram is the standard starting point for translating interactions into equations. Source

• the object experiencing the force (the “on” object)

• the object exerting the force (the “by” object)

A clear notation is: $\vec{F}_{\text{by A on B}}$, meaning “the force exerted by A on B.” This helps prevent common errors such as listing a force without identifying its interaction partner.

Interaction forces come in pairs across the two objects

If object A exerts a force on object B, then object B exerts a force on object A at the same time.

This figure shows an interaction where one object pushes on another and receives an equal-and-opposite force in return, exemplifying Newton’s third law. Because the two forces act on different objects, they are not combined as forces on a single system unless you intentionally choose the combined system. The diagram helps prevent the common mistake of pairing forces that do not act on the same free-body diagram. Source

These two forces:

• are the same type of interaction (e.g., contact, gravitational)

• have equal magnitude and opposite direction

• act on different objects, so they do not “cancel” on a single object

(You do not add an interaction pair together unless you are analysing both objects at once.)

Why an object cannot exert a net force on itself

The specification statement “an object cannot exert a net force on itself” means you should never include a “self-force” in an analysis of one object. Any forces that parts of an object exert on other parts of the same object are internal to that object and cannot change the motion of the object as a whole without an external interaction.

Ways this idea shows up in reasoning:

• Pulling on your own hands with a rope creates forces within the rope-hand system, but there is no external agent providing an unbalanced interaction on the system as a whole.

• If you isolate a single object as your system, you only include forces exerted by the environment (other objects outside that system). The object itself is not an “outside object,” so it cannot contribute a net external force on itself.

This is a common conceptual checkpoint: if your force list includes “force of the object on itself,” you have misidentified the interacting objects.

Common interaction partners to look for

When you identify forces, think in terms of “What other object is touching it or influencing it?”

Typical interaction partners include:

• Earth (gravitational interaction)

• a surface (contact interaction)

• a rope/cable (interaction through tension)

• a spring (spring interaction)

• a fluid like air or water (drag, buoyant interaction)

• another moving or stationary object (push/pull contact interaction)

Vector nature: direction matters because interactions have direction

Because forces are vectors, the direction of the interaction is as important as its size.

Two equal forces applied in different directions are not equivalent, and multiple interactions combine according to vector addition. Keeping careful track of direction is part of correctly modelling the physical interactions present.