Fnet

Fnet, short for net force, is the vector sum of all external forces acting on a body or system. It is the primary quantity that determines motion in classical mechanics. By definition Fnet equals the sum of all individual forces acting on the object: Fnet = Σ Fi. For a single object with constant mass, this leads to Newton's second law: Fnet = m a, where a is the object's acceleration; more generally, Fnet = dp/dt, with p the momentum.

Because forces are vectors, Fnet must be computed by vector addition, taking into account directions and magnitudes. Free-body diagrams are commonly used to identify the contributing forces, such as gravity, normal force, friction, tension, and applied forces. Components along coordinate axes yield Fnet,x and Fnet,y, equal to the sum of the corresponding components of each force.

In equilibrium, Fnet = 0, and the body experiences no acceleration. When Fnet is nonzero, it produces motion changes: Fnet = m a implies acceleration is in the direction of the net force. Examples include a block being pushed on a surface with friction, an object sliding with kinetic friction, or a projectile under gravity with air resistance neglected or included.

Conceptual notes: Fnet relates to impulse and momentum through J = Fnet Δt and Δp = J. The idea extends to systems of particles, where the net external force determines the acceleration of the system's center of mass.