Net Force & Equilibrium
Combine all force vectors acting on one system. The vector sum, not any single force, determines whether the system's velocity changes.
What matters when several forces act at the same time?
Force vectors combine. If the vector sum is zero, the system is in translational equilibrium and its velocity does not change. If the sum is nonzero, the system accelerates in the direction of the net force.
means zero acceleration, not necessarily zero velocity.
Add two opposite horizontal forces.
Horizontal acceleration is zero. The object could be at rest or moving with constant horizontal velocity.
Zero net force can hide many forces
A system can have several nonzero forces that cancel vectorially. Equilibrium does not mean 'no forces.'
Equilibrium preserves velocity
At rest stays at rest; constant velocity stays constant velocity. Both have zero acceleration.
Sum components by direction
In more dimensions, add x-components together and y-components together. A vector sum can vanish even when individual forces do not.