Concept reading
Distance alone does not determine work
A person holds a bag motionless: the supporting force may be large, but the bag has no displacement. In the mechanical model, the force does zero work on the bag because displacement is zero. When the same bag moves, whether a particular force does positive, negative, or zero work depends on the angle between the force and the object's displacement, not merely the existence of a force.
The constant-force equation
For constant force of magnitude F acting through straight displacement of magnitude d, the work by that force is W=Fd cosθ, with θ the angle between the force vector and displacement vector. Positive work transfers energy into the chosen system through that interaction; negative work removes kinetic energy when it is part of the net work balance. If θ=90°, work is zero although the force can still change direction of velocity.
A joule is a transfer unit
The SI work unit is the joule (J), equal to 1 newton-meter (N·m). It is also the SI energy unit. The work done by a particular force can be positive or negative, while a body's kinetic energy never becomes negative. A displacement of 5 m with a 12 N force aligned with it gives W=60 J for that force. With the force opposed to displacement, the work is −60 J.
Defining a system prevents confusion
Always identify which object moves and which interaction does work. The normal force on a horizontally moving cart does no mechanical work when it stays perpendicular to displacement, even though it acts on the cart. Friction opposing motion generally does negative work. If several forces act, add each work contribution algebraically to obtain net work.