Key Concepts: Work & Energy Transfer

Work

• Work is done when a force causes displacement: W = Fd cos θ, where θ is the angle between the force and displacement.
• SI unit: Joule (J). 1 J = 1 N·m.
• Work is positive if force and displacement are in the same direction, negative if opposite, and zero if perpendicular (θ = 90°).

Energy Transfer

• Work is the mechanism for transferring energy between objects or systems.
• Positive work → energy is added to the object (it speeds up).
• Negative work → energy is removed from the object (it slows down).

Examples

• Pushing a box across the floor: you do positive work; friction does negative work.
• Carrying a box horizontally at constant height: gravity does zero work (force perpendicular to displacement).
• Lifting a box: you do positive work against gravity; gravity does negative work on the box.