Work, Energy and Power — Scientific Principles

Scientific Principles

Keep three accounts separate

The work done by a specified force is one contribution to an object's energy change. Net work adds the contributions of the forces and equals the change in kinetic energy for a constant-mass particle. Total-energy conservation concerns the chosen wider system and can include heat or other internal changes that are outside a simple mechanical-energy sum.

Use the formula with its conditions

For constant force, multiply displacement by the force component along it. Parallel force gives positive work, opposite force gives negative work and perpendicular force gives zero work along the stated displacement. A stationary object can have zero mechanical work done on it even while a person supporting it uses metabolic energy.

Kinetic energy depends on speed squared. Calculate the initial and final values before subtracting. Near Earth's surface, gravitational potential-energy change can be written as m g Δh when g is treated as constant. An ideal spring stores energy proportional to extension squared within its elastic model. State the model and reference before treating a relation as universal.

Rate and efficiency answer different questions

Average power divides transferred energy by elapsed time. A faster device can deliver the same useful energy at a greater rate; that does not determine its total electrical input. Efficiency compares useful output with input under a consistent boundary. A kilowatt-hour is energy because it multiplies power by time. Check signs, units and whether the question concerns one force, net work, useful output or total input.