Work, energy, and power
Work is energy transferred when a force moves an object, energy is the capacity to do work, and power is the rate at which work is done. Together they form the framework physics uses to track how energy moves and transforms.
Work is done when a force displaces an object: W = Fd cos θ, where θ is the angle between the force and the displacement. Only the force component along the motion counts — carrying a box horizontally at constant speed does zero work against gravity, and a force perpendicular to motion (like the centripetal force in circular motion) does no work at all. Work is measured in joules (J).
Energy is the capacity to do work, and mechanics focuses on two forms. Kinetic energy, KE = ½mv², belongs to moving objects; gravitational potential energy, PE = mgh, is stored by position. The work-energy theorem connects the concepts: the net work done on an object equals its change in kinetic energy. When only conservative forces like gravity act, total mechanical energy (KE + PE) is conserved — energy shifts between forms but the sum stays constant, which is what makes energy methods so powerful for problems where forces vary or paths are complicated.
Power is how fast work is done or energy is transferred: P = W/t, measured in watts (1 W = 1 J/s). An equivalent form, P = Fv, gives the instantaneous power a force delivers to an object moving at speed v. Two motors lifting the same load do the same work; the more powerful one simply does it faster.
AP Physics 1 devotes a full unit to work, energy, and power — expect questions applying the work-energy theorem, conservation of energy, and power calculations, along with interpreting energy bar charts and force-versus-displacement graphs, where the area under the curve equals the work done.
Key takeaways
- Work is W = Fd cos θ — only the force component along the displacement transfers energy.
- Kinetic energy is ½mv²; gravitational potential energy is mgh; both are measured in joules.
- The work-energy theorem states net work equals the change in kinetic energy, and mechanical energy is conserved when only conservative forces act.
- Power is the rate of doing work: P = W/t = Fv, measured in watts.
- AP Physics 1 tests these ideas with energy conservation problems, bar charts, and area under force-displacement graphs.