Work and power
In physics, work is the energy transferred when a force moves an object through a distance (W = Fd cos θ), and power is the rate at which that work is done (P = W/t). Work is measured in joules and power in watts.
Work is the transfer of energy that occurs when a force moves an object. For a constant force, W = Fd cos θ, where F is the force, d is the displacement, and θ is the angle between them. Only the force component along the direction of motion counts: pushing a crate 5 m with a 40 N horizontal force does 200 joules of work, while carrying a bag horizontally at constant speed does zero work on it, because the upward force is perpendicular to the motion.
Power measures how fast work gets done: P = W/t, expressed in watts (one joule per second). Lifting a box in two seconds requires twice the power of lifting it in four, even though the work is identical. For an object moving at constant velocity against a force, power can also be written P = Fv, a form that explains why maintaining speed against greater resistance demands a stronger engine — or a stronger athlete.
The work-energy theorem ties work directly to motion: the net work done on an object equals its change in kinetic energy. This connection makes work and power the accounting system of mechanics, linking forces to energy changes without tracking every instant of the motion.
The AP Physics 1 exam tests work and power through calculations with W = Fd cos θ, the work-energy theorem, and P = W/t or P = Fv. The CSCS exam applies the same definitions to resistance training, where the ability to produce mechanical power — force times velocity — distinguishes explosive strength from maximal strength and shapes how athletes are trained and tested.
Key takeaways
- Work is energy transferred by a force acting through a displacement: W = Fd cos θ, in joules.
- A force perpendicular to the motion does zero work.
- Power is the rate of doing work: P = W/t (or P = Fv), measured in watts.
- The work-energy theorem says net work equals the change in kinetic energy.
- AP Physics 1 tests the formulas directly, while the CSCS applies power to explosive athletic performance.