Work, Energy, and Power

The Currency of Physics

Energy is the universal capacity to do work, and work is the transfer of energy by a force: $W = Fd\cos\theta$. Work is measured in joules.

A force perpendicular to motion does no work. Friction converts mechanical energy into heat, making real systems seem to lose energy.

Conservation and Power

The work-energy theorem states that net work equals the change in kinetic energy. In an isolated system without friction, total mechanical energy ($KE + PE$) remains constant.

Power is the rate of doing work: $P = \frac{W}{t}$, measured in watts. Two engines can do the same work, but the more powerful one finishes faster.

Common pitfall: Confusing energy (total work capacity) with power (the rate of transfer). A low-power device can deliver high total energy given enough time.