Use this as a quick reference for power as a rate, average power from energy or work, instantaneous power P = Fv cos θ, and reading power from a graph.
🧭 Plot Summary
Power is how fast energy moves. It is the rate at which energy is transferred into or out of a system, or converted from one type to another inside it, and it is measured in watts (1 W = 1 J/s). Two machines can do exactly the same work with very different power: the one that finishes sooner has more. Average power is the energy transferred divided by the time, Pavg = ΔE/Δt, and since work is energy transferred by a force, also Pavg = W/Δt. For a constant force, the instantaneous power is P = F∥v = Fv cos θ: only the part of the force along the velocity counts, and the same force delivers more power at higher speed. A car, a crane, an elevator, and a light bulb are all power problems.
What you'll do in this lesson
- Describe power as the rate of energy transfer or conversion, measured in watts.
- Calculate average power from energy and time (P = ΔE/Δt) and from work and time (P = W/Δt).
- Calculate instantaneous power with P = Fv cos θ for a constant force.
- Compare scenarios: how power changes when the time, force, or speed changes.
- Read power as the slope of an energy-versus-time graph.
Why it matters
Power finishes Unit 3. Work, kinetic and potential energy, and conservation of energy tell you how much energy moves. Power tells you how fast. It decides how big an engine, motor, or battery has to be, and what a household pays on its electric bill. Expect it on the AP exam in both calculation and comparison form, often together with the work-energy theorem.
✅ Self-Check Before You Roll On
Check off each item as you get there. These aren't grades — they're your own signal.