Everything you need to master Unit 3 — kinetic and potential energy, work and the work-energy theorem, conservation of mechanical energy, and power. The "energy approach" makes many problems much easier than using F = ma.
18–23% of the AP exam
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Seven free resources for Unit 3 — pick the one that fits how you learn.
Unit 3 introduces the most powerful idea in physics: energy conservation. Instead of tracking forces over time (Unit 2), you can track how energy moves between forms — and often solve problems in two lines that would take ten with F = ma. This unit ties together kinetic energy, potential energy, work, and power.
The College Board breaks Unit 3 into 5 topics: (3.1) Translational Kinetic Energy, (3.2) Work and the work-energy theorem, (3.3) Potential Energy (gravitational and elastic), (3.4) Conservation of Energy, and (3.5) Power. Topic 3.4 is the heart of the unit — energy is never created or destroyed, only transferred or transformed.
Unit 3 makes up roughly 18–23% of the AP Physics 1 exam — tied with Unit 2 as one of the heaviest units. Energy concepts also show up everywhere else: momentum (Unit 4), rotation (Units 5–6), and oscillations (Unit 7) all use energy methods.
Key terms preview
A taste of what you'll find in The Essentials and Flashcards.
Kinetic Energy
K = ½mv². The energy an object has because of its motion. Scalar; never negative.
Work
W = F·d·cos(θ). Energy transferred when a force acts on an object over a distance. Can be positive, negative, or zero.
Work-Energy Theorem
W_net = ΔK. The net work done on an object equals its change in kinetic energy.
Potential Energy
Stored energy from position or configuration. Gravitational: U = mgh. Elastic spring: U = ½kx².
Conservation of Energy
Energy is never created or destroyed — only transferred or transformed. The most powerful problem-solving idea in physics.
Power
P = ΔE/Δt. The rate at which energy is transferred or transformed. Units: watts (W = J/s).
Energy is never created or destroyed. It only moves between forms (kinetic ↔ potential ↔ thermal) and between systems. This is the single most powerful problem-solving tool in physics — when forces and motion are messy, energy is almost always cleaner.
2. Work is the bridge between forces and energy
Work = (parallel component of force) × (distance). It tells you how much energy a force transfers. The work-energy theorem says the NET work equals the CHANGE in kinetic energy. Perpendicular forces (like a normal force on a horizontal motion) do zero work.
3. Mechanical energy is conserved when only conservative forces act
Mechanical energy = kinetic + potential. When only conservative forces (gravity, springs) do work, mechanical energy stays constant. When non-conservative forces (friction, drag) act, mechanical energy is dissipated — usually into heat or sound.