Work, Energy and Power: every key term you need (+ practice quiz)
24 flashcard terms for AP Physics 1 Unit 3, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 24-question quiz — free, no account needed.
W = F·d·cos(θ); force applied over distance. W = F·Δx only if F parallel to motion. Units: joules (J).
Work-Energy Theorem
Work done on object = change in kinetic energy. W_net = ΔKE = ½mv² - ½mv₀².
Kinetic Energy
KE = ½mv². Energy of motion. Doubles velocity → 4x kinetic energy (depends on v²).
Potential Energy (gravitational)
PE = mgh (taking reference point as h=0). Changes with height. Moving up increases PE.
Potential Energy (elastic)
PE = ½kx² for spring; k = spring constant, x = displacement. Stores energy when stretched/compressed.
Conservation of Energy
Total energy conserved (no non-conservative forces). KE + PE = constant. Can transform between types.
Non-Conservative Forces
Friction, air resistance do negative work. Mechanical energy decreases. Heat released = work by friction.
Power
P = W/t = work per time unit. Units: watts (W). Also P = F·v; force times velocity.
Energy Transfer in Collisions
Elastic collision: KE conserved. Inelastic: KE lost (converts to heat, sound, deformation).
Unit 3 Summary
Work changes energy. KE = ½mv², PE = mgh or ½kx². Energy conserved; only transforms between types.
Work by a Variable Force
Work equals the area under an F-vs-x graph. For a spring from x₁ to x₂, W = ½k(x₂² − x₁²); you cannot use average-force shortcuts unless the force is linear.
Sign of Work
W = Fd cosθ. Force components parallel to displacement do positive work, antiparallel components do negative work, and perpendicular forces (normal force on a slope, tension in circular motion) do zero work.
Work Done by Friction Along a Path
Friction always does negative work equal to −μ_k N × path length. Unlike gravity, the energy loss depends on the actual path, not just the endpoints.
Choosing the System
Whether gravity does work or gravitational potential energy changes depends on whether Earth is inside your system. Both bookkeeping methods give the same answer if used consistently.
Potential Energy Curves
On a U-vs-x graph, force is the negative slope: F = −dU/dx. Minima are stable equilibria, maxima are unstable, and turning points occur where U equals total energy.
Energy Bar Charts
AP problems ask you to sketch KE, PE, and thermal energy bars at different instants. Total height stays constant for an isolated system; work done by external forces changes the total.
Power as F·v
Instantaneous power delivered by a force is P = Fv cosθ. A car at constant speed on a hill delivers power equal to the resistive plus gravitational component times speed.
Loop-the-Loop Minimum Height
To stay on a frictionless loop of radius R, the top speed must satisfy v² ≥ gR, so the release height must be at least 2.5R above the bottom.
Compressing a spring twice as far stores four times the energy because U = ½kx². Half the energy is stored in the last 29% of the compression.
Non-Conservative Work Equation
W_nc = ΔKE + ΔPE. When friction acts, the missing mechanical energy appears as thermal energy; when a person pushes, chemical energy is converted.
Work-Energy for Speed, Not Direction
The work-energy theorem gives magnitude of speed but not direction, so it is ideal for 'how fast at the bottom' questions and useless for 'which way' questions.
Vertical Circle Energy
Speed at any point on a vertical circle follows from energy conservation; tension or normal force at that point then follows from centripetal analysis. Two steps: energy first, then forces.
Efficiency
Efficiency = useful energy out / energy in. A 40 % efficient engine that consumes 1000 J does 400 J of useful work; the rest becomes thermal energy.
Zero-Net-Work Situations
Lifting a box at constant velocity: you do +mgh, gravity does −mgh, net work zero, kinetic energy unchanged. Carrying it horizontally at constant velocity: net work zero as well.