Common physics misconceptions
The Error Atlas
66 recurring reasoning mistakes. For each one: what it looks like, why it feels right, how to catch it, and the correct physics. Your diagnostic report links to the entries that match your answers.
Featured entries
mechanics illustrated
Constant speed is not zero acceleration
detect · You set a = 0 for something moving in a curved path at steady speed.
mechanics illustrated
Static friction is not always at its maximum
detect · You wrote f = μs·N for a body that isn't on the verge of slipping.
mechanics illustrated
Centripetal acceleration points inward
detect · Your free-body diagram has a net outward arrow for something moving in a circle.
Full index
63 of 66 entries
Mechanics 16
NL-01 A free-body diagram shows real forces only Adding a phantom “force of motion” in the direction of travel. Constraint missed NL-03 Zero velocity is not zero acceleration Concluding a = 0 at the instant an object is momentarily at rest. Force / acceleration confusion NL-06 Internal tension needs the right system choice Trying to find an internal force by writing ΣF for the whole system. Constraint missed NL-07 The normal force is not always mg Setting N = mg on an incline, in an elevator, or under an applied force. False equilibrium assumption ▣ NL-08 Third-law pairs act on different bodies Cancelling an action–reaction pair on a single free-body diagram. Force / acceleration confusion NL-09 Mass and weight are different quantities Using kilograms as a force, or reporting weight in kg. Dimensional inconsistency NL-10 Axes are bookkeeping, not physics Thinking a tilted coordinate system changes which forces are real. Vector / scalar confusion NL-11 Tension is uniform only in the ideal string limit Assuming every connected segment has the same tension even with mass or friction in the connector. Constraint missed NL-12 Kinetic friction opposes relative slipping Drawing friction opposite the object's ground-frame velocity instead of opposite slipping at the contact. Force / acceleration confusion NL-13 Constraint equations need signed directions Using equal acceleration magnitudes where the constraint requires opposite signed accelerations. Sign convention error NL-14 Apparent weight is the normal force Treating scale reading and gravitational weight as always identical. False equilibrium assumption NL-15 A spring force depends on displacement from equilibrium Using F = kx without defining x from the spring's relaxed length or equilibrium point. Force / acceleration confusion NL-16 Terminal velocity means zero acceleration, not zero forces Assuming a falling object at terminal speed has no forces acting. Force / acceleration confusion NL-17 The center of mass ignores internal forces Trying to change a system's center-of-mass motion using only internal pushes. Constraint missed NL-18 Banked-curve friction direction depends on speed Assuming friction always points up or down a banked curve regardless of speed. Circular-motion direction error NL-19 Projectile components are independent only after forces are split Letting horizontal velocity affect vertical acceleration in ideal projectile motion. Vector / scalar confusion Rotation 10
ROT-01 Equilibrium needs Στ = 0, not just ΣF = 0 Balancing forces but forgetting the torque condition. Rotation / torque-arm error ROT-02 Torque uses the perpendicular lever arm Writing τ = F·r regardless of the force's direction. Rotation / torque-arm error ▣ ROT-03 Moment of inertia depends on the axis Treating I as a fixed property of the object. Rotation / torque-arm error ROT-04 Rolling without slipping links v and ω Treating translation and rotation as independent while rolling. Constraint missed ROT-05 Angular momentum needs a no-external-torque axis Conserving angular momentum about an axis with a nonzero external torque. Wrong conservation law ROT-06 Static friction can cause rolling without doing work Assuming any friction force always removes mechanical energy. Wrong conservation law ROT-07 Torque sign follows your rotation convention Flipping torque signs because a force points down or left, rather than because of its rotational tendency. Sign convention error ROT-08 Angular acceleration follows net torque, not angular velocity Assuming a spinning object must have a net torque because ω is nonzero. Force / acceleration confusion ROT-09 Radius and diameter are not interchangeable Using diameter where a rolling or torque relation requires radius. Dimensional inconsistency ROT-10 The parallel-axis theorem adds Md² Adding Md or shifting moment of inertia by a linear distance. Dimensional inconsistency Energy 11
EN-01 Energy conservation fails when external work is hidden Applying KE + PE = const when friction, drag, or a motor is present. Wrong conservation law ▣ EN-02 The normal force usually does no work Counting the normal force in the work–energy budget. Wrong conservation law ▣ EN-03 Kinetic energy is a scalar — it has no direction Assigning a direction to KE or adding energies as vectors. Vector / scalar confusion EN-04 Work is path-dependent only with friction Assuming gravitational work depends on the route taken. Path vs. state confusion EN-05 Power is not the same as energy Using watts and joules interchangeably. Dimensional inconsistency EN-06 Spring energy uses displacement squared Using kx or mgx as spring energy instead of ½kx². Dimensional inconsistency EN-07 Friction work depends on path length Treating friction like gravity and using only vertical height change. Path vs. state confusion EN-08 Potential energy needs a reference level Treating gravitational potential energy as an absolute number instead of a difference. Path vs. state confusion EN-09 Work-energy uses net work Equating the work by one force to ΔK while ignoring other forces that also do work. Wrong conservation law EN-10 Energy diagrams need every storage mode Calling energy 'lost' instead of moving it into thermal, spring, rotational, or internal storage. Wrong conservation law EN-11 Bound orbits have negative total energy Assuming a satellite in orbit has positive total mechanical energy because it is moving. Energy vs. momentum misuse Momentum 9
MOM-01 Momentum is conserved only when external impulse is negligible Applying conservation of momentum with a large external force present. Wrong conservation law MOM-02 Energy and momentum are not interchangeable Conserving kinetic energy in an inelastic collision (or momentum via energy). Energy vs. momentum misuse MOM-03 Impulse is a vector — signs matter Computing Δp with speeds instead of signed velocities. Sign convention error MOM-04 Choose the system before applying conservation Applying conservation without defining what's inside the system. Constraint missed MOM-05 Center-of-mass velocity is the system momentum divided by total mass Averaging speeds without weighting by mass. Constraint missed MOM-06 Explosions conserve momentum while kinetic energy can increase Assuming an explosion violates momentum conservation because pieces fly apart. Energy vs. momentum misuse MOM-07 Momentum components conserve separately Conserving the speed-like magnitude of momentum instead of x and y components. Vector / scalar confusion MOM-08 Impulse is area under force-time, not peak force Using the maximum force in a collision as if it were the impulse. Dimensional inconsistency MOM-09 A ballistic pendulum needs two separate models Using momentum conservation through the entire swing, or energy conservation through the collision. Energy vs. momentum misuse Circuits 8
CIR-01 Series and parallel don't combine by 'adding up' Adding resistances in parallel (or reciprocals in series). Circuit equivalence error ▣ CIR-02 Ideal meters: ammeter 0 Ω, voltmeter ∞ Ω Placing meters without accounting for their ideal resistance. Circuit equivalence error CIR-03 Electric potential is not electric field Using V and E interchangeably, or assuming E = 0 wherever V = 0. Dimensional inconsistency CIR-04 Kirchhoff loop signs follow your traversal Changing voltage signs based on how a battery or resistor looks, not on the chosen loop direction. Sign convention error CIR-05 Current is not used up Assuming less current remains after each resistor in a series loop. Circuit equivalence error CIR-06 Series shares current; parallel shares voltage Applying the series current rule to parallel branches or the parallel voltage rule to series elements. Circuit equivalence error CIR-07 A charged capacitor blocks steady DC Treating a capacitor as a wire after a long time in a DC circuit. Circuit equivalence error CIR-08 Changing equivalent resistance changes the source current Adding a parallel branch but keeping total current fixed. Circuit equivalence error Waves & SHM 5
SHM-01 SHM needs a linear restoring force Calling any back-and-forth motion simple harmonic. Force / acceleration confusion SHM-02 Wave speed is set by the medium, not the source Assuming shaking a rope faster makes the wave travel faster. Dimensional inconsistency SHM-03 In SHM, velocity and acceleration peak a quarter-cycle apart Assuming a mass on a spring is fastest where its acceleration is greatest. Force / acceleration confusion ▣ SHM-04 Mass-spring period does not depend on amplitude Assuming a larger oscillation must take longer because it travels farther. Force / acceleration confusion SHM-05 Phase is not the same as path distance Confusing where an oscillator is in its cycle with how far it has traveled. Path vs. state confusion Thermo 4
TH-01 Temperature is not heat Treating a hotter object as necessarily containing more heat. Dimensional inconsistency TH-02 Heat added is not all converted to work Assuming an engine turns all input heat into useful work. Wrong conservation law TH-03 Adiabatic means no heat transfer, not constant temperature Treating Q = 0 as if ΔT = 0. Wrong conservation law TH-04 PV-work sign depends on the convention Mixing work done by the gas with work done on the gas in the same first-law equation. Sign convention error Research basis
Where the taxonomy comes from.
The Atlas follows the physics-education-research tradition of treating a wrong answer as evidence of a specific alternative model, not carelessness. Several categories map to documented results:
- The phantom “force of motion” (NL-01) and impetus-style reasoningClement, Am. J. Phys. 50 (1982) · Halloun & Hestenes, Am. J. Phys. 53 (1985) · McCloskey, Scientific American 248 (1983)
- Velocity / acceleration confusion (the force–acceleration category)Trowbridge & McDermott, Am. J. Phys. 48 (1980) & 49 (1981)
- Distractors authored to detect specific misconceptions — the instrument design this diagnostic imitatesHestenes, Wells & Swackhamer, “Force Concept Inventory,” The Physics Teacher 30 (1992)
- Novices sorting problems by surface features instead of governing principles — why the ladder re-tests one principle in changing costumesChi, Feltovich & Glaser, Cognitive Science 5 (1981)
The entries, codes, and problems here are original. The method — coded distractors, named misconceptions, targeted repair — stands on this literature.