ConceptMechanics
← All conceptsWork, energy & power
W=Fd cos(θ) (sign of work by each agent), work-energy theorem, kinetic & gravitational PE, conservation of mechanical energy, power P=W/t=Fv, efficiency
11 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- Efficiency inverted/over 100%Computes efficiency as input over output (inverted), or accepts a value above 100%.→
- Friction loss omittedApplies mechanical-energy conservation when friction dissipates energy, ignoring the thermal loss term.→
- GPE reference inconsistentMeasures gravitational PE from inconsistent reference heights so the height change is wrong.→
- Gravity works in horizontal motionCounts gravity as doing work on a body moving horizontally, ignoring that the vertical displacement is zero.→
- KE linear in speedTreats kinetic energy as proportional to v (or mv) rather than (1/2)mv2, missing the square.→
- P=Fv uses distance not speedSubstitutes distance for velocity in P=Fv, or otherwise mismatches the instantaneous-power formula.→
- Perpendicular force does workAssigns work to a force perpendicular to motion (e.g. normal or centripetal force) where the work is zero.→
- Power confused with energy/workTreats power and work/energy as interchangeable, omitting the time division (P=W/t).→
- Sign of work by each agent droppedTreats all work as positive, missing that a force opposing the displacement (e.g. friction, braking) does negative work.→
- Work ignores angleComputes work as force times distance without the cos(θ) factor for non-aligned force and displacement.→
- Work-energy mixes net and single forceEquates the work of a single force to the change in KE instead of using the net work of all forces.→
