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Concept · Mechanics
Work, 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
Switches you to Physics
Named traps
11 ways this goes wrong.
Each is a named misconception the questions are built to catch. Open one for the move that tempts, and the fix.
- 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.→
Key formulas
No formula sheet is matched to this concept yet. Work it from first principles, and watch the traps.
Shaky underneath?
If the concept underneath still feels loose, patch it here before coming back to CSCA questions.
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