Equilibrium

Traps in Equilibrium

• Ignores distance in moments
  Moment = force × perpendicular distance. Balance needs clockwise moments = anticlockwise moments, so a smaller force further away can balance a bigger force close in.
• Inverts a formula
  Write the defining equation in words first (density = mass per unit volume), then rearrange algebraically. Check units: kg/m³.
• Uses the wrong arithmetic operation
  Write the formula symbolically and substitute only at the end. Check units: if the units of your answer are wrong, so is the operation.
• Mixes up a pair of similar physics terms
  Make a two-column table for each pair with one defining feature and an example. Underline the defining word in the stem before you look at the options.
• Reverses a relationship or direction
  Write the relationship with an arrow or proportionality sign first (E ∝ f; R ∝ L; cooling a metal → R falls). Then check the option follows the same direction.
• Picks a related law, device or effect with a different job
  Learn laws and devices as 'name → what it tells you' cards. Contrast neighbours: left-hand rule = motor force, right-hand rule = dynamo current, grip rule = field around a wire.
• Newton's laws misconceptions
  First law: no resultant force → constant velocity (or rest). Third law pairs act on DIFFERENT bodies, are equal and opposite and of the same type.
• Friction and viscosity misconceptions
  Limiting friction ∝ normal reaction (F = μR), independent of contact area. Static friction > kinetic. Friction can be useful (walking, brakes). Viscosity is fluid friction; terminal velocity reached when drag + upthrust = weight.
• Confuses energy, power and time
  Power = energy/time (W = J/s). Energy = power × time. kWh = kW × h.
• Uses everyday intuition instead of physics
  Name the law that applies (Newton's first law, Archimedes' principle, kinetic theory) and reason from it. If the option only 'feels' right, test it against the law.
• Takes part of the condition as the whole answer
  Check whether the option is enough on its own. Equilibrium needs zero resultant force AND zero total moment; a complete explanation names every factor in the stem.