Motion

Traps in Motion

• 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.
• Forgets to square a quantity
  Check dimensions: energy must be kg m² s⁻². If v is not squared the units are wrong.
• Inverts a formula
  Write the defining equation in words first (density = mass per unit volume), then rearrange algebraically. Check units: kg/m³.
• Converts only one unit of a compound unit
  Convert each unit separately: km → m (× 1000) and h → s (÷ 3600). Together: km h⁻¹ ÷ 3.6 = m s⁻¹.
• Fails to convert units to SI
  Convert every quantity to SI (m, kg, s) BEFORE substituting. 1 km/h = 1000/3600 m/s; 1 g = 10⁻³ kg; 1 cm² = 10⁻⁴ m².
• Uses 60 instead of 3600 seconds in an hour
  1 h = 60 min = 3600 s. Write the chain: 72 km h⁻¹ = 72 000 m ÷ 3600 s = 20 m s⁻¹.
• 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.
• Chooses a true but irrelevant statement
  Re-read the question word (why, which, what causes). A true fact that answers a different question is still wrong.
• 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.
• Over-generalises a rule
  Every rule has conditions. Before choosing, ask which waves, which media, which angles or which components the rule really applies to.
• 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.
• 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.
• Leaves a quantity out of the formula
  Write the full formula and tick off each symbol as you substitute. Every symbol needs a value.
• Field and charge misconceptions
  Like charges/poles repel, unlike attract. Field lines go from N to S (magnetic) and + to − (electric). Fleming's LEFT hand: motors (force); RIGHT hand: generators (induced current).
• 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.