Mechanics

Traps in Mechanics

• Leaves a quantity out of the formula
  Write the full formula and tick off each symbol as you substitute. Every symbol needs a value.
• Rearranges a formula wrongly
  Use the triangle or do the algebra: from V = IR, divide both sides by R → I = V/R. Check with units (V/Ω = A).
• Miscounts steps or makes a counting slip
  Count the steps explicitly on paper (list each half-life, each strand), then check the total.
• 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.
• 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.
• 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.
• 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.
• 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.
• Writes a quantity in the wrong unit
  Carry units through every line. A percentage is a ratio × 100; joules and newtons cannot become % without dividing by a quantity of the same kind.
• 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.
• Omits the ½ in s = ut + ½at²
  Displacement under constant acceleration is the AREA under the v–t graph: a rectangle (ut) plus a triangle (½at²). Never drop the ½.
• Calculates a different quantity
  Name the quantity asked for and its unit before calculating. If your number has the unit of a different quantity, you used the wrong formula.
• Inverts a formula
  Write the defining equation in words first (density = mass per unit volume), then rearrange algebraically. Check units: kg/m³.
• Adds quantities with different units
  Only quantities with the same unit can be added. If the units differ, the formula multiplies or divides them.