MTLE Physics (Grades 9-12) common mistakes
MTLE Physics (Grades 9-12) · Understand the traps and how to avoid them.
On free-response problems, writing the final numeric answer without the unit or with the wrong unit (e.g., reporting a spring constant as "40" instead of 40 N/m, or giving gravitational potential energy in newtons).
Candidates treat the number as the answer and drop the unit as an afterthought, often because they compute in a hurry and copy the raw calculator display. Many also never check whether the requested quantity is a vector magnitude, a rate, or a per-unit quantity like N/m.
What to do instead
Adopt a habit of writing the unit at the moment you set up the calculation, not after. Before finalizing, do a one-line dimensional check: substitute base units into your formula and confirm the result matches the target quantity (kg·m/s² for force, N/m for spring constant, J for energy). Practice by rewriting every worked problem in your review set with units carried through each step.
Choosing a kinematics equation by pattern-matching a keyword (e.g., seeing "distance" and grabbing x = vt) rather than first listing which of the five variables (v₀, v, a, t, Δx) are given and which is unknown.
Under time pressure, candidates recognize surface features of a problem instead of its structure. Constant-acceleration problems look alike, so a familiar-looking setup triggers a memorized formula that assumes a = 0 or v₀ = 0 when neither holds.
What to do instead
Use a forced inventory step: write the five kinematic variables in a row, fill in the three knowns (including explicit zeros), and mark the unknown. Only then select the equation that contains exactly those four. Drill with mixed problem sets where some items are constant-velocity and some are accelerating, so the inventory step becomes automatic.
Applying Newton's second law to a system without drawing or reasoning about the free-body diagram, so tension, normal force, or friction is omitted or double-counted (e.g., treating the normal force on an incline as mg rather than mg·cosθ).
Candidates jump to ΣF = ma with a memorized net-force expression and skip identifying individual forces. Components on inclines and the distinction between weight and normal force are frequent weak spots because the geometry is done mentally rather than written out.
What to do instead
Make an isolated free-body diagram mandatory for every dynamics item: one dot, every force labeled with its source (gravity by Earth, normal by surface, tension by string). Resolve into chosen axes algebraically on paper, including the cosθ/sinθ factors, before writing ΣF = ma. Practice incline, elevator, and two-body-pulley setups until the component breakdown is routine.
Treating conservation of momentum and conservation of kinetic energy as interchangeable, or assuming kinetic energy is conserved in every collision, including perfectly inelastic ones where the objects stick together.
Both are "conservation" principles taught near each other, and candidates conflate them. They also forget that momentum is conserved in all isolated collisions while kinetic energy is conserved only in elastic ones, so they set up a spurious energy equation and get inconsistent results.
What to do instead
Before solving, classify the collision explicitly: elastic (both momentum and KE conserved), inelastic (momentum only), perfectly inelastic (momentum only, common final velocity). Write the conservation statement you are entitled to use and cross out the one you are not. Verify by checking whether the computed final KE is less than or equal to initial KE.
Mixing up the direction of induced current or magnetic force by misapplying a right-hand rule, or reversing the sign in Lenz's law so the induced field is drawn in the same direction as the change rather than opposing it.
There are several distinct right-hand rules (force on a moving charge, force on a current-carrying wire, field around a wire, induced current), and candidates memorize gestures without the underlying relationship. Lenz's law requires reasoning about the change in flux, which is easy to invert under pressure.
What to do instead
Anchor each rule to its physical statement rather than a hand gesture: F = qv × B, and induced EMF opposes the change in flux. For Lenz's law problems, explicitly state whether flux is increasing or decreasing, then which field direction would oppose that change, then the current direction. Work through a magnet approaching and receding from a coil and confirm the two cases give opposite current directions.
Setting up a circuit problem with series and parallel rules swapped, such as adding resistances directly for parallel branches or claiming current is the same through parallel branches and voltage the same across series elements.
The two rule sets are mirror images and are learned close together, so retrieval under exam pressure flips them. Candidates also fail to simplify a mixed network step by step and instead try to apply one rule to the whole circuit.
What to do instead
Memorize the invariants by their physical basis: in series, charge has one path so current is shared and voltages add; in parallel, branches share the same nodes so voltage is shared and currents add. For mixed networks, redraw and collapse the innermost parallel or series group first, labeling each equivalent resistance, then work back out to find branch currents and voltages.
Confusing wave properties when answering: reporting frequency when asked for period (or vice versa), treating wave speed as changed by a medium boundary without changing wavelength, or mixing up the conditions for constructive and destructive interference.
Frequency, period, wavelength, and speed are tightly coupled through v = fλ, and candidates apply the equation mechanically without tracking which quantity is fixed by the source and which changes with the medium. Path-difference conditions for interference are memorized as bare numbers without the half-wavelength logic.
What to do instead
State the invariant explicitly in every wave problem: frequency is set by the source and does not change when entering a new medium; speed and wavelength change together. Derive interference conditions from path difference in wavelengths (whole number for constructive, half-integer for destructive) rather than reciting them. Practice converting between period and frequency until it is reflexive.
On thermodynamics items, applying the first law with the wrong sign convention, or asserting that efficiency can reach 100% or that entropy decreases in a spontaneous isolated process.
Sign conventions for work done on versus by the system differ across textbooks, and candidates carry a convention from one context into another without checking. The second law is often reduced to a slogan, so quantitative efficiency and entropy questions expose shallow understanding.
What to do instead
Pick one convention at the start of each problem (e.g., ΔU = Q − W with W done by the system) and write it down before substituting. For efficiency, always compute as useful work out divided by heat in and check the result is below the Carnot limit. For entropy, verify the sign against whether the process is spontaneous and isolated.
Making significant-figure and rounding errors that are avoidable: reporting ten digits from a calculator, rounding intermediate steps and compounding error, or ignoring that g is given to two significant figures.
Candidates equate calculator precision with measurement precision and do not track the least precise given value. Intermediate rounding is a time-saving shortcut that quietly shifts final answers outside the accepted tolerance.
What to do instead
Carry at least one extra digit through intermediate steps and round only the final answer to the significant figures of the least precise given value. Before computing, note the precision of each given quantity. On numeric free-response items, sanity-check magnitude (e.g., a dropped object should not take 0.02 s to fall 2 m).
On graph-interpretation items, reading the slope or area of a motion or force graph as the wrong quantity, for example taking the slope of a position–time graph as acceleration or the area under a velocity–time graph as jerk.
Candidates memorize "slope means rate" without mapping each specific axis pair to its physical meaning, and they do not distinguish slope from area. Graph items are also answered quickly because they look easy.
What to do instead
For every graph, write the axis labels and then the units of slope (rise units over run units) and of area (product of units). Let the units tell you the quantity: m/s over s gives m/s², so slope of velocity–time is acceleration; m/s times s gives m, so area under velocity–time is displacement. Practice a mixed set of position, velocity, acceleration, force, and momentum graphs until unit-based identification is automatic.