Armenia Physics Unified Exam common mistakes
Armenia Physics Unified Exam · Understand the traps and how to avoid them.
Applying kinematics equations with g = 9.8 m/s² but plugging in a value in cm or km without converting to metres, then reporting a speed or displacement that is off by a factor of 10, 100 or 1000.
The formulas are memorised symbolically, and unit conversion is treated as an afterthought done at the end rather than before substitution; mixed-unit problem statements (e.g. distance in cm, time in minutes) encourage this.
What to do instead
Before substituting, write every given quantity on a separate line with its SI conversion shown (e.g. 40 cm = 0.40 m, 2 min = 120 s), and carry units through the calculation so the final unit is checked against the quantity asked for.
Treating the normal force as always equal to mg, even on an incline, in an accelerating lift, or when another vertical force acts, and then computing friction as μmg in those situations.
The equation N = mg is over-learned from flat-surface examples, and the habit of drawing a free-body diagram is skipped when the problem looks familiar.
What to do instead
Always draw the free-body diagram and resolve forces perpendicular to the surface; write N from the perpendicular force balance (e.g. N = mg cosθ on a frictionless incline, N = m(g + a) in an upward-accelerating lift) before using it in friction.
Using conservation of mechanical energy on a system where friction or an inelastic collision occurs, or using momentum conservation for a system with a net external force, without checking the conditions.
Both conservation laws are taught as general shortcuts, and the validity conditions (no non-conservative work; zero net external force along the relevant direction) are not routinely tested before applying them.
What to do instead
Before writing any conservation equation, state the condition explicitly in one line: 'no friction/air resistance, so mechanical energy is conserved' or 'net external force along x is zero during the short collision, so p_x is conserved'. If the condition fails, use the work-energy theorem with the friction term or account for the external impulse.
Mixing Celsius and Kelvin in the ideal gas law or in the first law of thermodynamics, e.g. substituting 27 instead of 300 for T, or computing ΔT correctly but then using Celsius in pV = nRT.
Everyday familiarity with Celsius makes it the default, and the gas constant R is remembered with its numerical value but not with the requirement that T be absolute.
What to do instead
Adopt the rule that any temperature entering a gas or thermodynamic formula is written as T = t + 273 first; keep a habit of writing the conversion line above the equation so the absolute value, not the Celsius reading, is substituted.
Sign-confusing the first law of thermodynamics, especially writing Q = ΔU + A with the wrong sign for work done on the gas versus by the gas, or claiming ΔU = 0 for an isothermal process while also adding a work term incorrectly.
Different textbooks and notations (A vs W, work done by vs on the system) coexist, and students memorise one form without tracking which convention the problem uses.
What to do instead
Fix one convention at the start of every problem (e.g. Q = ΔU + W, where W is work done by the gas) and write it at the top of the page; then check each special case against it (isothermal: ΔU = 0 so Q = W; adiabatic: Q = 0 so ΔU = −W).
Combining resistors or capacitors with the series/parallel rules swapped, or applying series rules to a circuit where a component is shorted or bridged.
The rules are memorised as formulas (series adds R, parallel adds 1/R) without the underlying idea of shared current versus shared voltage, so under time pressure the wrong rule is retrieved.
What to do instead
Before computing, trace the current path and label which junctions share current and which nodes share voltage: elements in the same current path are in series, elements across the same two nodes are in parallel. Redraw the circuit with nodes labelled A, B, C so the topology is explicit.
Determining the direction of the magnetic force or induced current by guessing, or applying the right-hand rule to the wrong vector (e.g. using the velocity instead of the field as the first finger) and getting a reversed direction.
The right-hand rule has several variants (F = qv×B, F = IL×B, induced current opposing flux change) and students blend them; also, negative charges reverse the force direction, which is easily forgotten.
What to do instead
State the rule being used in words before applying it ('fingers point along v, curl toward B, thumb gives F on a positive charge; reverse for an electron'), and for induction always first determine whether the flux is increasing or decreasing, then apply Lenz's law to set the current direction.
Using the period of a pendulum or the resonance condition with the wrong variable, e.g. T = 2π√(l/g) with l in cm, or confusing angular frequency ω with frequency f and dropping the factor 2π in ω = 2πf.
The formulas look similar and the 2π factors are easy to lose; the distinction between ω (rad/s) and f (Hz) is often only nominal in students' notes.
What to do instead
Write the defining relations ω = 2π/T = 2πf explicitly at the start of oscillation problems and keep a units check: if the answer should be in Hz, the expression must not contain a stray 2π; convert lengths to metres before using pendulum formulas.
Applying the photoelectric equation with the wrong quantity as the work function or confusing threshold frequency with the frequency of incident light, e.g. writing hf = A + K with f taken as the threshold frequency.
The equation is memorised as a string of symbols without a physical picture of the energy balance (photon energy in, work function out, remainder as kinetic energy), and threshold quantities are not distinguished from incident quantities.
What to do instead
Write the energy-balance sentence first: 'photon energy hf minus the escape energy A equals the maximum kinetic energy', then substitute only the incident frequency for f and only the minimum escape energy for A; check the threshold case f = f_threshold gives K = 0 as a sanity test.
Reading a value off a graph or scale without accounting for the scale division and significant figures, e.g. reporting a gradient with more digits than the data support or misreading the axis units (mA versus A, ms versus s).
Graph reading is treated as trivial, so the axis labels and the smallest scale division are not checked; calculator output is copied verbatim without rounding.
What to do instead
Before reading, note the axis quantity, unit and value per smallest division; compute gradients from two well-separated points with coordinates written out, and round the final answer to the precision justified by the scale (typically half a division on the last read value).