Tamil Nadu HSE First Year Physics common mistakes
Tamil Nadu HSE First Year Physics · Understand the traps and how to avoid them.
Writing the measured value without limiting significant figures to the instrument's least count, e.g. reporting a vernier caliper reading as 2.34 cm when the vernier least count is 0.01 cm but the main scale is read in mm, or dropping the trailing zero in 2.50 cm.
Students learn the significant-figure rules abstractly but never tie them to the actual instrument. They treat digits as decoration and round at the end out of habit, and the trailing zero feels 'unnecessary' after years of arithmetic where 2.5 equals 2.50.
What to do instead
Before every numerical answer, write down the least count of the instrument used. Then state the rule aloud: the last recorded digit must sit at the least-count place. Practise 20 readings from vernier caliper (LC 0.01 cm), screw gauge (LC 0.01 mm), and metre scale (LC 0.1 cm) until you automatically keep the trailing zero. Mark the significant digits with a dot under each when revising.
Adding or subtracting percentage errors instead of adding absolute errors, or adding percentage errors when multiplying/dividing instead of combining them correctly; for example computing the error in (a - b) as the sum of percentage errors of a and b.
The rules for combination of errors are memorised as a list without the underlying reason. Students confuse 'when do percentages add' with 'when do absolute errors add', and the formula sheet gives no cue about which operation is in play.
What to do instead
Anchor on one principle: absolute errors always add (worst case), and percentage errors add only for products and quotients. For sums and differences, convert to absolute errors first, combine, then convert back to percentage if asked. Drill mixed expressions such as Z = A²B/C and Z = A - B side by side so the contrast becomes automatic. Always show the intermediate absolute-error line in your working.
Substituting g = 9.8 m/s² into equations written in centimetre-gram-second units, or mixing metres and centimetres within a single kinematics calculation (e.g. u in m/s but displacement in cm), producing answers off by powers of ten.
School problems alternate between SI and cgs without warning, and students copy the previous problem's numbers. The habit of writing the formula first and plugging numbers second hides the unit clash until the final answer looks 'weird'.
What to do instead
Adopt a fixed three-step layout for every kinematics numerical: (1) list given quantities with units, (2) convert everything to SI in that list, (3) substitute. Never substitute directly into the formula line. If a problem is naturally in cgs, use g = 980 cm/s² deliberately and write 'cgs' at the top of the page. Do ten conversion drills (km/h to m/s, cm to m, g to kg) before attempting numericals.
Treating the relative velocity of two bodies moving in the same direction as v1 + v2, or applying the projectile range formula R = u²sin2θ/g to a launch from a height or to a target on an incline.
Relative velocity and projectile motion are taught as formula sets, and the sign conventions are glossed over. Students pattern-match on 'two velocities given' and 'projectile', ignoring direction and launch conditions.
What to do instead
Draw the velocity vectors before writing anything; same direction means subtraction, opposite means addition, and perpendicular means vector addition. For projectiles, always write the two component equations x = ucosθ·t and y = usinθ·t - ½gt² and solve from them rather than recalling R and H. Only invoke R = u²sin2θ/g after confirming launch and landing are at the same height on level ground, and state that condition explicitly.
Writing F = ma for a system and forgetting that tension differs on the two sides of a pulley, or omitting the component of weight along an incline (using mg instead of mg sinθ).
Free-body diagrams are skipped under time pressure. Students memorise 'tension is the same in a string' without the massless, frictionless-pulley caveat, and incline problems are attempted by analogy with horizontal ones.
What to do instead
Make a free-body diagram compulsory for every Laws of Motion problem, one diagram per body, with every force labelled and a chosen positive direction marked. For pulleys, write separate equations for each mass and add them to eliminate tension. For inclines, resolve weight into mg sinθ along the plane and mg cosθ perpendicular before writing Newton's second law. Check that the number of equations equals the number of unknowns.
Applying conservation of mechanical energy to a block sliding down a rough incline or through friction, and equating initial and final kinetic energy without accounting for work done against friction.
Conservation of energy is over-learned as 'KE + PE is constant' and the condition 'only conservative forces do work' is forgotten. Friction is treated as a detail rather than a term in the work-energy theorem.
What to do instead
Before using energy conservation, list every force doing work and classify it as conservative or non-conservative. If any non-conservative force acts, use the work-energy theorem: W_net = ΔKE, with friction contributing -f·d. Practise the same block-on-incline problem twice, once frictionless and once rough, and compare the final speeds so the difference in method is visible.
Using the equations of translational motion (v = u + at, s = ut + ½at²) for a rotating body instead of the angular analogues, or computing moment of inertia about the wrong axis (e.g. using I = ML²/12 for a rod about its end).
Rotational kinematics mirrors translational kinematics so closely that students substitute ω for v inconsistently, mixing linear and angular quantities in one equation. Moment-of-inertia formulae are memorised as a table without the axis attached.
What to do instead
Write a two-column table at the top of your rough sheet: translational quantity on the left, angular analogue on the right. Convert every given quantity into its angular form before substituting. For moment of inertia, always sketch the body, mark the axis, and write the axis beside the formula. Use the parallel-axis theorem I = I_cm + Md² whenever the axis is not through the centre of mass, and verify the result is larger than I_cm.
Stating that gravitational potential energy at a height h is mgh for large h, or confusing escape velocity (independent of mass and direction) with orbital velocity, and using g = 9.8 at altitude without variation.
The approximation mgh holds only near the Earth's surface, but textbooks introduce it first and it sticks. Escape velocity and orbital velocity formulae look similar (√(2GM/R) versus √(GM/r)), and students do not check which radius applies.
What to do instead
For any gravitation problem, first ask whether h is small compared with Earth's radius (use mgh) or comparable (use -GMm/r). Write the general expression U = -GMm/r and only approximate. For escape and orbital velocity, derive from energy and centripetal force respectively rather than recalling, and note that escape velocity is independent of the body's mass and launch angle. Keep Earth's mass, radius, and GM = 3.98 × 10¹⁴ m³/s² on a reference card.
Confusing stress with strain, or using Young's modulus without converting diameter to radius when computing cross-sectional area, and forgetting that strain is dimensionless.
The terms sound alike and the units (N/m² versus none) are the only clear discriminator. Area calculations invite the diameter/radius slip, and students rush past it because the arithmetic looks simple.
What to do instead
Write the definition of each quantity next to its symbol before substituting: stress = force/area (N/m²), strain = change in dimension/original dimension (no unit). For any wire or rod problem, compute the area explicitly as πd²/4 from the given diameter and circle it, so the radius error becomes visible. Check the final answer's unit against the expected unit of Young's modulus (N/m²).
Applying the wrong sign convention to the first law of thermodynamics, e.g. writing ΔQ = ΔU - ΔW so that work done by the gas comes out negative, or using Celsius temperatures in gas-law and efficiency calculations instead of kelvin.
Different textbooks use different sign conventions for W, and the student's notes may mix them. Temperature conversion is treated as trivial and skipped under exam pressure.
What to do instead
Pick one convention and write it at the top of every thermodynamics answer: heat absorbed by the system is positive, work done by the system is positive, so ΔQ = ΔU + ΔW. Apply it to four standard processes (isothermal, adiabatic, isochoric, isobaric) until each ΔQ, ΔU, W triple is automatic. Convert every temperature to kelvin in the 'given' list before substituting, and check that efficiency η = 1 - T₂/T₁ uses kelvin values.
Using the rms speed formula with molar mass in grams instead of kilograms, or confusing rms speed with average speed and most probable speed in kinetic theory problems.
The three speeds differ only by constants (√3, √(8/π), √2) and the molar mass unit slip is invisible in the arithmetic. Students memorise one formula and apply it to all three questions.
What to do instead
Write the three expressions side by side with their constants: v_rms = √(3RT/M), v_avg = √(8RT/πM), v_mp = √(2RT/M), and note v_rms > v_avg > v_mp. Always convert M to kg/mol (e.g. 32 g/mol = 0.032 kg/mol) in the given list. Sanity-check the magnitude: for nitrogen at 300 K, v_rms is about 517 m/s, so an answer in the thousands signals a unit error.
Writing the time period of a simple pendulum as T = 2π√(l/g) without checking that the amplitude is small, or confusing angular frequency ω with frequency f and substituting f where ω belongs in x = A sin ωt.
The pendulum formula is memorised as universally valid and the small-angle condition (sinθ ≈ θ) is forgotten. The ω versus f confusion arises because both appear in SHM without the factor 2π being emphasised.
What to do instead
Attach the condition to the formula in your notes: T = 2π√(l/g) holds only for small oscillations (typically under about 4°). For ω and f, write ω = 2πf = 2π/T every time before substituting, and check that the argument of sine is dimensionless. For wave problems, use v = fλ and v = ω/k together so the relationship is reinforced.
In the practical record, omitting the least count and zero error of the instrument, not applying zero-error correction with the correct sign, or failing to plot the graph with labelled axes and a stated scale.
Practical marks are lost on presentation rather than physics. Zero-error correction has a sign rule (positive error subtracted, negative added) that students apply backwards, and graph work is rushed at the end of the exam.
What to do instead
Keep a fixed practical template: instrument name, least count, zero error with sign, observed readings, mean, corrected value, and unit. Practise ten zero-error corrections until the sign rule is automatic. For graphs, always label both axes with quantity and unit, state the scale, plot points with a sharp pencil, and draw the best-fit straight line. Compute the slope from two widely separated points on the line, not from data points.