MTTC Physics (019) common mistakes
MTTC Physics (019) · Understand the traps and how to avoid them.
On a scenario asking whether a lab supports a claim, the candidate accepts a low-scatter data set as proof even when the instrument has a nonzero zero error and the trend is offset from the accepted value.
The candidate conflates precision with accuracy and does not build an uncertainty budget that separates random scatter from systematic offset.
What to do instead
Have the candidate classify each error source as random or systematic, check calibration/zero error, compare the result with the accepted value using uncertainty, and only then judge whether the claim is supported.
When converting a research question into an experiment, the candidate labels the quantity being changed as the dependent variable and the measured outcome as the independent variable, or fails to operationally define either variable.
The candidate memorised the variable labels but has not practised mapping a question onto a design with operational definitions and controls.
What to do instead
Require a written design statement: 'I change X, I measure Y, and I hold Z constant,' with an instrument, unit, and procedure-level definition for each variable; then identify controls and plausible confounds.
In a projectile or incline problem, the candidate substitutes into kinematic equations without first fixing a positive axis, then reports a negative speed or adds g in the wrong direction when the motion reverses.
The candidate is formula-matching rather than carrying a vector sign convention through the solution.
What to do instead
Draw a sketch and free-body diagram, declare the positive direction in writing, keep signs through every equation, interpret a negative result as a direction rather than a negative speed, and check the answer against the physical motion.
The candidate claims mechanical energy is conserved for a block sliding with friction because 'energy is always conserved,' then uses ½mv² = mgh to find the speed.
The candidate conflates conservation of total energy with conservation of mechanical energy and overlooks work done by nonconservative forces.
What to do instead
Audit the energy transfers and use ΔK + ΔU = W_nc; when friction is present, calculate f_k d and include it. Use mechanical-energy conservation only after showing W_nc = 0.
In a refraction problem, the candidate measures the angle from the water/glass surface instead of from the normal, or writes n1/n2 = sin θ2/sin θ1.
The candidate follows the drawn interface rather than the normal and recalls Snell's law as a ratio without checking which index pairs with which angle.
What to do instead
Draw the normal at the interface, label both angles from the normal, write n1 sin θ1 = n2 sin θ2, and sanity-check that light bends toward the normal when entering a higher-index medium.
For a standing wave in a pipe or string, the candidate uses L = nλ/2 for a pipe open at one end or counts antinodes as nodes, producing the wrong harmonic.
The candidate memorised a standing-wave formula without mapping boundary conditions to nodes and antinodes.
What to do instead
Sketch the displacement pattern first: a closed end is a node and an open end is an antinode. Derive L in half-wavelengths or quarter-wavelengths from the sketch, then solve for λ and identify the harmonic number.
In a series-parallel circuit, the candidate says current is 'used up' by the first resistor or assigns the same current to parallel branches with unequal resistance.
The candidate overextends a water-flow analogy and does not apply the junction rule and Ohm's law at circuit nodes.
What to do instead
Mark nodes on the diagram, apply the junction rule at each node, assign equal voltage across parallel branches, and let current divide inversely with resistance; verify branch values with V = IR and power checks.
The candidate finds the direction of magnetic force on a moving charge with a right-hand rule but ignores the sign of the charge, or predicts an induced current that reinforces the flux change.
The candidate memorised the hand motion without attaching the charge sign to F = qv × B or using Lenz's law as an opposition principle.
What to do instead
Use F = qv × B and reverse the force for a negative charge. For induction, identify the change in magnetic flux and choose the induced current whose field opposes that change; confirm with energy conservation.
In a first-law problem, the candidate mixes sign conventions by giving work done by the gas and work done on the gas the same sign, or calls an isothermal process adiabatic because the temperature does not change.
The candidate has not stated a convention before calculating and conflates process definitions with observable effects.
What to do instead
Write the convention explicitly, such as ΔU = Q − W_by, and list the sign of each Q and W term. Distinguish isothermal (T constant, ΔU = 0 for an ideal gas) from adiabatic (Q = 0), and check the process on a PV diagram.
The candidate computes photon energy with E = hc/λ but substitutes λ in nm while using h in J·s, reports eV without converting, or says brighter light increases the maximum kinetic energy of photoelectrons below threshold.
The candidate mixes unit systems and confuses photon intensity with photon frequency in the photoelectric effect.
What to do instead
Use hc = 1240 eV·nm for quick photon-energy work or convert all quantities to SI before calculating. For the photoelectric effect, maximum kinetic energy depends on frequency, while intensity changes the number of emitted electrons; apply the threshold condition hf = φ.