ILTS Science: Physics (243) common mistakes
ILTS Science: Physics (243) · Understand the traps and how to avoid them.
On incline problems, substitutes mg for the component of weight along the plane and sets the normal force equal to mg even when the surface is tilted.
F = ma is applied with a single unrotated axis, and the flat-surface result N = mg is memorized rather than derived.
What to do instead
Draw a free-body diagram, rotate axes parallel and perpendicular to the incline, then write ΣF∥ = mg sinθ and N = mg cosθ. Check limiting cases θ = 0° and θ = 90°.
Predicts an induced current whose magnetic field points in the same direction as the change in flux, or applies the right-hand rule to the external field instead of the induced field.
Lenz's law is reduced to the word 'oppose' without tracking whether flux is increasing or decreasing, and right-hand-rule practice is shaky.
What to do instead
Use a three-step routine: compute ΦB before and after, state whether flux is increasing or decreasing, then choose the induced field that opposes that change. Apply the right-hand rule only to the induced field.
Assumes the frequency of light changes when it enters a new medium, or predicts that light entering a higher-index medium bends away from the normal.
Mechanical-wave intuition (v = fλ with f changing) is misapplied, and Snell's law is memorized without a normal-line sketch.
What to do instead
Anchor on f being fixed by the source, v = c/n, and λ = λ0/n. Sketch the normal and use n1 sinθ1 = n2 sinθ2 to show that higher n gives a smaller angle from the normal.
Applies Q = mcΔT during melting or boiling, or flips the sign of work midway so that ΔU = Q + W silently becomes ΔU = Q − W.
Textbooks use two different sign conventions, and phase-change plateaus are not visualized as constant-temperature intervals.
What to do instead
Write the chosen convention on scratch paper before solving. Split the process into heating segments (Q = mcΔT) and phase-change segments (Q = mL), and sanity-check that ΔU = 0 for an isothermal ideal gas.
Says an experiment 'proves' a hypothesis, or calls a well-tested explanation a 'law' once enough evidence accumulates.
Everyday speech conflates proof, theory, and law, and school labs often oversell confirmation of a single result.
What to do instead
Use 'supports' or 'fails to refute.' Define a theory as an explanatory framework and a law as a descriptive regularity. Require identification of alternative explanations and controls before accepting a conclusion.
Predicts that brighter light gives photoelectrons more maximum kinetic energy, or that a higher-intensity sub-threshold beam will eventually eject electrons.
Wave-model intuition is carried into the photoelectric effect, confusing photon energy with photon number.
What to do instead
Write KEmax = hf − φ. Intensity sets the current (photons per second), while frequency sets KEmax; the threshold f0 = φ/h is independent of intensity.
Uses conservation of kinetic energy for a collision in which objects stick together, or applies momentum conservation without vector components in a two-dimensional collision.
Energy conservation is over-learned, and the word 'collision' triggers a memorized equation without classifying the interaction first.
What to do instead
Classify the collision as elastic, inelastic, or perfectly inelastic before writing equations. Use momentum conservation always, kinetic energy conservation only when elastic; resolve p into x and y components.
Says current is 'used up' as it passes through resistors, so the current before and after a series resistor is treated as different.
An energy or water-pressure analogy is misapplied, replacing the charge-flow picture with a consumption picture.
What to do instead
Invoke charge conservation at each node: series current is identical everywhere, and parallel branches share the same voltage. Energy is dissipated as P = I²R, not charge.
Reports a result with more significant figures than the instrument allows, or adds percentage uncertainties directly when multiplying measured quantities.
Calculator output is treated as data, and uncertainty propagation rules are memorized without their underlying logic.
What to do instead
Match significant figures to the least precise measurement. For multiplication add percentage uncertainties; for addition add absolute uncertainties. Always state the uncertainty with its unit.
Uses L = nλ/2 for an open pipe, or counts antinodes instead of half-wavelength loops when identifying the harmonic number.
Standing-wave formulas are memorized per system rather than derived from boundary conditions, and nodes and antinodes are confused.
What to do instead
Derive from boundary conditions every time: a fixed end is a node and an open end is an antinode. Count half-wavelength loops along the medium to determine n.