Praxis Physics 5265 common mistakes
Praxis Physics 5265 · Understand the traps and how to avoid them.
When solving projectile or inclined-plane problems, the candidate plugs numbers into kinematic equations without first resolving initial velocity or acceleration into components parallel and perpendicular to the motion, then treats the vertical and horizontal motions as independent only after making sign errors.
Under time pressure, candidates default to memorized formulas like v = v0 + at without drawing a free-body diagram or coordinate system. They also forget that g is always downward, so a positive upward convention must be applied consistently to every term.
What to do instead
For every mechanics problem, first write a one-line coordinate choice (e.g., 'up positive, right positive') and list the knowns with signs. Then write the component equations separately (x: ax=0; y: ay=-g) before substituting any numbers. Practice 10 problems where you only set up the equations and check signs, not solve them.
On problems involving Newton's second law for systems (Atwood machines, blocks connected by strings, stacked blocks), the candidate applies F=ma to each object using the same tension or acceleration value without accounting for the direction of the string force on each body, or omits the normal force between stacked blocks.
Candidates memorize 'tension is the same throughout a massless string' but forget that the tension force points in opposite directions on the two ends. They also skip drawing separate free-body diagrams for each object and instead try to write one equation for the whole system.
What to do instead
Always draw one free-body diagram per object, label each force with its direction, and write ΣF=ma separately for each. Use a consistent sign convention per object. For stacked blocks, explicitly include the contact normal force between them. Drill 5 system problems where the solution requires at least three separate equations.
In electrostatics, the candidate computes the electric field at a point due to multiple charges by adding the magnitudes algebraically instead of adding the field vectors, ignoring that fields from positive and negative charges point in different directions.
Candidates conflate electric field (a vector) with electric potential (a scalar) and rely on the formula E=kq/r² without a direction. They also forget that the field due to a negative charge points toward the charge, not away.
What to do instead
Before calculating, sketch the point and draw an arrow for each field contribution showing its direction (away from positive, toward negative). Then resolve into components and add. Practice by computing both the net field and the net potential for the same configuration and noting that only the potential adds algebraically.
When analyzing DC circuits with multiple resistors, the candidate assumes that current is used up as it passes through resistors, or that the current through series resistors differs, or that voltage is the same across parallel branches without checking.
Everyday language ('the battery runs out of current') reinforces the misconception that current is consumed. Candidates also memorize 'series: same current; parallel: same voltage' but cannot apply it to a mixed circuit because they do not redraw the circuit in simplified stages.
What to do instead
Redraw the circuit step by step: first identify series and parallel groups, replace each group with its equivalent resistance, and track the total current from the source. Then work backward to find the current and voltage for each element, explicitly stating which conservation rule (junction rule or loop rule) justifies each value. Solve 3 mixed circuits this way per study session.
In wave and optics problems, the candidate confuses frequency and wavelength when a wave crosses from one medium to another, assuming both change, or applies the thin-lens equation without a sign convention for object and image distances.
The formula v=fλ is memorized as a single relationship, but candidates forget that frequency is set by the source and remains constant across media while speed and wavelength change. For lenses, the sign convention (real/virtual, converging/diverging) is often learned as a list of rules without a physical picture.
What to do instead
For refraction, write 'f fixed by source; v and λ change such that v/λ = f' before solving. For lenses, draw a ray diagram for each problem and assign signs based on the diagram (object distance positive for real object, image distance positive for real image on opposite side). Practice 5 problems where you predict whether the image is real or virtual before calculating.
In thermodynamics, the candidate applies the first law as ΔU = Q + W or ΔU = Q − W inconsistently, or assumes that in an isothermal process no heat is exchanged, or that in an adiabatic process temperature is constant.
Different textbooks use different sign conventions for work, and candidates memorize one formula without tracking whether work is done on or by the system. They also conflate the names of processes (isothermal, adiabatic, isobaric) with their defining conditions.
What to do instead
Adopt one convention (e.g., ΔU = Q − W, where W is work done by the gas) and write it at the top of every thermodynamics problem. For each process, state the defining condition (isothermal: ΔT=0; adiabatic: Q=0; isobaric: ΔP=0; isochoric: W=0) before using any equation. Work through a table comparing all four processes for an ideal gas.
In kinetic theory, the candidate uses the average kinetic energy formula (3/2 kT) for a gas that is not monatomic, or forgets that rms speed depends on molar mass and uses molecular mass in the wrong units.
The formula KE_avg = (3/2)kT is taught first for monatomic gases, and candidates overgeneralize it to diatomic and polyatomic gases where the equipartition theorem gives different degrees of freedom. Unit conversion between kg/mol and g/mol is also a common slip.
What to do instead
Memorize the equipartition result: each quadratic degree of freedom contributes (1/2)kT; monatomic has 3 translational, diatomic has 5 (3 translational + 2 rotational) at moderate temperatures. For rms speed, use v_rms = sqrt(3RT/M) with M in kg/mol. Always write the molar mass in kg/mol before substituting.
In modern physics, the candidate calculates the photoelectric threshold or de Broglie wavelength using the wrong mass (electron vs. proton) or forgets to convert electronvolts to joules when using SI formulas.
Problems mix units (eV for photon energy, J for kinetic energy) and candidates plug eV directly into E=hf without converting. They also grab the electron mass out of habit even when the problem specifies a proton or an unknown particle.
What to do instead
Before calculating, write a unit plan: list every quantity with its unit and convert all energies to joules (1 eV = 1.60×10⁻¹⁹ J) unless the problem explicitly asks for eV. For de Broglie wavelength, circle the particle named in the problem and use its mass. Practice 5 problems that require at least one unit conversion.
In laboratory and measurement questions, the candidate reports a calculated result with more significant figures than the least precise given value, or ignores systematic uncertainty by averaging repeated trials that share the same calibration error.
Candidates treat significant figures as a formatting afterthought and believe that more digits imply more accuracy. They also conflate precision (repeatability) with accuracy (closeness to true value) and assume repeated measurements eliminate all error.
What to do instead
For every calculation, identify the measured quantity with the fewest significant figures and round the final answer to that place. Distinguish random error (reduced by averaging) from systematic error (not reduced by averaging; requires calibration or correction). Practice by evaluating a data set where the mean is precise but offset from the accepted value.
When designing or interpreting an experiment, the candidate fails to identify the independent, dependent, and controlled variables, or proposes a procedure that changes more than one variable at a time and then attributes the effect to one of them.
Scientific inquiry questions are often presented as scenarios, and candidates jump to the conclusion without explicitly listing variables. They also confuse correlation with causation when analyzing graphed data.
What to do instead
Use a three-column table for every experiment: independent variable (what you change), dependent variable (what you measure), and controlled variables (what you keep constant). Before accepting a causal claim, check that only the independent variable differed between trials. Practice with 3 sample experiment descriptions by filling in the table and stating one possible confounding variable.