Georgia Unified Physics common mistakes
Georgia Unified Physics · Understand the traps and how to avoid them.
Applying a kinematic equation such as v = u + at or s = ut + ½at² to a situation where acceleration is not constant (for example a block on a spring, an object with air resistance, or a car whose braking force varies), then reporting the answer as exact.
The standard four kinematic equations are memorized as a default tool, and the constant-acceleration condition is treated as a formality rather than a checkable premise. Under time pressure, candidates grab the most familiar equation before reading the force description.
What to do instead
Before writing any kinematic equation, explicitly write the acceleration function or state 'a is constant because the only force is a constant weight'. If the net force changes with position or speed, switch to energy methods (½mv², spring potential ½kx²) or Newton's second law as a differential equation. Practise sorting ten short scenarios into 'constant a' and 'variable a' in under two minutes.
Treating a force as if it were a velocity or a momentum when resolving vectors: adding a normal force to a velocity in the same diagram, or claiming the centripetal force 'points outward' on a car rounding a curve.
Free-body diagrams and motion diagrams are drawn on the same page, and arrows are copied between them without relabeling. The centrifugal pseudo-force feels intuitive from the passenger's frame but is not a real force in the inertial frame used for Newton's second law.
What to do instead
Use two physically separate diagrams: one labeled 'forces on the object' and one labeled 'velocity and acceleration'. In the force diagram, only include forces from identifiable interactions (gravity, normal, tension, friction, applied, electric, magnetic). State the frame of reference at the top of every problem and never draw a centrifugal arrow in an inertial frame.
Sign-confusing work and heat in the first law: writing ΔU = Q + W with the wrong convention, or concluding that a gas that expands adiabatically must cool because 'work is done on it'.
Two sign conventions (physics: W is work done by the gas; chemistry: W is work done on the gas) circulate in textbooks, and candidates mix them mid-problem. Adiabatic expansion is also misread because 'no heat' is confused with 'no energy change'.
What to do instead
At the start of every thermodynamics item, write the convention you will use, e.g. 'ΔU = Q − W_by gas', and keep it for the whole solution. For adiabatic processes, set Q = 0 and reason from ΔU = −W_by gas: expansion means W_by gas > 0, so ΔU < 0 and T falls. Check the sign against the ideal-gas temperature at the end.
Applying the ideal-gas law to a process without checking which quantity is held fixed, or using degrees Celsius in pV = nRT and in ratio calculations.
Boyle's, Charles's, and Gay-Lussac's laws look like the same equation, and candidates substitute whatever numbers are given without identifying the constant. Celsius values are often the ones printed in the stem, so they get plugged in directly.
What to do instead
Before substituting, write 'constant: n and T' (or whichever applies) and cross out the variable that is fixed. Convert every temperature to kelvin in the first line of work and write the conversion explicitly (T/K = θ/°C + 273.15). Sanity-check ratios: doubling absolute temperature at constant volume must raise pressure by a factor of two, not by 1.5.
Mixing up the wave quantities in refraction and thin-film problems: using the vacuum wavelength in a film, or using the frequency to compute path difference inside a medium.
Frequency is invariant across boundaries while speed and wavelength change, and formulas such as n = c/v are applied mechanically without tracking which quantity is being altered. Thin-film problems add the extra trap of a possible half-wavelength phase shift on reflection.
What to do instead
For each medium, write the triplet (v, λ, f) and state that f is unchanged; compute λ_medium = λ_vacuum / n before any path-difference calculation. For thin films, check both interfaces: a phase shift of π occurs when light reflects from a higher-index medium. Then set 2nt = mλ or (m + ½)λ according to the number of shifts, and state m's parity.
Drawing a ray diagram or applying the mirror/lens equation with the wrong sign for a virtual image, or placing the image on the same side as the incoming light for a converging lens.
The Cartesian sign convention and the 'real is positive' convention are both taught, and the image-side rule differs between mirrors and lenses. Candidates also trust a memorized formula over the diagram they just sketched.
What to do instead
Choose one convention per problem and write it down: for a converging lens with light left to right, a real image is on the right and has positive image distance; for a concave mirror, a real image is on the same side as the object. Always draw the three principal rays first and use the diagram to predict the sign, then confirm with 1/f = 1/u + 1/v.
Adding resistor values in a mixed series–parallel circuit as if all were in series, or assuming the current through each branch of a parallel network is equal.
The circuit is often described in prose rather than drawn, and candidates simplify the first pair they see without redrawing. Equal-current intuition comes from everyday 'splitting' language and from confusing parallel resistors with identical lamps.
What to do instead
Redraw the circuit at every simplification step, marking nodes with letters so that components between the same two nodes are visibly in parallel. Compute the equivalent resistance bottom-up, then find the total current, then work back through each branch using V = IR at each node. Verify with a power balance: total power supplied equals the sum of I²R in each resistor.
Using the right-hand rule inconsistently for magnetic force and induced current: predicting the force on a moving charge with the wrong hand, or getting the direction of an induced current without applying Lenz's law to the change in flux.
Three separate right-hand rules (force on a charge, field of a wire, induced current) are learned close together and blur. Lenz's law requires reasoning about whether flux is increasing or decreasing, which is skipped when candidates jump straight to a formula.
What to do instead
Label which rule you are using before you place your hand: 'F = qv × B, right hand, fingers along v, curl to B, thumb gives F for positive q'. For induction, write the flux Φ = BA cos θ, state whether it is rising or falling, then choose the induced current direction that opposes that change. Check the result against the mechanical work done against the magnetic force.
Treating time dilation and length contraction as symmetric without tracking which frame is the proper frame, and applying the formulas when the relative speed is small enough that the effect is negligible.
The two formulas look interchangeable, and the phrase 'moving clocks run slow' is remembered without the reciprocal statement. Candidates also forget that proper time is measured by a clock at rest with respect to the two events.
What to do instead
For every relativity item, list the two events and identify the frame in which they occur at the same place; the time in that frame is the proper time τ, and the dilated time is γτ. Identify the frame in which the object is at rest to get proper length L₀, and use L = L₀/γ for the frame in which it moves. Compute γ numerically first and note whether it differs from 1 by more than the precision of the data.
Reporting a measurement with more significant figures than the instrument supports, or quoting an uncertainty without stating whether it is the resolution of the scale or the standard deviation of repeated readings.
Calculators return many digits and candidates copy them through. The distinction between resolution (half the smallest division, or the division itself) and statistical uncertainty from repeats is rarely made explicit in the stem.
What to do instead
State the instrument resolution before recording data, and report each reading to that place value. When repeats are available, compute the standard deviation and quote it as the uncertainty; otherwise quote half the smallest division. Carry one extra digit through intermediate calculations and round the final answer to match the uncertainty, writing it as (value ± uncertainty) unit.