MTEL Physics (69) common mistakes
MTEL Physics (69) · Understand the traps and how to avoid them.
On a mechanics constructed-response item, begins substituting into kinematics equations before choosing a positive axis or drawing a motion diagram, then reports a negative time or a displacement with the wrong sign.
Timed conditions reward recalling a formula quickly; candidates skip the setup step that fixes signs and constant-acceleration assumptions.
What to do instead
Write a one-line coordinate choice (e.g., up positive), sketch a motion map with velocity and acceleration arrows, list knowns with signs, and only then select the equation that contains the unknown.
Writes N = mg for a block on an incline or a person in an accelerating elevator, then carries that value into friction or apparent-weight calculations.
Everyday experience makes normal force feel identical to weight; the perpendicular component and acceleration are easy to overlook.
What to do instead
Draw a free-body diagram, resolve forces perpendicular to the surface, and solve ΣF_perp = ma_perp; on a frictionless incline N = mg cosθ, and in a vertical elevator N = m(g ± a).
Applies ½mv² = mgh to a rough incline or a spring problem without including work done by friction or by the spring, or changes the potential-energy zero between initial and final states.
Conservation of energy is overgeneralized as 'energy before equals energy after' without tracking nonconservative work and reference levels.
What to do instead
Write W_nc = ΔK + ΔU, choose one reference level for the whole problem, and compute friction work as −f_k d with d the actual path length.
Adds a separate 'centripetal force' arrow to a free-body diagram alongside tension, gravity, or friction, instead of treating the inward components of real forces as the centripetal force.
The phrase 'centripetal force' sounds like an additional force; in an inertial frame it is a net-force role, not a new interaction.
What to do instead
Draw only real forces, choose the radial direction toward the center, and set the sum of inward components equal to mv²/r; never add an extra centripetal arrow.
Uses ΔU = Q + W in one thermodynamics item and ΔU = Q − W in another without stating which work sign convention is being used.
Physics and chemistry textbooks define W oppositely, and candidates mix formulas from different sources under time pressure.
What to do instead
At the top of the solution write the convention (e.g., W = work done by the gas, so ΔU = Q − W), then keep that sign for every term and check an adiabatic or isothermal case.
When light crosses from air into glass, changes the frequency along with the speed and wavelength, then uses the altered frequency in E = hf or in a photoelectric threshold comparison.
The relation v = fλ is applied as if all three quantities change, rather than recognizing that frequency is fixed by the source.
What to do instead
State that f stays constant across a boundary; compute v = c/n and λ = λ0/n, and use the original f in E = hf.
Uses the thin-lens or mirror equation with object and image distances measured from inconsistent surfaces, or assigns a positive image distance to a virtual image.
Sign conventions differ among textbooks and between lenses and mirrors, and a quick sketch is often omitted.
What to do instead
Adopt one published sign table, label the object side and image side on a ray diagram, and test the result with a limiting case (e.g., magnifying glass gives a virtual, upright image).
Says current is 'used up' as it passes through series resistors, or assumes parallel branches always split current equally regardless of resistance.
The water-flow analogy is misapplied; charge conservation and the reciprocal resistance rule are not practiced enough.
What to do instead
Apply Kirchhoff's junction rule (same current in series) and loop rule; for parallel branches use I_i = V/R_i, so smaller resistance carries larger current.
Applies the right-hand rule to a current-carrying wire by pointing fingers along B and thumb along I, which reverses the force direction, or forgets to reverse the result for a negative charge.
Two similar rules (q v × B and I L × B) are conflated, and the negative charge sign is easy to drop.
What to do instead
Use F = I L × B: fingers along current, curl toward B, thumb gives force; for a moving charge use q v × B and reverse the final direction if q is negative.
Concludes that brighter light increases the maximum kinetic energy of photoelectrons, or substitutes frequency in Hz into E = hf while using h in eV·s without converting.
Intensity and photon energy are conflated, and mixed unit systems produce plausible but wrong numbers.
What to do instead
Use K_max = hf − φ; intensity changes the number of photons and saturation current, not K_max; convert h to J·s or frequency to appropriate units before calculating.