MATSEC IM Physics common mistakes
MATSEC IM Physics · Understand the traps and how to avoid them.
Writing the final numerical answer without the unit, or converting a quantity into SI units only at the last step while intermediate values stay in cm, g, or minutes.
Candidates copy numbers straight from the stem and substitute them into a formula without first building a consistent unit system, so mixed prefixes (cm and m, g and kg, minutes and s) silently corrupt the arithmetic and the answer loses its meaning.
What to do instead
Before substituting, write a one-line 'given' list converting every quantity to SI (m, kg, s, A, K, mol, cd) and record the unit beside each symbol. Carry units through the calculation as algebra so the final unit is forced to come out right; reject any answer whose unit does not match the quantity asked for.
Treating a vector quantity as a scalar: adding speeds without direction, or using a scalar equation like v = u + at along a curved path where acceleration changes direction.
Most early kinematics practice is one-dimensional, so candidates memorise magnitudes and forget that velocity, acceleration, force, and momentum are vectors; resolving components feels like extra work and is skipped.
What to do instead
Adopt a fixed sign convention (e.g. up/right positive) written at the top of every solution, and draw a small arrow diagram before choosing equations. For two dimensions, split into x and y components and solve each axis separately; check that the vector sum of forces or momenta has the direction the physical situation demands.
Confusing mass with weight, and using g = 9.81 N/kg as if it were a constant force rather than a field strength that converts mass to weight.
Everyday language uses 'weight' for mass, and the numerical value of g is memorised as a number to multiply by without attaching a unit meaning; candidates then report weight in kg or mass in N.
What to do instead
State the definition explicitly each time: mass is the quantity of matter in kg and is invariant; weight is the gravitational force W = mg in N and depends on the local field strength. When a question says 'weight', write W and the unit N; when it says 'mass', write m and the unit kg, and check the unit of the final line against the wording of the question.
Reading a distance–time or velocity–time graph as if the vertical axis were the other quantity, or claiming the gradient of a displacement–time graph is acceleration.
The shapes of the two graph types look similar, and candidates learn 'gradient = rate' as a slogan without mapping it onto the specific axes in front of them; area under the curve is often ignored entirely.
What to do instead
Before answering, label both axes in words and write 'gradient = ...' and 'area = ...' for that specific pair of axes. For displacement–time, gradient is velocity; for velocity–time, gradient is acceleration and area is displacement. Sanity-check by units: gradient units must be (y-unit)/(x-unit).
Applying conservation of momentum to a collision without checking whether it is elastic, and separately assuming kinetic energy is always conserved.
Momentum and energy conservation are taught together, and candidates conflate the two conditions; they also forget that momentum is a vector so signs matter in one-dimensional collisions.
What to do instead
Write two separate statements: momentum is conserved in all isolated collisions (vector, so assign signs), kinetic energy is conserved only in elastic collisions. Compute total kinetic energy before and after to test elasticity rather than assuming it; for inelastic cases, account for the missing energy as heat, sound, or deformation.
Mixing up the conditions for total internal reflection: using the critical angle formula with the wrong refractive index ratio, or applying it when light travels from a less dense to a denser medium.
The formula sin C = 1/n is memorised for the air-to-glass case and then misapplied to glass-to-air, and candidates forget that total internal reflection requires the ray to be inside the optically denser medium and above the critical angle.
What to do instead
State the condition first: light must travel from higher n to lower n, and the angle of incidence must exceed C. Use sin C = n2/n1 with n1 the medium the light is in; for glass to air this becomes sin C = 1/n. Check the direction of the ray on a sketch before substituting, and confirm the computed C is less than 90 degrees.
Treating the image formed by a converging lens or concave mirror as always real, or placing the object inside the focal length and still predicting a real image.
The thin-lens and mirror formulae give a sign for the image distance that candidates ignore, and ray diagrams are drawn from memory rather than from the actual focal length and object position.
What to do instead
Use a consistent sign convention (real-is-positive or Cartesian) written beside the equation, and interpret the sign of the image distance explicitly. Draw two principal rays from the object: for an object inside the focal length of a converging lens, both rays diverge and the image is virtual, upright, and magnified. Cross-check the magnification sign against the diagram.
Adding resistances in parallel by simple summation, or treating the terminal voltage of a cell as equal to its EMF while current flows.
Series addition is learned first and generalised incorrectly; the internal resistance of a cell is treated as a textbook abstraction rather than a real series resistor that drops voltage when current flows.
What to do instead
Write the parallel rule as reciprocals: 1/R = 1/R1 + 1/R2, and check that the combined resistance is smaller than the smallest branch. For a cell, model it as an ideal EMF in series with internal resistance r, so terminal voltage V = EMF − Ir; test the limiting cases I = 0 (V equals EMF) and short circuit (V falls to zero).
Confusing the direction of the magnetic field around a current-carrying wire with the direction of the force on a charge or wire in that field.
The right-hand grip rule and Fleming's left-hand rule are both 'hand rules' and are memorised as gestures without attaching each to its physical situation, so candidates apply the field rule when asked for the force.
What to do instead
Separate the two rules explicitly: the grip rule gives the field circling a current (thumb along current, fingers along field); Fleming's left-hand rule gives the force on a current in an external field (first finger field, second current, thumb force). Always draw the external field direction as arrows first, then apply the force rule, and note that reversing either the current or the field reverses the force.
Applying the ideal gas equation or a thermodynamic relation without converting temperature to kelvin, or mixing up heat capacity with specific heat capacity.
Room temperatures are habitually quoted in Celsius, and the symbols c and C differ only by case, so candidates substitute 20 instead of 293 and lose the proportionality entirely, or multiply by C when the mass is given in grams.
What to do instead
Convert every temperature to kelvin at the 'given' stage and write T = θ + 273 explicitly. Distinguish the symbols: c is specific heat capacity in J/(kg·K) and needs mass in kg, while C is heat capacity in J/K for a whole object. Check units of Q = mcΔT: kg × J/(kg·K) × K must give J.
Mixing up the photoelectric equation: writing hf = work function + maximum kinetic energy with the terms in the wrong places, or claiming emission occurs at any frequency provided the intensity is high enough.
The equation is memorised as a string of symbols without a physical picture of energy conservation at the surface, so the threshold frequency concept is lost and intensity is wrongly assumed to control emission.
What to do instead
Write the energy balance in words first: photon energy hf = work function Φ + maximum kinetic energy of the emitted electron. State that emission requires f above the threshold f0 = Φ/h, that intensity controls the number of electrons per second, and that maximum kinetic energy depends on frequency, not intensity. Sketch the straight-line graph of maximum kinetic energy against frequency with gradient h and intercept −Φ.
Applying the inverse-square law for gravitational or electric field strength without squaring the distance, or measuring distance from the surface of a planet rather than its centre.
The formula is remembered as 'inverse distance' because the squared symbol is easily dropped in recall, and diagrams show the surface of a sphere rather than its centre, so r is taken as the height above the surface.
What to do instead
Write the law with the square shown: g = GM/r² and E = Q/(4πε0r²), and define r explicitly as the distance from the centre of mass or the centre of charge. When a height above a planet's surface is given, add the planetary radius to obtain r, and check with a limiting case: doubling r must quarter the field strength.