Estonian Basic-School Physics Exam common mistakes
Estonian Basic-School Physics Exam · Understand the traps and how to avoid them.
Writing a speed or density answer as a bare number (e.g. "5") without the unit, or mixing units within one calculation (km with s, grams with cm³) and reporting the result in whatever unit falls out.
Candidates treat the unit as decoration added at the end rather than part of the quantity, and they copy numbers from the task text without first converting them to a single coherent system (SI or a consistent cm–g–s set).
What to do instead
Before substituting, write every given value with its unit, convert to SI in a separate line (km/h ÷ 3.6 = m/s; g ÷ 1000 = kg; cm³ ÷ 1 000 000 = m³), then carry units through the arithmetic as if they were symbols. Check the final unit by cancelling: m ÷ s gives m/s, kg ÷ m³ gives kg/m³. Train with 5 short conversions daily until they are automatic.
Confusing mass and weight, or using g = 9.8 N/kg as if it were a force, and stating that a body's weight is the same on the Moon as on Earth.
In everyday Estonian speech "kaal" is used for both mass and weight, so the textbook distinction (mass in kg is constant; weight in N depends on gravitational field strength) never gets practised.
What to do instead
Force yourself to write the two sentences every time: "Mass m = 2 kg everywhere; weight F = m·g = 2 × 9.8 = 19.6 N on Earth, about 3.3 N on the Moon." Always attach N to forces and kg to masses in your working, and mark any answer in N that you obtained by multiplying by g.
Treating pressure as if it depended only on the total force, ignoring area - for example claiming a wide snowshoe sinks deeper because it is bigger, or that a nail is easier to drive with the head than the point.
Everyday intuition links "more force" with "more effect", and the inverse relation p = F/A is easily memorised as a formula without a mental picture of the same force spread over a larger area.
What to do instead
For every pressure question, draw the contact area and write p = F/A with the same F and two different A values, computing both. State the rule in words: same force, smaller area → larger pressure. Apply it to three concrete cases (skis, knife edge, syringe needle) so the direction of the effect is anchored.
Mixing up heat, temperature and internal energy: saying a cup of boiling water "contains more heat" than a bathtub of warm water, or that a thermometer measures heat.
In ordinary language "hot" and "heat" are interchangeable, and the distinction between intensive temperature and extensive thermal energy (which depends on mass and specific heat capacity) is abstract.
What to do instead
Use Q = mcΔT explicitly and compare two bodies numerically: 0.2 kg water cooling 80 °C releases about 67 kJ, while 100 kg water cooling 2 °C releases about 840 kJ. Write the sentence "temperature tells how hot; thermal energy tells how much" before each answer, and never write "heat" where you mean temperature.
Assuming a good conductor of heat is also a good conductor of electricity, or that all metals behave identically, and predicting that a plastic-handled spoon heats through as fast as a metal one.
Both properties are taught as "metals conduct", and the different mechanisms (free electrons versus lattice vibration) are rarely separated in the candidate's mind.
What to do instead
Make a two-column table (thermal conductor? electrical conductor?) and fill it with metal, plastic, glass, salt water, graphite, dry wood. Note explicitly that salt water conducts electricity but is a poor thermal conductor, and that graphite conducts electricity but not heat well. Memorise one counterexample in each direction.
Adding resistances in parallel by simple addition, or adding voltages around a parallel circuit as if it were a series circuit, then reporting a current that violates Kirchhoff's rules.
The series rules are learned first and are arithmetically simpler, so they are over-applied; the reciprocal formula for parallel resistors is also error-prone under time pressure.
What to do instead
Before calculating, redraw the circuit and label each junction with a node letter. Write the two rule sentences (current into a node = current out; voltage rises = voltage drops) and then choose the formula. For two parallel resistors use R = R₁R₂/(R₁+R₂) and sanity-check: the result must be smaller than the smallest resistor.
Treating a ray diagram as decoration and placing the image on the same side of a converging lens regardless of object distance, or claiming a virtual image can be projected on a screen.
Candidates memorise "lens makes an image" without the three special rays, and the sign conventions for real versus virtual images are not practised.
What to do instead
Draw all three principal rays every time (parallel ray through the focus, central ray undeviated, focal ray emerging parallel) and let their intersection decide the position. Then state in words: real image = rays actually meet, can be caught on a screen, inverted; virtual image = rays only appear to meet, upright, cannot be projected. Test with object outside 2F, between F and 2F, and inside F.
Confusing the frequency, wavelength and speed of a wave when it crosses into another medium - claiming the frequency changes on refraction, or that a louder sound has a higher pitch.
Amplitude, frequency and speed are all described as "how big the wave is", and the everyday words loud/high are used loosely for both intensity and pitch.
What to do instead
Write the relation v = f·λ and the boundary rule: at a boundary frequency stays fixed (set by the source), speed and wavelength change together. Then attach one word to each quantity - amplitude ↔ loudness/ brightness, frequency ↔ pitch/colour, speed ↔ medium. Practise with a sound entering water (f same, v larger, λ larger).
Writing a nuclear equation that balances mass number but not charge (or vice versa), or stating that beta decay emits an electron from the electron cloud rather than from the nucleus.
The conservation rules are treated as one rule ("numbers add up") and the transformation of a neutron into a proton is not linked to the emitted beta particle.
What to do instead
For every decay equation write two separate balance lines, top row mass numbers and bottom row atomic numbers, and check both. For beta-minus decay write the neutron change explicitly: n → p + e⁻ + antineutrino, so Z increases by 1 and A is unchanged. Verify with a known example such as carbon-14 → nitrogen-14 + e⁻.
Reporting a measured value with more decimal places than the instrument allows (e.g. 12.3456 cm from a millimetre ruler) or omitting the uncertainty and repeating a single measurement as if it were precise.
Calculators display many digits and candidates copy them, and the idea that a reading carries an uncertainty of about half the smallest division is rarely applied in practice.
What to do instead
Before writing any result, note the smallest division of the instrument and round to that (mm ruler → 0.1 cm, uncertainty ±0.05 cm). Repeat at least three times, take the mean, and state the result as mean ± uncertainty with the same number of decimals. Round the final answer only after the calculation, never intermediate values.