Saga Public High School Entrance Examination - Science common mistakes
Saga Public High School Entrance Examination - Science · Understand the traps and how to avoid them.
When calculating average speed, the candidate adds the two speeds and divides by two instead of dividing total distance by total time, so a trip out at 40 km/h and back at 60 km/h is reported as 50 km/h rather than 48 km/h.
The word 'average' triggers a reflex to average the numbers that appear in the problem. Students have practised arithmetic means far more often than the harmonic-style reasoning that average speed requires, and the two speeds look symmetric and harmless.
What to do instead
Before any average-speed question, write the definition v = total distance / total time at the top of the working. Compute the time for each leg separately, add them, then divide. Use a fixed total distance such as 120 km so both legs give clean times, and check that the answer lies closer to the slower speed than to the midpoint.
The candidate treats a force as something a moving object carries with it, so they claim a ball thrown upward still has an upward force acting on it at the top of its flight, or that a constant push is needed to keep a sled moving at steady speed on frictionless ice.
Everyday experience of pushing heavy objects builds an intuitive 'motion implies force' model that survives instruction unless it is confronted directly. Textbooks' force diagrams are often copied without asking which object exerts each arrow.
What to do instead
For every force arrow, name the object exerting it. If no object can be named, erase the arrow. Practise the pair of contrasting cases: a pushed block on rough ground at steady speed (net force zero, push balances friction) and a puck on frictionless ice (no forward force at all). Draw the top-of-flight free-body diagram for a thrown ball with only weight present.
When a circuit is changed, the candidate assumes current stays the same when a resistor is added in series, or assumes voltage stays the same when cells are added, then applies Ohm's law to the wrong quantity and reports a current that violates conservation of charge.
Students memorise V = IR as three interchangeable symbols and never anchor which quantity is fixed by the battery and which is fixed by the components. Series and parallel rules are recalled as formulas without a mental model of the single path versus branching path.
What to do instead
Adopt a fixed procedure: identify the source voltage, redraw the circuit as a single equivalent resistance, compute total current, then work backwards through each element. Check the result against two invariants: current is the same everywhere in a series loop, and voltages across series elements must sum to the source voltage. Do three numeric drills changing one element at a time.
The candidate reads a wave diagram and confuses wavelength with amplitude, or measures wavelength from a trough to the next crest, and then uses the wrong value in v = f x lambda.
Both quantities are horizontal or vertical distances on the same drawing, and the standard sketch labels are small. Students also remember 'distance between two peaks' without the requirement that the two points be in the same phase.
What to do instead
Always mark one full cycle on the diagram with a bracket: crest to crest, or any point to the next point in the same phase and same direction of travel. Separately draw a vertical double arrow for amplitude. Practise converting a period to frequency with f = 1/T before touching the wave equation.
The candidate explains dissolving, melting, or diffusion by saying particles 'melt', 'expand', or 'disappear', or states that particles themselves get bigger when a substance is heated, rather than describing increased separation and increased speed of motion.
The particle model is taught with static diagrams, so students attach macroscopic words like melting and boiling to the particles themselves. The distinction between a change of state and a chemical change is also blurred because both are described as 'changes'.
What to do instead
Use one sentence frame for every particle explanation: 'The particles gain kinetic energy, move faster, and the spaces between them increase; the particles themselves are unchanged.' Apply it to expansion, melting, evaporation and diffusion in turn, and contrast it with a reaction where new particles with new arrangements are formed.
The candidate balances a chemical equation by changing subscripts, for example writing H2 + O2 -> H2O2 to make the oxygen atoms match, instead of placing coefficients in front of the formulas.
Balancing feels like a puzzle to be solved by adjusting numbers, and subscripts are the most visible numbers on the page. Students do not yet treat a formula as an identity that defines the substance.
What to do instead
State the rule explicitly before each equation: subscripts belong to the substance and may never be altered; only coefficients may change. Balance in a fixed order (metals, non-metals, hydrogen, oxygen last), and finish by recounting every element on both sides. Include one worked counterexample showing that H2O2 is a different compound from water.
The candidate writes the neutralisation product as 'salt + hydrogen' or names the salt incorrectly, for example giving sodium sulfate when hydrochloric acid reacts with sodium hydroxide, because they match the wrong acid anion to the metal.
The reaction of a metal with an acid does produce hydrogen, so the pattern is over-generalised to all acid reactions. Salt naming requires tracking which anion came from which acid, a step students skip when writing quickly.
What to do instead
Build a two-column table pairing each common acid with its anion (hydrochloric/chloride, sulfuric/sulfate, nitric/nitrate) and use it whenever naming a salt. Write the general pattern acid + base -> salt + water and acid + metal -> salt + hydrogen side by side, and classify each practice reaction before predicting products.
The candidate describes mitosis and meiosis as interchangeable, claiming meiosis produces identical body cells or that mitosis halves the chromosome number, and so mispredicts the chromosome count in gametes.
Both are division processes with similar-looking stage diagrams, and the vocabulary is learned as a list of definitions rather than as two processes with different purposes and different products.
What to do instead
Anchor each process to its purpose and product: mitosis for growth and repair, two identical diploid cells; meiosis for gametes, four genetically varied haploid cells. Track chromosome number numerically through a worked example (human cell 46 -> gametes 23 -> zygote 46), and draw both outcomes side by side from the same starting cell.
The candidate treats a dominant allele as the more common allele in the population, or assumes a recessive trait must have disappeared from a family, and so misreads pedigree and cross diagrams.
The everyday meaning of 'dominant' as 'prevailing' conflicts with the genetic meaning of 'expressed when present'. Probability reasoning about small families is also weak, so students expect exact ratios in every family.
What to do instead
Define dominance only in terms of expression: a dominant allele is expressed when one copy is present. Work a cross between two heterozygous parents and list all four offspring combinations, stressing that the 3:1 ratio is a probability over many offspring, not a guarantee per family. Use a pedigree where a recessive trait skips a generation to show carriers.
The candidate describes energy flow in a food chain as a cycle, or states that energy is lost because organisms 'use it up', rather than recognising that energy is transferred to the environment as heat and that matter cycles while energy does not.
Carbon and nitrogen cycles are drawn as loops, so the circular drawing is generalised to energy as well. The word 'lost' is used loosely in teaching, which hides the conservation principle.
What to do instead
Draw energy as a one-way arrow with decreasing width at each trophic level and label the losses as heat from respiration. Draw matter as a closed loop on a separate diagram. State the contrast in one line: matter is recycled, energy flows through and is eventually dispersed as heat.
The candidate explains seasons by the Earth being closer to the Sun in summer, or explains day and night by the Earth moving around the Sun, rather than by axial tilt and by rotation respectively.
Distance to the Sun is the most obvious variable in a solar-system picture, and the diagrams used in class are not to scale, so the small variation in orbital distance is invisible while the tilt is easy to overlook.
What to do instead
Use two separate demonstrations: a globe and lamp to show that tilt, not distance, changes the angle and duration of sunlight, and a spinning globe to show day and night. Add the decisive fact that the northern and southern hemispheres have opposite seasons at the same moment, which distance cannot explain.
The candidate classifies a rock by colour alone, calling any dark rock igneous or any layered rock sedimentary without checking texture and mineral composition, and confuses weathering with erosion when describing how a landscape formed.
Colour is the most salient feature in a hand specimen and is the easiest to remember, while texture requires careful observation. Weathering and erosion are taught together as 'breaking down rocks', so the distinction between in-place breakdown and transport is lost.
What to do instead
Use a three-question key for rocks: does it have layers or grains that can be seen, does it have interlocking crystals, or is it glassy? Then assign sedimentary, igneous or metamorphic. For landscapes, ask whether material was broken in place (weathering) or carried away (erosion) and name the transporting agent.