Shizuoka Public High School Entrance Examination - Science common mistakes
Shizuoka Public High School Entrance Examination - Science · Understand the traps and how to avoid them.
When calculating work or power, the student multiplies force by the distance the object moves in any direction, instead of using only the component of displacement along the force; for example, for a cart pulled by a rope at an angle, the student computes W = F × s and ignores cos θ, or uses the distance travelled along the slope for a lifting problem.
Textbook problems often present horizontal forces, and the student memorises W = Fs as a formula to plug numbers into. The idea that work is a scalar product, and that a force perpendicular to motion does no work, is never made explicit, so inclined ropes and circular motion expose the gap.
What to do instead
Before substituting numbers, always draw the force vector and the displacement vector, and mark the angle between them. Compute W = Fs cos θ, and check the two limiting cases: θ = 0° gives maximum work, and θ = 90° gives zero work. Practise with a suspended object moving horizontally (the tension does no work) and with a box pushed at 30° to the horizontal.
The student treats current as something that is 'used up' as it passes through a series circuit, writing that the current before a resistor is larger than after it, or adding currents at a junction in a simple series loop. In parallel-circuit problems, the student also assumes the current divides equally regardless of resistance.
Everyday language about 'consuming electricity' and the visible brightness of bulbs encourage a mental model in which charge is consumed. Kirchhoff's current law is stated in class but not connected to the idea that charge is conserved and cannot accumulate in a wire.
What to do instead
State explicitly that current is the rate of flow of charge and is the same at every point in a single series loop. For parallel branches, compute I = V/R for each branch separately and verify that the branch currents sum to the total. Solve one numerical problem both ways and check the sum, so that the conservation check becomes a habit.
When drawing ray diagrams for a convex lens or concave mirror, the student places the image on the same side as the object for a real image, or draws the ray through the focal point parallel to the axis after refraction. The student also reports the image as virtual when it is real, or vice versa, without checking whether the rays actually converge.
The three standard rays are memorised as a list without the underlying rule that each ray is reversible and that a real image forms where refracted or reflected rays actually meet. Sign conventions for object distance, image distance, and focal length are applied mechanically.
What to do instead
Draw the diagram first and decide real or virtual from whether the outgoing rays converge or only appear to diverge. Then apply 1/f = 1/a + 1/b with a fixed sign convention written at the top of the page, and check the sign of b against the diagram. Repeat for object positions outside 2f, at 2f, between f and 2f, and inside f.
The student classifies a mixture as a pure substance, or a compound as a mixture, because both look uniform. In separation questions, the student chooses filtration for a dissolved salt or distillation for a suspension, and cannot justify the choice by the difference in particle size or boiling point.
The terms 'pure substance', 'mixture', 'compound', and 'element' are learned as vocabulary lists rather than as categories defined by composition and by whether the components can be separated by physical means. The particle model is not used to predict behaviour.
What to do instead
For any sample, ask two questions: can it be decomposed by a chemical change, and can its components be separated by a physical method? Build a decision tree: filtration for an insoluble solid in a liquid, evaporation or crystallisation for a dissolved solid, distillation for liquids with different boiling points, chromatography for dissolved coloured substances. Justify each choice with a particle-level reason.
When writing or balancing a chemical equation, the student changes subscripts to balance atoms (writing H2O as H2O2, or Fe2O3 as FeO), or omits state symbols and reaction conditions. In oxidation and reduction questions, the student equates oxidation only with combination with oxygen and misses oxidation by loss of electrons or by removal of hydrogen.
Balancing is treated as an arithmetic puzzle about making the numbers match, so the student adjusts the most convenient symbol. The definition of oxidation is learned in one context (combustion) and not generalised to reactions such as copper oxide being reduced by hydrogen.
What to do instead
Balance only by placing coefficients in front of formulae, and never alter a subscript; after balancing, count each element on both sides as a check. Write state symbols (s), (l), (g), (aq) and conditions such as heating. For redox, identify the change in oxidation number or the transfer of oxygen and hydrogen, and label which substance is oxidised and which is reduced.
In neutralisation problems, the student assumes that equal volumes of acid and base always neutralise each other, or that a solution with pH 7 contains no ions. When asked what happens to the ions, the student says they disappear rather than forming water and a dissolved salt.
The word 'neutralisation' suggests that everything cancels. The ionic equation H+ + OH− → H2O is memorised but not connected to the spectator ions that remain in solution, and concentration is confused with amount.
What to do instead
Write the full ionic equation and circle the spectator ions. State that at the equivalence point the solution still contains the salt's ions, and that pH 7 reflects equal H+ and OH− concentrations, not the absence of ions. For calculations, convert volume and concentration to moles before comparing acid and base, using the coefficients in the balanced equation.
The student describes mitosis and meiosis as interchangeable, saying meiosis produces two identical cells or that mitosis halves the chromosome number. In genetics problems, the student cannot distinguish genotype from phenotype, or writes the gametes of an AaBb individual as AB and ab only, ignoring recombination of independent assortment.
Both processes are summarised as 'cell division' and the diagrams look similar. The purpose of each process, growth versus gamete formation, is not linked to the chromosome behaviour, so the outcomes are confused.
What to do instead
Make a comparison table with rows for number of divisions, daughter cells, chromosome number, and genetic variation. For meiosis, trace one pair of homologous chromosomes through both divisions. For dihybrid crosses, list all four gamete types by taking one allele from each gene pair, and use a Punnett square to check the 9:3:3:1 ratio.
When explaining homeostasis, the student writes that the body 'keeps everything constant' without naming the sensor, the control centre, and the effector, or describes only one direction of correction. For example, for body temperature the student says sweating cools the body but cannot explain what happens when body temperature falls.
Homeostasis is learned as a slogan. Negative feedback is described in one direction only, so the student does not see that the response reverses the original change and must work in both directions.
What to do instead
For each example (body temperature, blood glucose, water balance), write the stimulus, receptor, control centre, effector, and response, and then write the opposite response for the reversed stimulus. Use the phrase 'the response cancels the original change' as a check, and practise with insulin and glucagon as a paired example.
In food-web questions, the student counts all organisms at every trophic level as the energy available to the next level, ignoring that most energy is lost through respiration, excretion, and heat. The student also treats decomposers as a trophic level competing with herbivores, or claims that removing one predator has no effect because other species remain.
Arrows in food webs are read as 'eats' rather than 'energy flows to'. The quantitative loss at each transfer, roughly one tenth retained, is memorised as a number without being applied to a pyramid or to a prediction about population change.
What to do instead
Redraw the food web with arrows labelled by direction of energy flow, and add a pyramid of numbers or biomass. Calculate available energy level by level using the 10% rule. For removal questions, trace the effect along every connected path, including indirect effects through competing species and through decomposers.
When reading a geological cross-section, the student applies the principle of superposition to folded or faulted strata without first restoring the original order, or identifies a fault as an intrusion. In earthquake questions, the student confuses the focus with the epicentre, or states that P-waves and S-waves arrive at the same time.
The principles of superposition, original horizontality, and cross-cutting relationships are memorised individually but not applied in the correct order. Wave terminology is learned as definitions without the physical picture of wave speed and travel through different media.
What to do instead
Work through cross-sections in a fixed order: identify folding and faulting, then apply cross-cutting relationships to order events, then apply superposition to the restored sequence. For earthquakes, draw the focus below ground and the epicentre directly above it, and compare arrival times using the fact that P-waves travel faster and pass through liquids while S-waves do not.