Shimane Public High School Entrance Examination - Science common mistakes
Shimane Public High School Entrance Examination - Science · Understand the traps and how to avoid them.
In speed, work, or energy calculations, substituting centimetres and grams directly into formulas that require metres and kilograms, then writing the answer with the original unit (for example, 30 cm ÷ 2 s = 15 m/s).
The numbers in the question are often given in cm or g, and candidates concentrate on getting a numerical answer rather than aligning the units with the formula.
What to do instead
Before substitution, write a conversion line such as 30 cm = 0.30 m and 200 g = 0.20 kg. After calculating, check the unit by dimensional analysis: m ÷ s gives m/s, not cm/s.
Treating the normal force from a table on a book and the gravitational force on the book as an action–reaction pair because they are equal in size and opposite in direction.
Diagrams often draw both arrows on the same object and everyday language calls them 'balanced', so the distinction between forces on one object and forces on two objects is lost.
What to do instead
Draw a separate free-body diagram for each object and label every force as 'on ___ by ___'. For an action–reaction pair, confirm that the two forces act on different objects and are the same type, such as both gravitational.
On a current–voltage graph, calculating resistance as the slope ΔV/ΔI when voltage is on the horizontal axis, or reading the intercept as resistance.
Students memorise V = RI but do not re-derive the slope from the axis labels, so they apply a fixed 'slope = resistance' rule regardless of the graph orientation.
What to do instead
Write the graph equation as vertical-axis quantity divided by horizontal-axis quantity. With voltage horizontal on an I–V graph, slope = ΔI/ΔV = 1/R, so R = 1/slope; verify with one point using R = V/I. This reciprocal-slope rule assumes an ohmic component with constant physical conditions and a proportional graph.
Balancing a chemical equation by changing subscripts, for example turning H2O into H2O2 to add oxygen atoms, instead of changing coefficients.
Changing a subscript makes the atom count match quickly, and the difference between a coefficient and a subscript is not firmly fixed in memory.
What to do instead
Balance only with coefficients in front of formulas. After each change, recount each element in a table and confirm that no subscript has been altered.
In a precipitation reaction, writing every ion as a product or including spectator ions in the net equation, so the equation shows ions that do not actually combine.
Students equate 'a reaction occurs' with 'all ions react' and do not separate the ions that remain dissolved in the solution.
What to do instead
List all ions present before mixing, mark the pair that forms the precipitate, gas, or water, and write the net ionic equation with only the reacting ions and the product. Check that atoms and charge balance.
When a reaction in an open container loses mass, concluding that mass was not conserved or that the missing mass simply disappeared.
The balance reading decreases and the gas product is invisible, so the visible evidence seems to contradict conservation of mass.
What to do instead
Identify which matter leaves or enters before explaining the balance change. If a specified gas product escapes, include its mass in the total; do not assume every open-container mass change has the same cause.
For a cross between two heterozygotes, reporting the genotype ratio as 3:1 or the phenotype ratio as 1:2:1, and not stating which ratio the question asks for.
The 3:1 ratio is strongly memorised, but the labels genotype and phenotype are often read quickly and treated as interchangeable.
What to do instead
For a single locus with complete dominance and equal viability, draw the Aa×Aa Punnett square. Its expected genotype ratio is1AA:2Aa:1aa and expected phenotype ratio3dominant:1recessive; an observed small family need not match exactly.
In an experiment on transpiration or photosynthesis, comparing two setups that differ in more than one condition, such as light and temperature, and attributing the result to one factor.
The experimental setup looks convincing and the intended variable is obvious to the student, so other differences are overlooked.
What to do instead
Write a control-variable table listing light, temperature, humidity, and plant size. Change only the factor named in the question and keep all others identical, including a control group.
Ordering a fault or an igneous intrusion as older than the layers it cuts across, or assuming that all tilted layers were deposited after folding.
The superposition principle is applied mechanically to every boundary, and cross-cutting relationships are less familiar.
What to do instead
Apply the cross-cutting principle: a fault or intrusion is younger than the layers it cuts. Folding occurred after the folded layers were deposited. Mark relative ages on a sketch before answering.
On a weather map, reading the wind direction as the direction the wind blows toward, or confusing the symbols for warm and cold fronts and their direction of advance.
Everyday phrasing such as 'the wind is going east' and the triangular or semicircular front symbols are easy to mix up under time pressure.
What to do instead
State wind direction as the direction from which the wind comes: a northwest wind blows from the northwest. Draw the front symbols: triangles mean a cold front, semicircles mean a warm front, and the side with the symbols shows the direction of advance.