Models in science: what to keep and what to ignore
A metro map shows only stations and lines, not buildings or the real bends of the track. Yet it helps you travel.
A model is a simplified way of looking at a real system. It keeps what matters for your question and leaves out the rest. These choices are called assumptions.
In the figure : From the full real system we keep only the details that change the answer.
Rule 1: Keep what changes the answer, ignore what does not — First ask: what do I want to find out? Then keep the details that affect it.
- Easy : To find the time to cycle to school, keep the distance and speed. Ignore the colour of the cycle.
- Medium : A stone falls from a roof: keep the height and gravity. In a first model, ignore air and the stone's colour.
- Exam-type : Why is a car treated as a point on a long road? Its size is tiny compared with the distance it travels.
Rule 2: Every model has limits — Assumptions work only in some conditions. If the model fails, add back the detail you ignored.
- Easy : A feather falls slower than a stone because of air, so a no-air model fails for it.
- Medium : A bridge model ignores paint. If tonnes of paint were added, the model would have to include it.
- Exam-type : Saha treated star matter as a hot gas and focused on temperature, pressure and ionisation. This explained star colours.
A model is a tool to answer ONE question. Leaving out details is a choice, not a mistake.
Thinking a model must show everything, or that it is an exact copy of reality.
Ask: 'Does this detail change my answer?' If not, drop it. Memory clue: KEEP-DROP — keep what changes the answer, drop what does not.
Spot the difference
| Point | Real system | Model |
|---|---|---|
| Details | All of them | Only the important ones |
| Use | Hard to study fully | Easy to study and predict |
| Accuracy | Exact | Good within its assumptions |
Practice questions
- 1What is a model in science?
- 2Name one detail you can ignore when modelling the time a school bus takes to reach school.
- 3Which shape is used as a simple model of the Earth in earth science?
- 4Why is air resistance ignored in a basic model of a falling stone?
- 5A metro map ignores the real bends of the track. Why is it still useful?
- 6Rohan adds the brand of a bell to his model of cycling time. Is it needed?
- 7A model with no air predicts that a feather and a stone fall together. In real air they do not. What does this show?
- 8When would ignoring the weight of paint on a bridge become wrong?
Show answers (try them yourself first)
- 1A simplified way of looking at a real system that keeps only what matters for a question.
- 2The colour of the bus.
- 3A smooth sphere made of layers.
- 4To understand the basic effect of gravity first.
- 5It keeps only the stations and connections a traveller needs.
- 6No. The brand of the bell does not change the time.
- 7The model has limits: its assumption fails for the feather.
- 8When the paint layer is so heavy that it adds a large load.
Language of science: terms, symbols, units and mathematics
Two shopkeepers in different cities both mean the same amount by 1 kg. Without shared units, trade and science would be confusing.
Science uses precise terms, symbols and standard units so that everyone understands a measurement in the same way. Mathematics is its language for relationships between quantities.
In the figure : Every quantity has a symbol and a unit.
Rule 1: Always write a number with its unit — A number alone is not a measurement. 5 could be 5 m, 5 kg or 5 s.
- Easy : 2.5 kg = 2.5 × 1000 = 2500 g.
- Medium : 36 km/h = 36 × 1000 ÷ 3600 = 10 m/s.
- Exam-type : A cook reads '250' for milk. Is it 250 mL or 250 cups? The missing unit causes confusion.
Rule 2: Use standard (SI) units and agreed symbols — Kilogram for mass, metre for length, second for time. The symbol c for the speed of light comes from the Latin word celeritas (speed). Its value is exactly 299792458 m/s.
- Easy : The SI unit of mass is the kilogram.
- Medium : 5 m/s = 5 × 3600 ÷ 1000 = 18 km/h.
- Exam-type : An aircraft once ran short of fuel because two kinds of units were mixed up. Standard units avoid such errors.
Rule 3: Understand first, then use the equation — Understand the situation, identify the quantities, then use the relation between them.
- Easy : Distance = speed × time: at 20 m/s for 5 s the distance is 100 m.
- Medium : A car at 10 m/s covers 600 m. Time = 600 ÷ 10 = 60 s.
- Exam-type : Why not just memorise equations? An equation is a compact statement of how quantities are related. Understanding the situation first makes it a guide, not an obstacle.
A number without a unit is incomplete.
Mixing units in one calculation (like km with m) without converting first.
Convert to SI units first, then calculate. Memory clue: symbols m, v, F, I — 'Many Visitors Find Interest' for mass, velocity, force, current.
Spot the difference
| Quantity | Symbol | SI unit |
|---|---|---|
| Mass | m | kg |
| Velocity | v | m/s |
| Electric current | I | A |
Practice questions
- 1Write the usual symbol for mass and for force.
- 2Convert 3.2 km into metres.
- 3What is the SI unit of time?
- 4Convert 36 km/h into m/s.
- 5Why are standard units needed in trade and in science?
- 6Convert 750 mL into litres.
- 7A cyclist moves at 5 m/s. Find the speed in km/h.
- 8Explain why the symbol c is used for the speed of light.
Show answers (try them yourself first)
- 1m and F.
- 23200 m.
- 3The second (s).
- 410 m/s.
- 5So that everyone means the same by a measurement and results can be compared.
- 60.75 L.
- 718 km/h.
- 8It comes from the Latin word celeritas, meaning speed.
Laws, theories and principles
When a bus stops suddenly you jerk forward. We can describe this pattern, and we can also explain why it happens.
A law describes a regular pattern in nature. A theory explains why the pattern happens, using tested evidence. A principle is a broad idea that helps us understand situations.
In the figure : Law tells what happens, theory tells why, principle is a broad idea.
Rule 1: Law means 'what happens' — Repeated observation of the same pattern leads to a law.
- Easy : A ball thrown up always comes back down. This is a pattern.
- Medium : Newton's laws of motion describe the jerk in a stopping bus.
- Exam-type : For a fixed amount of gas at constant temperature, pressure rises when volume is reduced. This is a law-type statement.
Rule 2: Theory means 'why it happens' — A theory explains patterns and is supported by evidence gathered over time.
- Easy : Atomic theory: matter is made of atoms that join to form molecules.
- Medium : In science a theory is NOT a guess.
- Exam-type : A good theory can be improved or replaced when new evidence appears.
Rule 3: A principle is a broad idea — It is used in many different situations.
- Easy : Conservation of energy: energy is never lost, it only changes form.
- Medium : Climbing stairs: energy from food becomes energy stored in height.
- Exam-type : A falling ball loses height and gains speed, but the total energy stays the same.
In science, 'theory' means a well-tested explanation, never a mere guess.
Saying 'it is only a theory' to mean 'it is only a guess'.
Law = WHAT, Theory = WHY. Memory clue: Law Looks, Theory Tells why.
Spot the difference
| Term | What it does | Example |
|---|---|---|
| Law | Describes a pattern | Thrown ball comes down |
| Theory | Explains why | Atomic theory |
| Principle | Broad idea | Conservation of energy |
Practice questions
- 1What does a law do?
- 2What does a theory do?
- 3Give one example of a principle.
- 4'It is only a theory' — what is wrong with this sentence?
- 5Which explains why gases expand on heating: a law or a theory?
- 6Why can scientific theories change?
- 7New careful experiments disagree with an old theory. What should scientists do?
- 8Differentiate a law and a theory with one example.
Show answers (try them yourself first)
- 1It describes a regular pattern in nature.
- 2It explains why a pattern happens, using evidence.
- 3Conservation of energy.
- 4In science a theory is a well-tested explanation, not a guess.
- 5A theory: gas particles move faster and spread out on heating.
- 6New evidence may improve or replace them. This makes science reliable.
- 7Re-examine the theory in the light of the evidence.
- 8Law: heated gases expand (what happens). Theory: the particles move faster and push apart (why).
Predictions, evidence and testing claims
The sky is dark, so you say 'it will rain'. A scientist asks: how humid was it the last time it rained?
A prediction is a reasoned expectation based on evidence, made using laws, theories and models. It can be checked against observations.
In the figure : Predict, test, compare and improve — again and again.
Rule 1: Test a prediction with measurable evidence — Ask about data and past patterns, not just yes or no.
- Easy : 'It will rain' becomes testable when we ask about humidity, wind and earlier rainy days.
- Medium : Yes/no questions give little. Questions about data and patterns give much more.
- Exam-type : Eclipse claim: what physical, chemical or biological change happens to food in a shadow? None, so the claim is not supported.
Rule 2: Match or mismatch — Match: confidence grows. Mismatch: re-check assumptions, models and measurements.
- Easy : Water boils at 100 °C at sea level, as predicted and observed.
- Medium : If a mixture does not warm up as predicted, test again. Never change the reading.
- Exam-type : Forecasts are reliable for hours to days only, because tiny differences grow with time.
Rule 3: Evidence decides — Ideas are accepted or rejected on evidence, not on belief, fame or popularity. No theory is final.
- Easy : A claim shared a million times is not proof.
- Medium : A wrong prediction is a strength of science: it shows where to improve.
- Exam-type : A viral post says an object glows 'because of a ghost'. Look for natural, testable explanations.
A prediction is reasoned, not a guess. Failure helps science improve.
Believing a claim because many people share it.
Ask: What is the evidence? Can it be tested? Memory clue: PTCI — Predict, Test, Compare, Improve.
Spot the difference
| Point | Guess | Scientific prediction |
|---|---|---|
| Basis | Feeling or luck | Evidence and reasoning |
| Can be tested? | Often not | Yes |
| If wrong | Ignored | Ideas are re-examined |
Practice questions
- 1What is a scientific prediction?
- 2Why do weather forecasts sometimes go wrong?
- 3What is an eclipse mainly?
- 4Give one question that makes 'it will rain today' more testable.
- 5What should be done when a prediction does not match the observation?
- 6Why is popularity not proof of a claim?
- 7A student alters the thermometer reading to match her prediction. What is wrong with this?
- 8A viral message says a certain tea cures every disease. How will you check it scientifically?
Show answers (try them yourself first)
- 1A reasoned expectation based on evidence.
- 2Tiny differences in conditions grow with time.
- 3A play of shadows.
- 4What was the humidity on earlier rainy days?
- 5Re-check assumptions, models and measurements, and test again.
- 6Many people can be wrong. Only repeated careful testing is proof.
- 7Data must never be changed to fit an idea. The mismatch must be investigated.
- 8Ask for evidence and measurements, look for repeatable tests and natural explanations.
Estimation and approximate reasoning
For a school function, 'about 150 chairs' is enough. But a baby's medicine dose must be exact.
Estimation means finding a rough but reasonable value by reasoning with round numbers. It tells us whether an answer makes sense.
In the figure : Understand, choose quantities, round, then check.
Rule 1: Break the problem into small known parts — Use quantities you know and round numbers.
- Easy : 40 students × 5 sheets = 200 sheets.
- Medium : 1 day = 24 × 60 = 1440 minutes. At 15 breaths a minute, a day has about 15 × 1440 = 21,600 breaths.
- Exam-type : A 2 L balloon takes about 4 breaths, so one breath is about 0.5 L. About 20,000 breaths × 0.5 L = 10,000 L of air a day.
Rule 2: Check whether the answer is reasonable — Compare with what you know from daily life.
- Easy : A cyclist at 400 km/h is unreasonable.
- Medium : Rice for a family of four for a month: a few tens of kg is sensible. 100 g is too little, tonnes are too much.
- Exam-type : Two methods giving 8640 L and 10,000 L agree closely, so the estimate is sensible.
Rule 3: When rough is fine and when exact is needed — Think about what a small error would cause.
- Easy : Rough is fine: number of chairs, walking time.
- Medium : Exact is needed: a medicine dose, tools that need precision.
- Exam-type : 3 km at 5 km/h takes about 36 minutes, which is good enough to plan a walk.
An estimate needs reasoning, not just a guess.
Trusting a calculated answer without checking whether it is reasonable.
Round the numbers, multiply, then do a sanity check. Memory clue: RMC — Round, Multiply, Check.
Spot the difference
| Situation | Estimate OK? | Why |
|---|---|---|
| Chairs for a function | Yes | A small error is harmless |
| Medicine dose | No | Health depends on the exact value |
| Walking time to a shop | Yes | It is only for planning |
Practice questions
- 1How many minutes are there in a day?
- 22 L of water a day for 30 days is how many litres?
- 3Is an estimate good enough to decide a baby's medicine dose?
- 4A person takes 15 breaths each minute. How many breaths in one hour?
- 540 students use 5 sheets each. How many sheets in all?
- 6A bus stop is 3 km away. At 5 km/h, about how long does the walk take?
- 7A tap drips 1 drop every second and 20 drops make 1 mL. Estimate the water wasted in an hour.
- 8A calculation says a person drinks 200 L of water daily. Is it reasonable? Explain.
Show answers (try them yourself first)
- 11440.
- 260 L.
- 3No. It needs an exact value.
- 4900 breaths.
- 5200 sheets.
- 63 ÷ 5 = 0.6 h, which is 36 minutes.
- 73600 drops ÷ 20 = 180 mL.
- 8No. A person drinks about 2 L a day, so 200 L must contain an error.
Branches of science and science as a human activity
A face mask uses physics (particles and electric attraction), chemistry (polymer fibres), biology (virus size) and mathematics (airflow) together.
Science is organised into branches — physics, chemistry, biology and earth science — but nature has no such boundaries. Science is also a human activity built on curiosity, questioning and sharing.
In the figure : The branches overlap in real objects and problems.
Rule 1: Branches are our way of organising knowledge — We divide science to keep knowledge in order.
- Easy : Physics: motion, force, energy. Chemistry: matter and its changes.
- Medium : Biology: living things. Earth science: the Earth and its systems.
- Exam-type : A pressure cooker involves physics (pressure), chemistry (changes in food) and biology (nutrition).
Rule 2: Real problems need many branches — Climate change, medicines and clean technology need ideas from several branches together.
- Easy : Climate change needs earth science, chemistry, physics and mathematics.
- Medium : New medicines need chemistry and biology.
- Exam-type : Traffic jam near a school: speeds (physics), pollution (chemistry), health (biology), counting (mathematics).
Rule 3: Science is a human activity — People ask questions, test ideas, share results and learn from mistakes.
- Easy : Researchers from different countries share results and ideas.
- Medium : Science grew through the work of many people, cultures and generations.
- Exam-type : Even if you do not study science after Grade 10, scientific thinking helps you judge information critically.
The divisions in science are made by us. Nature does not have them.
Thinking an object or problem belongs to only one branch.
For any object ask: 'Which branches are hiding here?' Memory clue: PCBE — Physics, Chemistry, Biology, Earth science.
Spot the difference
| Branch | Studies | Example |
|---|---|---|
| Physics | Motion, force, energy | Falling ball |
| Chemistry | Matter and its changes | Rusting |
| Biology | Living things | Digestion |
| Earth science | Earth and its systems | Monsoon |
Practice questions
- 1Name the four branches of science mentioned in this chapter.
- 2Which branch studies living things?
- 3What does the magnifying glass in the textbook's page numbers symbolise?
- 4Which branches help to explain a pressure cooker?
- 5Why do we divide science into branches?
- 6Name one way in which scientists collaborate.
- 7How do physics, chemistry, biology and mathematics all help in understanding a mask?
- 8Why is scientific thinking useful even if you do not study science after Grade 10?
Show answers (try them yourself first)
- 1Physics, chemistry, biology and earth science.
- 2Biology.
- 3Careful observation.
- 4Physics (pressure), chemistry (changes in food) and biology (nutrition).
- 5To organise knowledge. Nature itself has no boundaries.
- 6By sharing results and ideas with researchers across places and cultures.
- 7Physics: particle motion and electric attraction. Chemistry: polymer fibres. Biology: virus size and behaviour. Mathematics: airflow and filtering model.
- 8It helps you evaluate information critically and understand the technology around you.