NCERT Solutions Class 9 Science Chapter 1 Exploration Entering the World of Secondary Science

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 9 · SCIENCE

NCERT Solutions

Chapter 1: Exploration: Entering
the World of Secondary Science

NCERT Textbook — Exploration

BOOK PAGES SECTIONS QUESTIONS MEDIUM

1–7 11 13 English

Solutions, notes, sample papers & more at 18 pages

Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

CLASS 9 · SCIENCE · EXPLORATION

NCERT Solutions — Chapter 1: Exploration: Entering the
World of Secondary Science
The opening chapter of Exploration is about how science works rather than about any one topic. It explains
why science builds simplified models, why it insists on precise words, agreed symbols and SI units and on
mathematics as a language, what the words law, theory and principle actually mean, how prediction and
estimation are used, and why the boundaries between physics, chemistry, biology and earth science are
drawn by us and not by nature.

TEXTBOOK BOOK PAGES

Exploration (Class 9) 1–7

SECTIONS QUESTIONS

11 13

MEDIUM

English

Example 1.1 — Page 2
Why science uses models

EXAMPLE

Q1 A cricket shot. Think of a cricket ball being hit for a six. You want to make a simple
model. What details would you include? What would you ignore?

First fix the question the model has to answer: “Will the ball cross the boundary without hitting
the ground first?” Everything the model keeps or drops is decided by that one question.

Page 1 of 18

Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

KEEP IN THE WHY IT IS NEEDED IGNORE
MODEL

Mass of the ball Fixes how much the bat's blow speeds it up Brand of the bat

Speed just after the hit (u) Range grows as u² Colour of the ball

Direction (angle) of the Decides how the speed is shared between height Amount of grass on the
hit and distance field

Height of the bat above Small, but it adds a little to the range Stitching of the seam
ground

Distance to the boundary The number the answer is compared with Spin of the ball

With those few quantities the ball can be treated as a point moving under gravity alone. For a hit
at u = 30 m s⁻¹ at 45°:

range R = u² sin 2θ / g

R = (30 m s⁻¹)² × sin 90° / (9.8 m s⁻²)

R = 900 m² s⁻² / 9.8 m s⁻²

R = about 92 m — comfortably past a 70 m boundary

Why it happens: a model is not judged by how much of the real world it contains,
but by whether it answers the question. The colour of the ball cannot enter any
equation that links speed, angle and distance, so keeping it would add work without
adding accuracy. Air resistance, spin and the seam do have real effects — they can
shorten a big hit by ten metres or more — but they are small compared with the
effect of u and θ, so a first model leaves them out. As the model is made more
complex, these details are added back for greater accuracy.

Check it yourself: put u = 25 m s⁻¹ into the same formula. R = 625/9.8 ≈ 64 m — now
the ball lands inside a 70 m boundary. A drop of only 5 m s⁻¹ in bat speed turns a six
into a catch, which is exactly why the model keeps u and drops the bat's brand.

Activity 1.1: Let us model — Page 2

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Building your own model

ACTIVITY

Q1 Suppose you ride a bicycle from your school to your home. You want to model the
time it takes to go home from school. What details would you keep? What details
could you ignore? Suggest why ignoring some details may actually be useful.

The model only has to produce one number — the time t — so keep every quantity that changes
t by more than a minute or two, and drop the rest.

KEEP EFFECT ON THE TIME IGNORE

Distance from school to home t grows in direct proportion to it Colour and make of the
cycle

Your average cycling speed t is inversely proportional to it What you are wearing

Number and length of stops (signals, level Adds a few minutes directly Each individual pothole
crossing)

Slope of the road, strong headwind Changes the average speed by Songs playing in your
10–20% head

A worked model for a 3 km ride at an average 12 km h⁻¹ with two signals of about 90 s each:

riding time = distance / average speed
= 3 km / 12 km h⁻¹ = 0.25 h = 15 min

waiting time = 2 × 90 s = 180 s = 3 min

total time t = 15 min + 3 min = 18 min

Page 3 of 18

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

co m
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Why ignoring details is useful: three reasons, and all three matter.

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You can actually measure what is left. Distance and average speed can be
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found with an odometer and a watch. The exact wind at every metre of the road

a gl cannot be — a model that needs unmeasurable inputs gives no answer at all.
The ignored details are smaller than the natural scatter. Your own riding time

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varies by two or three minutes from day to day. A detail that changes the answer
by ten seconds is buried inside thatm .
s e scatter, so including it cannot make the
prediction better.
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A simple model can be tested and corrected. Time yourself for a week. If the
real time is always 22 min and not 18 min, you know exactly one thing is wrong —
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most likely the assumed speed — and you can fix it. In a model with twenty
em.
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inputs you would not know which one to blame.
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Try this: ride the same route on a holiday, when there is no traffic. If your time falls

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to about 15 min, the stops term in the model is confirmed. That is a model being

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tested against observation — the same a scientist uses.

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Threads of Curiosity — Page 3
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The shared language of science: symbols and units

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THREADS

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Q1 Why is the speed of light denoted by ‘c’?
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Because c comes from the Latin word celeritas, meaning speed — not from the first letter of any
English word.

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Scientific symbols usually come from history and from international agreement. They are not

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convenient abbreviations invented by each writer.
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m courant), q for charge, Fe for iron (Latin ferrum).
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exactly 299792458 m/s — not measured, defined.
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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Why it happens: a symbol has to mean the same thing to a student in Kerala, a
researcher in Japan and a textbook printed in Brazil. If everyone abbreviated in their
own language, an equation could not be read across borders. Once a symbol is
agreed internationally, changing it would break every book and paper already
written — so the historical choice survives even when it no longer matches the
language being used.

Did you know? Because c is now a defined number, it is the metre that depends on
it, not the other way round: one metre is the distance light travels in vacuum in
1/299792458 of a second. So an experiment can no longer measure c more
accurately — any improvement simply makes the metre more accurate.

Q2 Why is a kilogram used everywhere?

So that one kilogram means exactly the same amount of matter in every shop, every laboratory
and every country.

Measurements are based on agreed international standards, not on local objects or on
someone's opinion of what a “seer” or a “handful” should be.
Standard units let scientific results be compared: a reaction that needs 5 g of a substance
needs the same 5 g in Delhi and in Berlin.
They also make daily trade fair. When you buy a kilogram of rice or vegetables, you expect
the same quantity from every seller.

Why it happens: a measurement is really a comparison — you are asking “how
many times does the standard fit into this?” If two people use different standards,
their numbers cannot be compared at all, even though both may be perfectly
careful. The whole point of the SI system is to fix one standard for each quantity so
that the number carries information rather than confusion. The aircraft in this
chapter ran out of fuel for exactly this reason: the crew computed a density in
pounds per litre and used it as though it were kilograms per litre, leaving the plane
about 15,000 litres short of the 22,300 kg it needed.

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Did you know? Until 2019 the kilogram was defined by a metal cylinder kept near
Paris — a “local object” of exactly the kind this box warns against, and it was slowly
losing mass. Today the kilogram is defined from a fixed constant of nature instead,
so no country needs to hold the standard.

Pause and Ponder — Page 4
Laws, theories, principles and prediction

PAUSE AND PONDER

Q1 Think of a prediction you or your family made recently (for example, the outcome of
a cricket match). Was it based on evidence and reasoning, or mainly on guesswork?
How can scientific thinking improve such predictions?

Judge any prediction by one test: could someone else check it with data you can both look at? If
yes, it rests on evidence; if it rests only on a feeling, it is guesswork.
Sample answer: Before a one-day match, my uncle said “India will win, they always win at this
ground.” That was partly evidence and mostly guesswork. It used one real pattern — home
record — but ignored the playing conditions, who was injured, and which team was batting
second.
Scientific thinking improves such a prediction in four steps:

1. State it so it can be wrong. “India will win” is testable. “India will play well” is not, because
nobody can agree when it has failed.
2. Name the quantities that matter. Win record at that ground, average first-innings score,
dew after sunset, bowling average of the opening pair.
3. Use past data, not memory. If the last 20 matches at that ground were won 13 times by the
side batting second, that is a measured 65% — a far better base than “they always win”.
4. Check the outcome and revise. If the prediction fails repeatedly, the assumption behind it
is wrong and must be changed, not defended.

Why it works: a prediction becomes scientific not because it turns out right, but
because it is built from measurable quantities and can be shown to be wrong. A
guess that happens to come true teaches you nothing, since you cannot tell which
part of your reasoning did the work. A reasoned prediction that fails is genuinely
useful — it points straight at the assumption that needs correcting, which is exactly
how scientists use failed predictions.

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Example 1.2 — Page 4
Making a prediction testable

EXAMPLE

Q1 How do we check predictions? Varsha told her friend Meghna, “It will rain this
afternoon because the clouds look dark”. Think of some questions Meghna could
ask Varsha to make this prediction scientifically testable.

Meghna should ask for measurable evidence and past patterns — questions whose answers are
numbers, not opinions.

“What was the condition of the sky the last time it actually rained?”
“What is the humidity today? Was it above 80 per cent the last time it rained?”
“What is today's wind speed and direction?”
“Is the temperature dropping the way it did before the recent rains?”
“How many times in the last month did dark clouds appear and rain follow within three
hours?”

She should also ask Varsha to sharpen the claim itself, so that it can be scored afterwards:

vague: “It will rain this afternoon.”

testable: “Measurable rain will fall here between 2 p.m. and 6 p.m. today.”

Why it works: questions with a plain yes/no answer (“Do the clouds look dark?”) are
not very useful, because the answer is already contained in what Varsha said and
cannot separate a good prediction from a lucky one. Dark clouds are a genuine sign
— they mean thick, water-laden cloud — but they are not sufficient by themselves;
dark clouds often drift past without raining. A testable prediction needs a quantity
(humidity, temperature trend, wind), a place and time window, and a past record
to compare against. Only then can Meghna say afterwards, honestly, whether the
prediction succeeded or failed.

Tip: the same three ingredients — a measurable quantity, a stated time window, and
a record to check against — turn any everyday claim (“this bus is always late”, “this
soap lasts longer”) into something you can actually test.

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Ready to Go Beyond — Page 4
The limits of prediction

READY TO GO BEYOND

Q1 Why do weather forecasts sometimes go wrong?

Because weather depends on many quantities that keep changing together, and very tiny
differences in today's conditions grow into completely different weather a few days later.

The state of the atmosphere is set by temperature, pressure, humidity and wind — at every
point, and at every height.
Forecasting uses measurements fed into a model. But the measurements come from
stations and satellites spaced kilometres apart, so what happens between them has to be
filled in.
A small error in that starting picture does not stay small. It doubles, doubles again, and after
a few days it is as large as the weather itself.

error today ≈ 0.1 °C at one point
grows roughly by doubling every 1–2 days

after 8 days ≈ 0.1 °C × 2⁵ ≈ 3 °C — the size of a whole weather change

Why it happens: the atmosphere is not simply complicated, it is sensitive. The
equations of air flow feed back on themselves — a slightly warmer patch rises a little
faster, which pulls in more air, which shifts a cloud, which shifts where the sun heats
the ground next. This is why forecasts are usually reliable for a few hours or even a
few days, but far less certain further into the future. Note carefully what is not the
reason: the physics is not wrong, and the model is not badly built. The limit comes
from never being able to measure the starting state perfectly.

Did you know? Forecasters get around this by running the same model many times
from slightly different starting conditions. If 45 of 50 runs give rain, they announce
“90% chance of rain”. The spread of the runs is itself the honest measure of how far
ahead the forecast can be trusted.

Threads of Curiosity — Page 5

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Testing a claim with scientific questions
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THREADS OF CURIOSITY

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a s em ‘viral’ claims on social media: Is eating food harmful during an eclipse?
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No. The claim collapses as soon as you ask what
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Why it happens: food spoils for one reason — microbes multiply in it, helped by

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s warmth, moisture and time. An eclipse changes none of these; if anything the small
la drop in temperature slows microbes down slightly. So there is no physical, chemical
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survives not because of evidence but because eclipses are dramatic and rare, and

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Tip: use this three-question test on any viral claim — (1) What exactly is being

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claimed, in measurable terms? (2) What mechanism could produce it? (3) What data
would show it? A claim that fails all three is not “unproven”; iteismbaseless.
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Ready to Go Beyond — Page 5
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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Estimation as a scientific skill

READY TO GO BEYOND

Q1 How much rice would feed a family of four for a month?

About 70–80 kg of uncooked rice — under the deliberate assumption that all the family's
calories come from rice alone.

energy needed per adult ≈ 2000 – 2500 kcal per day, take 2250 kcal

family of four (2 adults + 2 children) ≈ 4 × 2250 kcal ≈ 9000 kcal per day

100 g of uncooked rice gives about 350 kcal when cooked

rice per day = 9000 kcal ÷ (350 kcal / 100 g)

= 9000 / 3.5 g ≈ 2570 g ≈ 2.6 kg per day

rice per month = 2.6 kg × 30 = about 77 kg

Now do what the box actually asks — check whether the answer makes sense:

100 g for a month is clearly far too little (that is one small meal).
A few tonnes is far too much (a tonne of rice would fill a small room).
77 kg is roughly one and a half of the 50 kg sacks sold in the market — a quantity a family
could actually store and finish. The estimate passes.

Why it happens: the answer comes out about twice what a real family buys, and
that is not an error — it is the assumption showing itself. Real meals also contain dal,
oil, vegetables and milk, and oil alone carries about 900 kcal per 100 g. Since only
part of the 9000 kcal now has to come from rice, the rice figure falls to roughly 35–40
kg a month, which is close to what households really use. That is the whole value of
an estimate: it is not meant to be exact, but it tells you the answer lies in tens of
kilograms, and it makes the assumption that produced it visible and correctable.

Check it yourself: find the calorie value printed on a rice packet and the number of
people in your home, and redo the three lines above. If your answer lands anywhere
between 30 kg and 100 kg a month, your reasoning is sound — the exact number
was never the point.

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Example 1.3 — Page 6
Estimating with two independent routes

EXAMPLE

Q1 Estimate how many litres of air you breathe in one day. Start by estimating how
many breaths you take per minute, and the volume of one breath. Your aim is not
to find an exact answer, but a reasonable estimate.

About 10,000 litres, or roughly 10 m³ of air a day.

Route 1 — breaths × volume per breath

breathing rate at rest ≈ 12 – 15 breaths per minute

minutes in a day = 60 × 24 = 1440 min

breaths per day = 12 × 1440 to 15 × 1440 = 17280 to 21600

≈ 18 – 22 thousand, say 20 thousand breaths a day
volume of one breath: a party balloon of about 2 litres takes 4 – 5 breaths to fill

so one breath ≈ 2 L ÷ 4 = 0.5 litre

air per day = 20000 × 0.5 L = 10,000 litres

Route 2 — the balloon, checked independently

one balloon takes about 20 s to blow up → 3 balloons per minute

(3 balloons / minute) × (2 litres / balloon) × (1440 minutes / day)

= 3 × 2 × 1440 L = 8640 litres per day

The two routes give 10,000 L and 8640 L — a difference of about 14%. For an estimate that is
excellent agreement, so the answer is trustworthy at the level of “about ten thousand litres”.

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Why the cross-check matters: a single estimate can be badly wrong without any
warning, because one careless factor of 10 changes everything. Two routes built on
different assumptions rarely go wrong in the same direction, so agreement between
them is real evidence. Notice also that the two routes are not truly independent —
both use the 2-litre balloon — so what the check really tests is the breathing-rate
step, and it confirms it. One honest caution: blowing up balloons continuously would
exhaust you within minutes, while restful breathing does not, because forced
breathing uses far more muscular work than quiet breathing.

Did you know? 10,000 litres is 10 m³ — a box about 2 m × 2 m × 2.5 m, roughly the
volume of a small bathroom. The 0.5 litre per breath in this estimate is not a lucky
guess either: the tidal volume of a resting adult really is about 500 mL, so the
everyday balloon reasoning lands on the medically measured value.

Pause and Ponder — Page 6
Estimation, exactness and the branches of science

PAUSE AND PONDER

Q2 Describe one situation where an approximate answer is good enough, and one
where you would need a very exact value.

The rule that decides between them: ask how much the answer may change before your
decision changes. That allowed slack is the precision you need — no more, no less.
Approximate answer is good enough — buying rice for a school picnic.

80 students × about 150 g cooked-rice serving ≈ 12 kg

buy a 15 kg bag — an error of 2 kg either way changes nothing

A 15% error here costs a little extra rice, and nothing else. Estimating is not laziness; measuring
each student's appetite would be pointless work.
A very exact value is needed — the dose of a medicine for a child.

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

paracetamol dose = 15 mg per kg of body weight

for a 20 kg child: 15 mg kg⁻¹ × 20 kg = 300 mg exactly

double this, repeated, and the liver is damaged

Here a 100% error is dangerous, so the child's mass and the syrup's concentration must both be
measured properly.

Why it happens: precision costs time, money and equipment, so science spends it
only where it changes the outcome. The picnic decision is tolerant — many answers
lead to the same action, buy one bag. The medicine decision is sensitive — a small
change in the number changes the outcome from cure to harm. The same
distinction runs through all of science: an estimate is enough to decide whether an
experiment is worth doing, but the final measurement it produces must be as exact
as the instrument allows.

More pairs to think about: approximate — how many buses for a school trip, how
much paint for a wall, whether a bridge design is even in the right range. Exact —
gold weighed at a jeweller's, the diameter of a machine part, the timing of a
spacecraft's engine burn.

Q3 Choose a real‑life object (maybe a pressure cooker or a mobile phone) or a problem
(maybe a traffic jam near your school). Make a sketch listing what kind of ideas
from physics, chemistry, biology, earth science, or mathematics are involved. Show
how at least two branches of science connect with your example.

Sample answer — the pressure cooker. First list the ideas each branch contributes, then show
the chain that links them.

Page 13 of 18

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

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BRANCH IDEAS IT CONTRIBUTES

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Physics Pressure heat

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aluminium and steel are chosen

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PHYSICS — the lid seals the vessel,
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100 °C, so the food gets hotter.

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cooking time, so roughly one-fourth.cthe LPG burnt and one-fourth the CO₂ released.
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One chain, four branches: a physics idea (pressure raises the boiling point) sets up a chemistry idea

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starch speed up sharply with temperature. Take away either link and the explanation

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Try this: build the same sketch for a mobile phone — physics (radio waves, battery
voltage, touchscreen capacitance), chemistry (lithium-ion cells, the glass), biology
(how the eye reads a 60 Hz refresh), earth science (the minerals mined for it, e-
waste), mathematics (data compression, error-correcting codes). Then trace one
chain across two branches, as above.

Ready to Go Beyond — Page 7
Real problems need several branches together

READY TO GO BEYOND

Q1 How does a mask really work?

Not by sieving. A mask works because particles are made to touch a fibre and stick to it, and
three different mechanisms do that for three different particle sizes.

1. Impaction
Heavy droplets (5 – 100 µm) are too
massive to follow the bending air, so
they run straight into the fibre.

2. Interception
A mid-size particle (about 1 µm) does
follow the air, but passes so close that
it grazes the fibre and is held.

3. Diffusion
Very small particles (below 0.1 µm) are
knocked about by air molecules and
wander onto a fibre within a few layers.

The grey circles are the cross-sections of single mask fibres. A mask is a deep tangle of thousands of
such fibres, so a particle that escapes one layer meets many more.

Now the branches, exactly as the box lists them:

Physics — particle motion and electrostatic attraction. The melt-blown middle layer carries a
permanent electric charge that pulls even neutral particles in, by inducing charge on them.
Chemistry — the properties of polymer fibres. Polypropylene is used because it is non-polar,
repels water, and holds a static charge for months.

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Biology — the size and behaviour of viruses. A virus is roughly 0.1 µm across, but it does not
travel alone: it rides in respiratory droplets 1 – 100 µm wide, which are far easier to trap.
Mathematics — modelling airflow and filtration efficiency. Each layer removes a fixed
fraction, so the efficiencies multiply, not add.

if one layer stops 60% of particles, it lets 0.4 through

three such layers let 0.4 × 0.4 × 0.4 = 0.064 through

overall efficiency = 1 − 0.064 = 93.6%

Why it happens: the gaps between mask fibres are tens of micrometres wide —
hundreds of times larger than a virus — so if a mask were a sieve it would be
useless. It is not a sieve; it is a trap. Impaction catches the big particles and diffusion
catches the very small ones, which leaves particles of about 0.3 µm as the hardest to
catch: too light to be thrown into a fibre, too heavy to wander into one. That size is
called the most penetrating particle size, and it is the size at which mask standards
are deliberately tested — a mask rated at its worst size performs better at every
other size.

Tip: this also explains why fit matters so much. Air that leaks around the nose or
cheeks meets no fibres at all, so the 93.6% calculated above collapses. Filtration
efficiency describes the material; a mask worn loosely does not use it.

Chapter at a glance
A model is a deliberately simplified picture of a real system that keeps only the quantities
that matter for the question being asked. Leaving out air resistance, or the individual cells
of the heart, is a choice made on purpose — not a mistake.
Science uses everyday words such as force, work, cell and reaction with narrow, agreed
meanings, and represents quantities by fixed symbols (m, v, F, I), each tied to a defined unit.
Mathematics is the language that states how quantities are related. An equation is a
compact statement about a relationship, not merely a recipe for calculating.
SI units are agreed international standards. Mixing units — pounds per litre for kilograms
per litre — cost an aircraft 15,000 litres of fuel and nearly cost lives.
A law describes a regular pattern in nature; a theory explains why the pattern occurs and
rests on evidence; a principle is a broad idea applied to many situations. In science a theory
is never a guess.

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Well-tested laws, theories and models let us predict what will happen in new conditions.
When a prediction fails, scientists re-examine the assumptions, the model or the
measurement — and that openness is science's strength, not its weakness.
Estimation — a rough calculation to check whether an answer is even possible — is a
genuine scientific skill. Careful reasoning matters more than a precise number in the early
stages.
Physics, chemistry, biology and earth science are divisions we made to organise knowledge.
Real problems such as climate change, medicines or how a mask works need several
branches together.

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Class 9 Science Chapter 1 Exploration: Entering the World of Secondary Science AglaSem · NCERT Solutions

Quick revision

TERM WHAT IT MEANS IN EXAMPLE FROM THIS WHY IT MATTERS
SCIENCE CHAPTER

Model A simplified representation A moving car treated as a single Makes an impossible
that keeps only what matters point; the Earth as a smooth problem solvable
for a given question layered sphere

Assumption A detail deliberately left out Neglecting air resistance for a Must be stated, so the
while building a model falling object model's limits are
known

Symbol An agreed letter that stands for m, v, F, I for mass, velocity, force, The same symbol
a physical quantity current means the same thing
worldwide

SI unit The internationally agreed unit kilogram (kg) for mass; m/s for Results can be
of a quantity speed compared; unit mix-
ups are avoided

Law A regular pattern observed in Newton's laws of motion explain Tells us what happens,
nature, stated in words or the jerk when a bus stops reliably
mathematics

Theory An evidence-based explanation Atomic theory explains how Not a guess — it has
of why a pattern occurs molecules form survived careful
testing

Principle A broad idea used to make Conservation of energy while Applies across physics,
sense of many situations climbing stairs chemistry and biology

Prediction A reasoned expectation from How far a kicked football will travel A failed prediction
established science, not a sends us back to the
guess assumptions

Estimation A rough calculation to test About 10,000 litres of air breathed Detects errors before
whether an answer is sensible per day precise work begins

Speed of light A defined physical constant Exactly 299792458 m/s; 'c' from Symbols come from
(c) Latin celeritas history and agreement

Simplifying A famous model: ignore most Meghnad Saha treated a star's Explained why a star's
the stars detail, keep the few quantities matter as a hot gas and kept only colour is tied to its
that decide the answer temperature, pressure and temperature
ionisation

Branches of Divisions made by us to Physics, chemistry, biology, earth Nature has no such
science organise knowledge science boundaries

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