NCERT Solutions for Class 9 Science Chapter 1 Exploration Entering the World of Secondary Science (PDF) – 2027

First fix the question the model must answer: “Will the ball cross the boundary without hitting the ground first?”

The brand of the bat, the colour of the ball and the grass make no difference to whether the ball clears the boundary. Air resistance, spin and the seam have only small effects, so a simple model ignores them; more complex models add such details back for greater accuracy.

the model has to give just one number — the time taken — so keep what changes the time noticeably and ignore the rest.

Why ignoring details is useful: the details we keep can actually be measured; the ignored ones change the time by only a few seconds, less than its normal day-to-day change; and a simple model is easy to test and correct.

The speed of light is denoted by c because the symbol comes from the Latin word celeritas, meaning speed. Scientific symbols often come from history and are based on international agreements; they are not just convenient abbreviations. Today the speed of light is one of the physical constants, defined to be exactly 299792458 m/s.

A kilogram is used everywhere so that it means exactly the same amount in every shop, laboratory and country. Measurements are based on agreed international standards, not on local objects or opinions. Standard units:

• allow scientific results to be compared anywhere in the world;
• ensure fairness in daily life and trade — when we buy a kilogram of rice or vegetables, we get the same quantity from every seller.

Before a one-day match my uncle said, “India will win — they always win at this ground.” This was mostly guesswork. It used one real pattern (the home record) but ignored the pitch, injured players, dew and which team would bat second.

Scientific thinking can improve such a prediction by:

1. stating it clearly so that it can be checked (“India will win”, not “India will play well”);
2. listing the things that matter — record at that ground, pitch, weather, players’ form;
3. using past data instead of memory (for example, in 13 of the last 20 matches here the team batting second won);
4. checking the result afterwards and changing the reasoning if the prediction fails.

Meghna should ask questions that look for measurable evidence and past patterns, not simple yes/no questions. For example:

• “What was the condition of the sky the last time it 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 like it did before the recent rains?”
• “How often in the last month did dark clouds appear and rain follow?”

Such questions ask about measurable data and past patterns, which go beyond just “the clouds look dark”.

Weather depends on many factors that keep changing together — temperature, pressure, humidity and wind. Forecasts use measurements and models, but the starting conditions can never be measured perfectly, and very tiny differences in conditions grow over time into completely different weather. So forecasts are usually reliable for a few hours or even a few days, but less certain further into the future.

No. No physical, chemical or biological mechanism can make food harmful during an eclipse. Asking simple scientific questions shows this:

About 70–80 kg of uncooked rice a month, assuming all the family’s calories come from rice alone.

Check: 100 g a month is far too little and a few tonnes far too much; about 77 kg (roughly one and a half 50 kg sacks) is a sensible amount, so the estimate makes sense.

About 10,000 litres of air a day.

8640 litres is close to 10,000 litres, so the estimate is reasonable.

Approximate answer is good enough — buying rice for a school picnic. For 80 students at about 60 g of uncooked rice each, we need about 80 × 60 g = 4800 g ≈ 5 kg. Buying a 5 kg or 6 kg bag is fine; half a kilogram more or less changes nothing.

A very exact value is needed — giving medicine to a child. The doctor decides the dose (often from the child’s body weight), and it must be measured exactly with the marked cup or dropper. Too much can harm the child and too little may not work.

Sample answer — the pressure cooker.

How two branches connect (sketch):

Physics (higher pressure raises the boiling point of water to about 120 °C) leads to chemistry (the changes that cook food go faster at this higher temperature), so food cooks sooner and less fuel is used.

A mask works as a trap, not a simple sieve: tiny droplets and particles carrying viruses are made to touch its fibres and stick to them. Understanding this needs several branches of science:

• Physics — particle motion (large droplets run straight into fibres; very small particles wander onto them) and electrostatic attraction (charged fibres pull particles in).
• Chemistry — properties of polymer fibres such as polypropylene, which repels water and holds an electric charge.
• Biology — size and behaviour of viruses: a virus is tiny (about 0.1 µm) but travels inside much larger droplets from breath, coughs and sneezes.
• Mathematics — modelling the airflow and how much each layer filters.