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

Explanation: An eclipse occurs when the Moon comes between the Sun and the Earth, so its shadow falls on part of the Earth; the only change is that less sunlight reaches that region for a short time. Food kept in shadow does not change significantly, and we routinely keep food in dark cupboards, in refrigerators, and overnight without any harm. Physically, no new rays or radiation are produced during an eclipse; in fact, the amount of sunlight reaching us decreases. Chemically, the brief absence of sunlight triggers no reaction in food, and biologically, the growth of microbes depends on temperature, moisture, and time, not on the position of the Moon. Since no mechanism supports the claim and there is no evidence of food spoiling faster during eclipses, scientific thinking tells us it is a superstition, not a fact.

Explanation: Follow the assumption the question sets up: rice supplies all the energy. An adult needs about 2000-2500 kcal per day, and raw rice gives roughly 350 kcal per 100 g, so one person needs about 2000/350 x 100 = 570 g to 2500/350 x 100 = 715 g per day. For four people that is about 2.3 to 2.9 kg per day, and over 30 days about 70 to 85 kg per month. An estimate like this is meant to land in the right range, not on an exact figure - note that a real family eats far less rice than this, because in practice dal, vegetables, oil and milk supply much of the energy.

Explanation: A surgical mask is made of several layers of non-woven fabric whose fibres criss-cross to form a very fine mesh. When we breathe, tiny air molecules pass easily through the gaps, but larger dust particles and the small liquid droplets released while coughing, sneezing, or talking get trapped in the fibre layers; this physical process is filtration. Disease-causing microbes such as viruses and bacteria travel riding on these droplets, so trapping the droplets protects both the wearer and the people nearby. Using multiple layers with small pore size improves the filtering, and special masks like the N95 additionally use electrostatically charged fibres that attract and hold very fine particles.

Explanation: This is an open-ended question, so any honest example analysed properly is acceptable. Most everyday predictions, like the result of a cricket match, mix a little evidence (recent form of the teams, playing conditions) with a lot of conjecture (support for a favourite team, gut feeling, or superstition). Scientific thinking asks us to rely only on relevant evidence and logical reasoning: collect data such as past performance and conditions, look for patterns, make the prediction while accepting its uncertainty, and update it when new information comes in. A prediction made this way is still not guaranteed to be correct, but it will be right far more often than one based on emotion or guesswork.

Explanation: Many symbols in science come from Latin or Greek roots. The speed of light in vacuum is denoted by c after the Latin word celeritas, which means swiftness or speed. It is also convenient to remember that c is a universal constant of nature, about 3 x 10⁸ m/s in vacuum, which is why the same symbol appears in famous relations such as E = mc².

Explanation: A model is a simplified representation of a real situation built to answer a specific question, here how long the ride will take. The factors that strongly affect the answer are the distance of the route and the average speed of cycling, perhaps adjusted slightly for slopes, traffic signals, and rest stops; with these, time = distance / average speed. Details like the bicycle's colour, the rider's clothes, minor wind, or the exact number of pedal strokes hardly change the result, so they are left out. Omitting them is beneficial because the model stays simple, quick to calculate, and easy to understand, while still being accurate enough for its purpose; including every detail would make it complicated without meaningfully improving the answer.

Explanation: To model the flight of the ball we keep only the factors that decide its path: the speed with which it leaves the bat, the angle of the hit, the height at which it is struck, and the downward pull of gravity. With just these, the ball can be treated as a simple projectile and the distance it travels can be estimated. Factors like the colour or brand of the ball, the bowler's run-up, and the crowd's noise do not affect the flight at all, while spin, wind, and air resistance change it only somewhat, so a first simple model can ignore them too. This is how scientific models work: keep what matters for the question being asked and drop the rest, adding refinements only if more accuracy is needed.

Explanation: This is a Fermi-style estimation built from reasonable assumptions. A person at rest breathes about 12-16 times a minute, so take about 15 breaths per minute, and each normal breath moves about half a litre of air. The air breathed per minute is therefore about 15 x 0.5 = 7.5 litres, which gives 7.5 x 60 = 450 litres per hour and 450 x 24 = 10,800 litres per day. Rounding off, a person breathes air of the order of 10⁴ litres, that is, roughly ten thousand litres a day. The exact figure varies with age, body size, and activity, so any nearby value reached with sound reasoning is acceptable.

Explanation: Weather is decided by a very large number of interlinked factors such as temperature, air pressure, humidity, and wind speed and direction, all changing continuously over huge regions. Forecasters feed measurements of these into computer models, but every model is a simplified version of the real atmosphere, and every measurement carries small errors. In such a complex system, tiny errors in the initial data grow quickly with time, so the forecast gradually drifts away from what actually happens. This is why forecasts are stated as probabilities, such as chances of rain, and why short-range forecasts are fairly reliable while longer-range ones often go wrong.

Explanation: The level of accuracy needed depends on the purpose. When buying rice for a month, judging how much paint a wall needs, or estimating the crowd at a fair, a rough figure is enough because a small error causes no real loss. But when a doctor decides the dose of a medicine, an engineer makes a machine part that must fit another, or a chemist weighs reactants for a reaction, even a small error can lead to harm or failure, so exact values are essential. Part of scientific thinking is recognising how much accuracy a given situation actually demands.