NCERT Solutions Class 9 Science Chapter 1: Exploration

Include the mass of the ball and the speed and direction of the hit. Ignore the brand of the bat, the colour of the ball and the grass on the field.

1. Decide the question firstThe question for the model is: “Will the ball cross the boundary without hitting the ground first?” The details we keep are the ones that change the answer to this question.
2. Details to include and ignore

   Detail	Include or ignore?	Reason
   Mass of the ball	Include	It decides how the ball moves after the hit.
   Speed of the hit	Include	A faster ball travels farther.
   Direction of the hit (angle)	Include	The angle decides the height and the distance.
   Air resistance	Ignore in a simple model	It has only a small effect.
   Spin of the ball and the stitching of the seam	Ignore in a simple model	These have smaller effects.
   Brand of the bat, colour of the ball, grass on the field	Ignore	They make no difference to whether the ball crosses the boundary.

3. Better modelsAs we build more and more complex models, we add the extra details, such as air resistance and spin, to get greater accuracy.

Meghna should ask questions that need measurable evidence and past patterns, not yes/no questions.

1. What makes a good questionA good scientific question asks for measurable evidence (numbers) and past patterns. A simple yes/no question is usually not very useful.
2. Examples of good questions

   Question Meghna can ask	What it checks
   What was the condition of the sky the last time it rained?	A past pattern
   What is the humidity today? Was it above 80 per cent the last time it rained?	A measured value compared with the past
   What are the wind speed and direction today?	Measured data
   Is the temperature dropping like it did before the recent rains?	A trend compared with earlier rain

3. Why “the clouds look dark” is not enoughIt is a feeling, not a measurement. The questions above turn it into data that can be checked. After the afternoon we can then compare the prediction with what actually happened. That is how a prediction is checked.

About 10,000 litres a day (about 20,000 breaths of about 0.5 litre each).

1. Breaths in a dayAt rest we take about 12 to 15 breaths a minute. A day has 60 × 24 = 1440 minutes.12 × 1440 = 17,280   and   15 × 1440 = 21,600So we take about 18,000 to 22,000 breaths, roughly 20,000 breaths a day.
2. Volume of one breathIt takes about 4 to 5 breaths to fill a typical rubber party balloon, which has a volume of about 2 litres when inflated. So one breath is about 2 ÷ 4 = 0.5 litre.
3. Total20,000 × 0.5 litre = 10,000 litres a day
4. Check the estimate another wayOne can blow up a balloon in about 20 seconds, so about 3 balloons a minute:3 balloons/minute × 2 litres/balloon × 1440 minutes/day = 8640 litresThis is reasonably close to 10,000 litres, so the estimate is sensible. (We would of course get tired very quickly blowing balloons all day, unlike normal restful breathing.)

Estimation question: the answer depends on the assumptions, so your numbers may be a little different. What matters is a reasonable estimate with clear assumptions.

If rice were the only food, a family of four would need about 70 to 80 kg of rice in a month. With other food as well, it is much less, perhaps 20 to 30 kg.

1. PlanAssume that all the energy comes from rice. An average adult needs about 2000 to 2500 kilocalories (kcal) a day. Find how many kcal 100 g of rice gives, and use it to find the daily need of the family.
2. Assumptions (sample)Each person needs 2200 kcal a day. Uncooked rice gives about 350 kcal per 100 g (3.5 kcal per gram).
3. CalculationFamily of 4: 4 × 2200 = 8800 kcal a dayRice a day: 8800 ÷ 3.5 ≈ 2500 g ≈ 2.5 kgOne month (30 days): 2.5 × 30 ≈ 75 kg
4. Does the answer make sense?100 g of rice for the whole month is clearly far too little, and a few tonnes is far too much. About 75 kg is sensible. In real life a family also eats dal, vegetables and roti, so the rice needed is less, roughly 20 to 30 kg a month.
5. Why such questions matterThey connect science to everyday questions about food and resources, and show why approximate reasoning is an important scientific skill.

Estimation question: the answer depends on the assumptions, so your numbers may be a little different. What matters is a reasonable estimate with clear assumptions.

A mask works by catching tiny particles in a layer of very fine fibres. Understanding it needs physics, chemistry, biology and mathematics together.

1. The main ideaSolving real problems requires knowledge from several branches of science. During the COVID-19 pandemic we all used masks for safety, and understanding how a mask works brings in many subjects.
2. What each subject tells us

   Subject	What it explains about a mask
   Physics	How tiny particles move, and how electrostatic attraction pulls particles towards the fibres.
   Chemistry	The properties of the polymer fibres the mask is made of.
   Biology	The size and behaviour of viruses, and how they travel in droplets.
   Mathematics	Models of the airflow and of the filtration efficiency (what fraction of particles is stopped).

3. Putting it togetherAir passes through layers of very fine plastic fibres. Particles are stopped by the fibres, and the electrostatic attraction helps to hold even very small particles. A well-fitting mask is important, so that the air goes through the filter and not round its edges.