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Class 9 Science · Chapter 1

1. Exploration: Entering the World of Secondary Science

Answers to Activity 1.1, Pause and Ponder and estimation tasks. CBSE Class 9 Science (NCERT Exploration).

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This is an introductory chapter. It has no Revise, Reflect, Refine exercise. Below are the answers to the activity, the Pause and Ponder questions and the estimation task given inside the chapter.

Key Ideas of the Chapter

ModelsSimplified ways of looking at real systems. They keep only what is important for a question and ignore other details on purpose.
Language of scienceWords like force, work and cell have exact meanings. Quantities have symbols (m, v, F, I) and standard (SI) units.
LawDescribes a regular pattern seen in nature, e.g. Newton’s laws of motion.
TheoryExplains why the pattern occurs, based on evidence, e.g. atomic theory.
PrincipleA broad idea used in a given situation, e.g. conservation of energy.
PredictionA reasoned expectation based on evidence, not a guess. It is tested by observation.
EstimationA rough answer that tells us whether a result is reasonable or impossible.

Activity 1.1: Let us model

ASuppose 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.

Details to keep:

  • The distance from school to home along the road.
  • My average speed on the bicycle.
  • Stops on the way, such as traffic signals and crossings.
  • Slopes (uphill roads slow me down).

Details that can be ignored:

  • The colour and brand of the bicycle.
  • The weight of my school bag (small effect).
  • Small bends in the road and the exact path of the wheels.
  • Air resistance on a calm day.

Why ignoring details is useful: it keeps the model simple. We can then use a simple relation, time = distance ÷ average speed, and get a quick, useful answer. Extra details can be added later if more accuracy is needed.

Pause and Ponder

1Think 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?

Example: My family predicted that India would win a cricket match. This was mainly guesswork, based on support for the team.

How scientific thinking improves it:

  • Look at evidence, such as past results of both teams.
  • Check the form of the players and the pitch condition.
  • Check the weather forecast and the toss result.
  • Compare the prediction with the actual result and learn from mistakes.

A prediction based on measurable data and past patterns is more reliable than a guess.

2Describe one situation where an approximate answer is good enough, and one where you would need a very exact value.

Approximate answer is enough: estimating how much rice a family needs for a month, or how long it takes to walk to school. A rough value like “about 15 minutes” is enough.

Exact value is needed: the dose of a medicine for a patient, or the amount of fuel filled in an aircraft. A small error here can be dangerous.

3Choose 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.

Example: Pressure cooker

PhysicsSteam builds up pressure. Higher pressure raises the boiling point of water. Heat flows through the metal base.
ChemistryFood cooks faster due to faster chemical changes at high temperature. The rubber gasket and steel body are chosen materials.
BiologyHigh temperature kills germs. Cooked food is easier to digest.
MathematicsCounting whistles, measuring time and quantity of water.
Earth scienceAt hill stations, air pressure is low and water boils below 100 °C. So a cooker is more useful there.

Connection: physics (high pressure → higher boiling point) and chemistry (faster cooking at higher temperature) work together. This also connects to biology, as the high temperature kills germs.

Draw this as a sketch in your notebook: the pressure cooker in the centre and arrows to each branch of science.

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Estimation Tasks

4Example 1.1: A cricket ball is hit for a six. You want to make a simple model. What details would you include? What would you ignore?

Include: the mass of the ball, and the speed and direction in which it is hit.

Ignore: the brand of the bat, the colour of the ball and the grass on the field. Air resistance, spin and the seam have small effects. They can be ignored in a simple model.

5Ready to Go Beyond: How much rice would feed a family of four for a month? (Assume all their calorie needs come from rice alone.)
Energy needed by one adult ≈ 2000 – 2500 kcal per day
For a family of four ≈ 4 × 2250 ≈ 9000 kcal per day
100 g uncooked rice gives about 350 kcal
Rice needed per day ≈ 9000 ÷ 350 × 100 g ≈ 2.5 kg
Rice needed per month ≈ 2.5 kg × 30 ≈ 75 kg

So the family needs roughly 70 – 80 kg of rice in a month. This answer makes sense. It is much more than 100 g and much less than a few tonnes.

Children need fewer calories than adults. So the real amount may be a little less. The aim is a reasonable estimate, not an exact value.

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