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ICSE Class 8 Physics Question 1 of 25

Energy — Question 2

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2
Question

Question 2(b)

Assertion (A) : When a ball is dropped from a height, its kinetic energy decreases and potential energy increases.

Reason (R) : The total mechanical energy remains conserved during the fall, if we ignore air resistance.

  1. Both A and R are true and R is the correct explanation of A
  2. Both A and R are true and R is not the correct explanation of A
  3. Assertion is true but reason is false
  4. Assertion is false but reason is true
Answer

Assertion is false but reason is true

Explanation

Assertion (A) is false because when the ball is released from a height, it falls down and the vertical height of the ball from the ground decreases. Therefore, the potential energy decreases and it changes to kinetic energy due to which the speed of ball increases.

Reason (R) is true because during the fall while transformation of mechanical energy from one form to the other desired form, some part of it changes to other undesirable form usually heat due to the presence of air resistance but when air resistance is absent then no losses occur and hence the total mechanical energy remains conserved.

Chapter Overview: Energy

Energy is the capacity to do work, measured in joules (J). It exists in various forms: kinetic (motion), potential (position), heat, light, sound, chemical (bonds), electrical (current), and nuclear (atomic nucleus). Energy can be transformed from one form to another — for example, an electric bulb converts electrical energy to light and heat, a solar panel converts light to electrical energy, and our body converts chemical energy (food) to kinetic and heat energy. The law of conservation of energy states that energy cannot be created or destroyed, only transformed; total energy remains constant. Energy sources are classified as renewable (solar, wind, hydro, geothermal, biomass, tidal — replenished naturally, low pollution) and non-renewable (coal, petroleum, natural gas, nuclear fuels — exhaustible, cause pollution). The growing energy crisis requires conservation: using LED bulbs, public transport, solar heaters, energy-efficient appliances, and reducing wastage.

Key Definitions & Concepts

Term Definition / Details
EnergyCapacity to do work. SI unit: joule (J)
Kinetic EnergyEnergy of a moving object
Potential EnergyEnergy stored due to position or configuration
Conservation of EnergyEnergy cannot be created or destroyed, only transformed
Renewable SourceEnergy source replenished naturally: solar, wind, hydro, biomass
Non-Renewable SourceExhaustible source: coal, petroleum, natural gas, nuclear fuels
Energy TransformationConversion of energy from one form to another

Must-Know Points for Exams

  • Energy = capacity to do work. Unit: joule (J).
  • 8 forms: kinetic, potential, heat, light, sound, chemical, electrical, nuclear.
  • Law of conservation: total energy of an isolated system is constant.
  • Renewable sources: inexhaustible, low pollution. Non-renewable: exhaustible, cause pollution.
  • Nuclear fuels (uranium) are NON-renewable despite being very powerful.
  • Conserve energy: LED bulbs, public transport, solar heaters, switch off when not in use.

Quick Self-Test

  1. Define the main concept of this chapter in one sentence.
  2. List the key types or categories discussed in this chapter.
  3. Give three real-life examples related to the main concept.
  4. Draw and label the key diagram of this chapter from memory.
  5. State the main law or principle covered in this chapter.