NCERT Solutions Class 9 Science Chapter 10 - Work and Energy

Work and Energy – Important Questions with Answers

1. Define work.
Answer: Work is said to be done when a force is applied on an object and the object is displaced in the direction of the force.
Work = Force × Displacement × cosθ

2. What is the SI unit of work?
Answer: The SI unit of work is joule (J).

3. When is work said to be positive, negative, or zero?
Answer:

• Positive work: Force and displacement are in the same direction.

• Negative work: Force and displacement are in opposite directions.

• Zero work: Displacement is zero or force is perpendicular to displacement.

4. Define 1 joule of work.
Answer: 1 joule is the work done when a force of 1 newton displaces an object by 1 metre in the direction of the force.

5. What is power?
Answer: Power is the rate of doing work.
Power = Work / Time

6. What is the SI unit of power?
Answer: Watt (W)

7. Define 1 watt.
Answer: Power is 1 watt when 1 joule of work is done in 1 second.

8. Define commercial unit of energy.
Answer: The commercial unit of energy is kilowatt-hour (kWh).
1 kWh = 1000 watts × 3600 seconds = 3.6 × 10⁶ joules

9. What is energy?
Answer: Energy is the capacity to do work. It exists in various forms like kinetic energy, potential energy, etc.

10. What is kinetic energy?
Answer: Kinetic energy is the energy possessed by a body due to its motion.
KE = ½mv²

11. What is potential energy?
Answer: Potential energy is the energy possessed by a body due to its position or configuration.
PE = mgh

12. A ball of mass 2 kg is moving at 3 m/s. Find its kinetic energy.
Answer:
KE = ½ × 2 × 3² = 9 J

13. A 5 kg object is lifted to a height of 4 m. Find its potential energy. (Take g = 10 m/s²)
Answer:
PE = mgh = 5 × 10 × 4 = 200 J

14. What is the Law of Conservation of Energy?
Answer: Energy can neither be created nor destroyed; it can only change from one form to another. The total energy remains constant.

15. What are the conditions for work to be done?
Answer:

• A force must be applied.

• The object must be displaced.

• The displacement must have a component in the direction of the force.

16. Name some forms of energy.
Answer: Heat energy, light energy, electrical energy, chemical energy, nuclear energy, etc.

17. What is mechanical energy?
Answer: The sum of kinetic and potential energy is called mechanical energy.

18. A body of 1 kg is lifted to a height of 10 m. What is the work done against gravity?
Answer:
W = mgh = 1 × 9.8 × 10 = 98 J

19. What do you mean by transformation of energy?
Answer: The process of changing one form of energy into another is called transformation of energy.
Example: Electrical energy to heat in an iron.

20. What is the average power?
Answer: Average power is the total work done divided by the total time taken.

21. What is instantaneous power?
Answer: Instantaneous power is the power at a particular moment.

22. What is the unit of energy in the SI system?
Answer: Joule (J)

23. Define kilowatt.
Answer: 1 kilowatt (kW) = 1000 watts

24. How is commercial energy consumption calculated?
Answer: Commercial consumption is usually measured in kilowatt-hour (kWh), also known as one unit.

25. State the relation between work, energy, and power.
Answer:

• Work and energy have the same unit and are numerically equal when work is done.

• Power = Energy / Time