Question Bank – Work, Energy, and Power

 

Question Bank – Work, Energy, and Power

1. Very Short Answer Questions (1 Mark)

1. Define work.

   • Answer: Work is said to be done when a force is applied to an object, and the object displaces in the direction of the applied force.

2. What is the SI unit of work?

   • Answer: Joule (J).

3. State the condition for work to be zero.

   • Answer: Work is zero if either the force or displacement is zero, or if the force and displacement are perpendicular to each other.

4. What is kinetic energy?

   • Answer: It is the energy possessed by a body due to its motion.

5. State the work-energy theorem.

   • Answer: The net work done on an object is equal to the change in its kinetic energy.

6. Define potential energy.

   • Answer: It is the energy possessed by a body due to its position or configuration.

7. What is the SI unit of power?

   • Answer: Watt (W).

8. What is meant by 1 watt of power?

   • Answer: When 1 joule of work is done in 1 second, the power is 1 watt.

9. What is the relation between work and energy?

   • Answer: Work done on a body results in a change in its energy.

10. Define mechanical energy.

• Answer: It is the sum of kinetic and potential energy of a system.

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2. Short Answer Questions (2 Marks)

11. Write the formula for kinetic energy and explain the terms.

    • Answer: $K.E. = \frac{1}{2} mv^2$, where $m$ is the mass of the object and $v$ is its velocity.

12. What is conservative force? Give an example.

    • Answer: A force is said to be conservative if the work done by it is independent of the path taken. Example: Gravitational force.

13. State the principle of conservation of mechanical energy.

    • Answer: In the absence of non-conservative forces, the total mechanical energy (kinetic + potential) of a system remains constant.

14. What is the work done by a force at an angle θ to the displacement?

    • Answer: $W = F d \cos\theta$, where $F$ is the force, $d$ is the displacement, and $\theta$ is the angle between them.

15. Define power and write its formula.

    • Answer: Power is the rate of doing work. $P = \frac{W}{t}$, where $W$ is work done and $t$ is time.

16. What is the relation between power and velocity?

    • Answer: $P = F \cdot v$, where $F$ is the force and $v$ is the velocity in the direction of the force.

17. What is a non-conservative force? Give an example.

    • Answer: A force is non-conservative if the work done depends on the path taken. Example: Friction.

18. What happens to the kinetic energy of an object when its velocity is doubled?

    • Answer: The kinetic energy becomes four times.

19. Explain the significance of negative work.

    • Answer: Negative work means the force is acting opposite to the displacement, reducing the energy of the system.

20. What is the work done by gravity on a satellite moving in a circular orbit?

    • Answer: Zero, because the force and displacement are perpendicular.

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3. Numerical Problems (3 Marks)

21. A force of 10 N displaces an object by 2 m along the direction of force. Calculate the work done.

    • Answer: $W = F \cdot d = 10 \times 2 = 20 \, J$.

22. Calculate the kinetic energy of a 2 kg object moving with a speed of 5 m/s.

    • Answer:
      $K.E. = \frac{1}{2} mv^2 = \frac{1}{2} \times 2 \times 5^2 = 25 \, J$.

23. Find the power if 200 J of work is done in 10 seconds.

    • Answer:
      $P = \frac{W}{t} = \frac{200}{10} = 20 \, W$.

24. A body of mass 10 kg is raised to a height of 5 m. Calculate its potential energy.

    • Answer:
      $P.E. = mgh = 10 \times 9.8 \times 5 = 490 \, J$.

25. A car of mass 1000 kg is moving with a velocity of 10 m/s. Calculate its kinetic energy.

    • Answer:
      $K.E. = \frac{1}{2} mv^2 = \frac{1}{2} \times 1000 \times 10^2 = 50,000 \, J$.

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4. Long Answer Questions (5 Marks)

26. Derive the work-energy theorem.
27. Derive the formula for potential energy of an object at a height h.
28. Explain the concept of conservative and non-conservative forces with examples.
29. Derive the relation between kinetic energy and momentum.
30. State and explain the law of conservation of mechanical energy with an example.
31. Explain the concept of power and derive the relation $P = F \cdot v$.
32. Explain positive, negative, and zero work with suitable examples.
33. Derive the expression for the kinetic energy of an object.
34. How does friction affect the conservation of mechanical energy? Explain.
35. Discuss the significance of work-energy theorem in practical situations.

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5. Higher Order Thinking Skills (HOTS) Questions

36. A man carries a bag horizontally. Is work done? Explain.

    • Answer: No, because the force and displacement are perpendicular.

37. Why do athletes bend their knees while landing?

    • Answer: Bending knees increases the time to stop, reducing the impact force.

38. Can kinetic energy be negative? Explain.

    • Answer: No, because mass and the square of velocity are always positive.

39. If two bodies have the same momentum, which one has more kinetic energy?

    • Answer: The body with smaller mass.

40. Explain why no work is done in uniform circular motion.

    • Answer: Because the force is always perpendicular to the displacement.

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6. Assertion-Reason Type Questions

41. Assertion (A): Work done by a force can be negative.
    Reason (R): Work is negative if the force opposes displacement.

    • Answer: Both A and R are true, and R is the correct explanation.

42. Assertion (A): Power is the rate of doing work.
    Reason (R): Power is the work done per unit time.

    • Answer: Both A and R are true, and R is the correct explanation.

43. Assertion (A): Mechanical energy is always conserved.
    Reason (R): Energy can neither be created nor destroyed.

    • Answer: A is false, but R is true (since mechanical energy may convert to other forms).

44. Assertion (A): Kinetic energy is proportional to mass.
    Reason (R): Kinetic energy is directly proportional to the square of velocity.

    • Answer: Both A and R are true, but R is not the correct explanation.