Work, Energy and Power

Physics, HSSC-Punjab First Year. This chapter is split into 37 topics and has 805 practice questions. The topics covered are listed below, followed by 10 free sample questions from the chapter.

Topics in this chapter

1. Electrical energy, kilowatt-hour and horse power 23 questions
2. Kinetic energy numericals 21 questions
3. Work done by a variable force 20 questions
4. Pendulum, swing and roller-coaster energy 22 questions
5. Positive, negative and zero work 24 questions
6. Chapter exercise: numerical problems with new data 23 questions
7. Kinetic energy: proportionality and ratios 20 questions
8. Units and dimensions in work, energy and power 23 questions
9. Assertion-reason items 20 questions
10. Power: definition and units 23 questions
11. Work-energy theorem: statement and derivation 23 questions
12. Gravitational potential energy near the surface 20 questions
13. Work-energy theorem numericals 20 questions
14. Energy conversion with friction and air resistance 20 questions
15. Work by gravity and friction along different paths 20 questions
16. Chapter exercise: derivations and explanations 22 questions
17. Boxes, tables and interesting information 20 questions
18. Conservative forces 22 questions
19. Absolute potential energy: concept and derivation 23 questions
20. Power numericals 21 questions
21. Stopping distances and resistive forces 20 questions
22. Energy: kinetic and potential forms 22 questions
23. Power and the product of force and velocity 20 questions
24. Free fall, projectiles and energy conservation numericals 24 questions
25. Non-conservative forces 24 questions
26. Kinetic energy: formula and derivation 23 questions
27. Work numericals with angles and components 21 questions
28. Chapter exercise: multiple choice items 24 questions
29. Interconversion of potential and kinetic energy 23 questions
30. Work from force-displacement graphs 23 questions
31. Escape velocity numericals 22 questions
32. Escape velocity: concept and derivation 22 questions
33. Work done by a constant force 25 questions
34. Absolute potential energy numericals 20 questions
35. Work-energy on inclined planes 20 questions
36. Work by a force at an angle to the displacement 20 questions
37. Chapter exercise: short and constructed response items 22 questions

Sample questions: Work, Energy and Power

10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.

Q1. 1 joule of work is done when:

• A. a body of mass 1 kg moves with a speed of 1 m s⁻¹
• B. a force of 1 N moves a body through 1 m in its own direction(correct answer)
• C. a force of 1 N acts on a body for 1 s
• D. a power of 1 W acts for 1 hour on a body

Answer: B.By definition 1 J = 1 N × 1 m: one newton of force moving a body one metre in the direction of the force does one joule of work.

Q2. A body moves with constant velocity on a rough horizontal surface under the action of a pulling force. The net work done on the body is:

• A. positive
• B. equal to the pulling work
• C. negative
• D. zero(correct answer)

Answer: D.The pulling work is positive and the friction work is equally negative, giving zero net (no change in kinetic energy).

Q3. A hand pulls a spring balance with a spring force opposing the pull. As the spring is stretched the work done by the spring force is:

• A. zero, since the force is variable
• B. positive, since it follows the pull
• C. negative, since it opposes the stretching displacement(correct answer)
• D. equal to the work by the hand

Answer: C.The spring force on the hand is opposite to the displacement, so its work is negative; the hand's work is positive.

Q4. The three examples of conservative forces mentioned in the book are:

• A. magnetic, air resistance and elastic spring force
• B. gravitational, frictional and electrostatic force
• C. frictional, air resistance and tension
• D. gravitational, electrostatic and elastic spring force(correct answer)

Answer: D.The text says gravitational force is conservative; other examples are the electrostatic and the elastic spring forces.

Q5. The book lists the following as non-conservative forces:

• A. gravity and the normal force of a surface
• B. air resistance, tension, normal force and rocket propulsion(correct answer)
• C. the elastic spring force and the electrostatic force
• D. gravity, air resistance and tension in a string

Answer: B.Other examples of non-conservative force given are air resistance, tension in a string, normal force and the propulsion force of a rocket.

Q6. An object with kinetic energy can perform work if it is allowed to push or pull on another object. The work done on the other object is:

• A. negative, and the kinetic energy of the first object increases
• B. positive, and the kinetic energy of the first object decreases(correct answer)
• C. zero, and the kinetic energy does not change
• D. infinite, and the kinetic energy becomes zero

Answer: B.Energy is transferred as the first object loses KE; the work it does is taken as positive on the other object (the work on the first is negative).

Q7. The absolute gravitational potential energy of an object at a certain position is defined by the book as:

• A. the product of mass and height measured from the ground
• B. the work done by the gravitational force in displacing the object from that position to infinity(correct answer)
• C. the kinetic energy gained in falling to the Earth
• D. the work done by the object against gravity in moving it round the Earth

Answer: B.At infinity the force of gravity becomes zero; the book defines U as the work done by gravity from that position to infinity.

Q8. How is the work done by a variable force calculated?

• A. as the product of the average time and force
• B. as the product of the largest force and the total distance
• C. as the area under the F cosθ versus d graph, i.e. the limit of the sum Σ Fᵢ cosθᵢ Δdᵢ(correct answer)
• D. as the force at the end of the path times the path length

Answer: C.For a variable force the work is the area under the F cosθ versus d graph, the limit of the sum of F cosθ Δd (Eq. 4.5).

Q9. Assertion (A): The work done by a force on a body is zero when the force is perpendicular to its displacement. Reason (R): The work is the product F d cosθ and cos 90° = 0. Select the correct option.

• A. A is true, but R is false
• B. A is false, but R is true
• C. Both A and R are true, and R is the correct explanation of A(correct answer)
• D. Both A and R are true, but R is not the correct explanation of A

Answer: C.cos 90° = 0, so the work Fd cosθ is zero when the force is perpendicular to the displacement, which is the reason R gives.

Q10. Assertion (A): A satellite in a circular orbit has constant kinetic energy. Reason (R): Gravity does positive work on the satellite all the time. Select the correct option.

• A. A is false, but R is true
• B. Both A and R are true, and R is the correct explanation of A
• C. A is true, but R is false(correct answer)
• D. Both A and R are true, but R is not the correct explanation of A

Answer: C.A is true but R is false: the gravitational force is perpendicular to the velocity, so it does zero work and the speed is constant.