📘 Chapter 5: Work and Energy – Exercise Short Answer Questions
Prepared by Muhammad Tayyab, Subject Specialist Physics, Govt Christian High School Daska. Based on PECTAA 2026 syllabus (National Curriculum 2023).
📖 What's Inside: This section covers exercise short answer questions from Chapter 5 Work and Energy including Work Done, Kinetic Energy, Potential Energy, Power, Efficiency, and Renewable vs Non-renewable Energy. Each question is presented with a detailed answer as per the official PECTAA 2026 Physics curriculum. Perfect for Punjab Boards (Lahore, Gujranwala, Multan, etc.) and all BISE boards across Pakistan.
📚 Related Resources – Chapter 5: Work and Energy
📋 Exercise Short Qs
📝 Short Qs
📖 Comprehensive Qs
🔢 Numerical Problems
✅ MCQs
✏️ Constructed Response
Work and Energy covers work, kinetic energy, potential energy, conservation of energy, power, and efficiency.
📖 Exercise Short Answer Questions & Answers (PECTAA 2026)
5.1 What is the work done on an object that remains at rest when a force is applied on it?
Answer:
Work done on an object is zero due to rest position displacement of body is zero. i.e \(S = 0\)
\[ W = F \times S = F \times 0 = 0 \]
OR
If some force is acting on a body but there is no displacement, then no work is done.
For example, a man is pushing hard a wall but the wall remains fixed in its place. In this case, the man is doing no work.
✅ Work done = 0 J (No displacement)
5.2 A slow-moving car may have more kinetic energy than a fast-moving motorcycle. How is this possible?
Answer:
The kinetic energy of a moving body depends upon its mass and velocity. A car has much greater mass than a motorcycle; therefore, a slow-moving car may have more kinetic energy than a fast-moving motorcycle.
\[ E_k = \frac{1}{2} mv^2 \]
✅ Kinetic energy depends on both mass and speed.
5.3 A force \(F_1\) does 5 J of work in 10 s. Another force \(F_2\) does 3 J of work in 5 s. Which force delivers greater power?
Answer:
For force \(F_1\):
\[ W_1 = 5\text{ J},\quad t_1 = 10\text{ s} \]
\[ P_1 = \frac{W_1}{t_1} = \frac{5}{10} = 0.5\text{ W} \]
For force \(F_2\):
\[ W_2 = 3\text{ J},\quad t_2 = 5\text{ s} \]
\[ P_2 = \frac{W_2}{t_2} = \frac{3}{5} = 0.6\text{ W} \]
✅ Force \(F_2\) delivers greater power (0.6 W > 0.5 W).
5.4 A woman runs up a flight of stairs. The gain in her gravitational potential energy is 4500 J. If she runs up the same stairs with twice the speed, what will be her gain in potential energy?
Answer:
The gain in gravitational potential energy depends upon the mass, gravitational field strength and height. Since she climbs the same stairs, her gain in potential energy will remain 4500 J.
\[ E_p = mgh \]
Speed does not affect potential energy.
✅ Potential energy gain = 4500 J (unchanged).
5.5 Define work and its SI unit.
Answer:
Work: Work is defined as the product of the magnitude of force and the distance covered in the direction of force. Mathematically,
\[ W = F \times S \]
SI Unit of Work: The SI unit of work is joule (J).
Joule: One joule work is done when a force of one newton acting on a body moves it through a distance of one metre in its own direction.
\[ 1\text{ J} = 1\text{ N} \times 1\text{ m} = 1\text{ Nm} \]
✅ 1 J = 1 N × 1 m
5.6 What is the potential energy of a body of mass \(m\) when it is raised through a height \(h\)?
Answer:
The gravitational potential energy of a body is equal to the work done in raising it through a height \(h\).
The applied force necessary to lift the body with constant velocity is equal to its weight \(w\). Therefore,
\[ E_p = wh = mgh \quad (\because w = mg) \]
✅ \(E_p = mgh\)
5.7 Find an expression for the kinetic energy of a moving body.
Answer:
Kinetic energy of a body is the energy that a body possesses by virtue of its motion.
To find how much kinetic energy a moving body has, we apply a force in the opposite direction to bring it to rest. The work done by this force is equal to the kinetic energy of the body.
\[ \text{Kinetic energy} = E_k = \text{Work done } (W) \]
Suppose a body of mass \(m\) is moving with velocity \(v\). An opposing force \(F\) brings the body to rest over a distance \(S\). Then,
\[ E_k = F \times S \quad \cdots (i) \]
By Newton's second law, \(F = ma\), where \(a\) is the deceleration.
The distance \(S\) traveled under constant deceleration:
\[ S = v_{\text{av}} \times t = \frac{v}{2} \times t \]
Putting values in (i):
\[ E_k = ma \times \frac{v}{2} \times t \quad \cdots (ii) \]
Using velocity-time graph, \(a = \frac{v}{t}\). Substituting in (ii):
\[ E_k = m \times \frac{v}{t} \times \frac{v}{2} \times t = \frac{1}{2} mv^2 \]
✅ \(E_k = \frac{1}{2} mv^2\)
5.8 Define efficiency of a working system. Why a system cannot have 100% efficiency?
Answer:
Efficiency: The ratio of useful output energy and the total input energy is called the efficiency of a working system. Thus
\[ \text{Efficiency} = \frac{\text{Useful output energy}}{\text{Total input energy}} \]
Efficiency is often multiplied by 100 to give percentage efficiency. Thus,
\[ \text{Percentage Efficiency} = \frac{\text{Useful output energy}}{\text{Total input energy}} \times 100 \]
Why not 100%: A system cannot have 100% efficiency because during any conversion of energy, some energy is wasted in the form of heat. No device has yet been invented that may convert all the input energy into required output.
✅ Efficiency < 100% due to energy losses (heat, friction).
5.9 What is power? Define the unit used for it.
Answer:
Power: Power is defined as the time rate of doing work.
\[ P = \frac{W}{t} \]
SI Unit of Power: SI unit of power is watt (W).
Watt: One watt is the work done at the rate of one joule per second.
\[ 1\text{ W} = \frac{1\text{ J}}{1\text{ s}} = 1\text{ J s}^{-1} \]
✅ 1 W = 1 J/s
5.10 Differentiate between renewable and non-renewable energy sources.
Answer:
| Renewable Energy Sources | Non-Renewable Energy Sources |
|---|---|
| The resources of energy which are replaced by new ones after their use are called renewable energy sources. | Non-renewable sources are those, which are depleted with the continuous use. |
| For example, solar energy, hydroelectricity, wind energy, tidal energy, wave energy and geothermal energy | For example, fossil fuels (coal, oil, natural gas) and nuclear energy |
✅ Renewable Examples
- Solar Energy
- Wind Energy
- Hydroelectric Energy
❌ Non-Renewable Examples
- Coal
- Oil (Petroleum)
- Natural Gas
📐 Key Concepts – Work and Energy (Exercise Short Questions)
Work: \(W = F \times S\)
Kinetic Energy: \(E_k = \frac{1}{2} mv^2\)
Potential Energy: \(E_p = mgh\)
Power: \(P = \frac{W}{t}\)
Efficiency: \(\eta = \frac{\text{Useful output}}{\text{Total input}} \times 100\)
💡 Exam Tip:
For exercise short questions, write concise and accurate answers. Use definitions, formulas, and examples where required. These questions test your understanding of fundamental concepts like work, kinetic and potential energy, power, efficiency, and energy sources. These questions follow the PECTAA 2026 pattern and are prepared by Subject Specialist Muhammad Tayyab.
Chapter 1Physical Quantities and Measurements
Chapter 2Kinematics
Chapter 3Dynamics
Chapter 4Turning Effect of Forces
Chapter 5✨ Work and Energy
Chapter 6Mechanical Properties of Matter
Chapter 7Thermal Properties of Matter
Chapter 8Magnetism
Chapter 9Nature of Science
📖 Complete syllabus coverage for Class 9 Physics (PECTAA 2026) – Chapters 1 to 9
Created by Hira Science Academy | Aligned with PECTAA 2026 Syllabus