📘 Chapter 5: Work and Energy – 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 short answer questions from Chapter 5 Work and Energy including Work, Kinetic Energy, Potential Energy, Conservation of Energy, Power, Efficiency, and Renewable Energy Sources. 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.
📑 Quick Jump to Questions
📖 Short Answer Questions & Answers (PECTAA 2026)
1 Define work.
Answer:
Work is defined as the product of magnitude of force and the distance covered in the direction of force. Mathematically,
\[ W = F \times S \]
✅ \(W = F \times S\)
2 When is work said to be done?
Answer:
Work is said to be done when a force acts on an object and moves it through some distance.
✅ Force causes displacement.
3 Under what condition is no work done by a force?
Answer:
If some force is acting on a body but there is no displacement, then no work is done. Similarly, if a force acting on the body is zero, work will be zero.
✅ No displacement → No work.
4 Write the expression for work done when force makes an angle with the direction of motion.
Answer:
When a force acts at an angle \(\theta\) with the direction of motion, only the horizontal component of the force in the direction of motion does the work. This component is \(F \cos \theta\). Therefore,
\[ W = F \cos \theta \times S = FS \cos \theta \]
✅ \(W = FS \cos \theta\)
5 How can work done be calculated using a force-distance graph?
Answer:
When a constant force acts through a distance, the event can be plotted on a force-distance graph. The area under the force-distance curve is equal to the product of force and distance.
\[ \text{Work} = F \times S \]
Hence, the area under a force-distance graph represents the work done by the force.
✅ Area under F-S graph = Work done.
6 What is the SI unit of work and how is it defined?
Answer:
The SI unit of work is joule (J).
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.
\[ 1\text{ J} = 1\text{ N} \times 1\text{ m} = 1\text{ Nm} \]
Bigger units of work are also used like \(1\text{ kJ} = 10^3\text{ J}\) and \(1\text{ MJ} = 10^6\text{ J}\).
✅ 1 J = 1 N × 1 m
7 What is energy? Give an example.
Answer:
Energy can be defined as the ability of a body to do work.
For example, a car cannot run without the energy obtained by burning fuel.
✅ Energy = ability to do work.
8 What is the SI unit of energy?
Answer:
Energy, like work, is a scalar quantity. The SI unit of energy is the joule (J).
When one joule of work is done on a body, the amount of energy spent is one joule.
✅ SI unit of energy = joule (J).
9 What is mechanical energy and what are its basic types?
Answer:
The combination of kinetic energy and potential energy is called mechanical energy.
There are two basic forms of energy:
(i) Kinetic energy
(ii) Potential energy
Kinetic Energy: Kinetic energy of a body is the energy that a body possesses by virtue of its motion.
\[ E_k = \frac{1}{2} mv^2 \]
Potential Energy: Potential energy is defined as the energy that a body possesses by virtue of its position or deformation.
\[ E_p = mgh \]
✅ Mechanical energy = \(E_k + E_p\).
10 What are the different forms of potential energy? Define them.
Answer:
There are many forms of potential energy. Some of the important ones are:
(i) Gravitational Potential Energy: The energy possessed by an object due to its position relative to the Earth. For example, water stored at a height in a dam.
(ii) Elastic Potential Energy: The energy stored in a compressed or stretched spring or elastic material. This energy is due to the deformation of the object.
(iii) Chemical Potential Energy: The energy stored in chemicals, such as in batteries or fuels. It is released during chemical reactions and may be converted into electrical or thermal energy.
(iv) Thermal (Internal) Energy: This is the energy released by burning fossil fuels like coal, oil, or gas through chemical reactions.
(v) Nuclear Energy: The energy stored in the nucleus of an atom. When the nucleus splits (nuclear fission), a large amount of energy is released in the form of heat and radiation.
✅ Types: Gravitational, Elastic, Chemical, Thermal, Nuclear.
11 State the principle of conservation of energy.
Answer:
Energy cannot be created or destroyed. It may be transformed from one form to another, but the total amount of energy never changes.
✅ Energy is conserved (cannot be created or destroyed).
12 Why does some energy appear to be lost during energy transfer?
Answer:
This loss of energy is due to work done against friction of the moving parts in the process. This energy appears as heat and is dissipated in the environment.
✅ Energy is lost as heat due to friction.
13 What is fossil fuel energy?
Answer:
Fossil fuel energy comes out from burning of oil, coal and natural gas. These materials are known as fossil fuels.
✅ Fossil fuels: oil, coal, natural gas.
14 What is hydroelectric generation?
Answer:
Hydroelectric generation is the electricity generated by using the kinetic energy of the falling water.
✅ Electricity from falling water's kinetic energy.
15 What is solar energy? Also write its uses.
Answer:
Solar energy is the energy of the Sunlight that can be converted into electricity.
Uses of Solar Energy: Solar energy can be used in two ways.
(i) Heating System
(ii) Electricity Generation
✅ Solar energy: heating and electricity generation.
16 What is difference between solar cell and solar panel?
Answer:
Solar Cell: Solar cells are also known as photo voltaic cells. The voltage produced by a single voltaic cell is very low.
Solar Panel: In order to get sufficient high voltage for practical use, a large number of such cells are connected in series to form a solar cell panel.
✅ Solar cell: low voltage; Panel: series of cells for high voltage.
17 What is Nuclear Energy?
Answer:
Nuclear energy is the heat energy released when the nucleus of an atom (e.g., uranium or plutonium) splits in a process called nuclear fission.
OR
The energy released by breaking the nucleus of an atom is known as nuclear energy.
✅ Energy from nuclear fission.
18 What is geothermal energy?
Answer:
Geothermal energy is the heat derived from hot, semi-molten rocks deep beneath the Earth's surface. This heat is generated by the decay of radioactive elements and can reach temperatures of \(250^\circ C\) or higher.
OR
Geothermal energy is the heat energy of the hot rocks present deep under the surface of the Earth.
✅ Heat from Earth's interior.
19 What is Wind Energy?
Answer:
Wind energy is the electrical energy produced by using the kinetic energy of the fast-blowing wind.
For thousands of years, people have been using windmills to draw water from the well or to grind grains into flour.
✅ Energy from wind (kinetic energy).
20 What is tidal energy?
Answer:
The gravitational force of the moon gives rise to tides in the seas. The rise and fall of water can be utilized to turn on turbine for electricity generation.
✅ Energy from tides (rise and fall of sea water).
21 What is wave energy?
Answer:
The tides and winds blowing over the surface of the sea produce strong water waves. Their energy can be used to generate electricity.
✅ Energy from sea waves.
22 What is Biomass Energy? Also define Sewage.
Answer:
Biomass Energy: It is that energy which is obtained from the biomass. Biomass consists of organic materials such as plants, waste foods, animals dung, sewage, etc.
OR
Biofuel energy is that energy which is obtained by fermentation of organic materials in the form of biogas or ethanol.
Sewage: Sewage is that dirt which is left over after staining dirty water.
✅ Energy from organic materials (biomass).
23 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
24 What are the advantages of hydroelectric power and the disadvantages of fossil fuel energy?
Answer:
Advantages of Hydroelectric Power: The production of hydroelectric power is more economical and pollution free.
Disadvantages of Fossil Fuel Energy: Power generation by fossil fuels adds to the pollution of environment. Burning of fossil fuels produces smoke, carbon dioxide gas and heat.
✅ Hydro: economical & clean; Fossil: polluting.
25 Define power. Write its formula.
Answer:
Power is defined as the time rate of doing work.
\[ P = \frac{W}{t} \]
It tells us how fast or slow work is done.
OR
Power of any agency can also be defined as energy transferred per unit time.
✅ \(P = W/t\) = rate of doing work.
26 What is SI unit of power? Define it. Also write some bigger units of power.
Answer:
SI unit of power is watt (W).
Watt: One watt is the work done at the rate of one joule per second.
\[ 1\text{ W} = \frac{1\text{ J}}{1\text{ s}} = 1\text{ J s}^{-1} \]
Bigger Units of Power: Bigger units of power are kilowatt (kW), megawatt (MW) etc.
\[ 1\text{ kW} = 10^3\text{ W} = 1000\text{ W} \]
\[ 1\text{ MW} = 10^6\text{ W} = 1000000\text{ W} \]
In British engineering system, the unit of power used is horse-power (hp).
\[ 1\text{ hp} = 746\text{ W} \]
✅ 1 W = 1 J/s; 1 kW = 1000 W; 1 hp = 746 W.
27 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).
28 What is a perpetual energy machine?
Answer:
It is a hypothetical machine that can do work indefinitely, without any external source of energy.
✅ Hypothetical machine, no external energy needed.
29 Why is a perpetual energy machine not possible?
Answer:
In any real mechanical system, some energy is always lost as heat due to friction between moving parts and air resistance. Therefore, it is impossible for a machine to keep moving without an external source of energy.
✅ Energy lost as heat due to friction; external energy needed.
📐 Key Concepts – Work and Energy (Short Answer Questions)
Work: \(W = F \times S\)
Work (at an angle): \(W = FS \cos \theta\)
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 short answer 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