📘 WORK AND ENERGY – CLASS 9 NOTES
1. Introduction
Work and Energy are fundamental concepts in Physics that explain how objects move and interact. In daily life, we use the word “work” in many ways, but in Physics, it has a specific meaning. Similarly, energy is the ability to do work. Everything around us — moving vehicles, flowing water, burning fuel, even our body — involves work and energy.
Understanding this chapter helps explain how machines function, how objects move, and how energy changes from one form to another.
2. Work
Meaning of Work in Physics
In Physics, work is said to be done when:
- A force is applied on an object, and
- The object moves (displacement occurs).
If there is no displacement, no work is done.
Examples:
- Pushing a table and it moves → Work is done.
- Pushing a wall and it does not move → No work is done.
- Holding a heavy bag without moving → No work is done (in Physics).
2.1 Mathematical Expression for Work
W = F × s
Where:
W = Work
F = Force applied
s = Displacement
SI Unit of Work
- SI unit: Joule (J)
- 1 Joule = Work done when 1 Newton force moves an object by 1 meter.
2.2 Conditions When Work is Zero
Work is zero when:
- Displacement is zero.
- Force is zero.
- Force is perpendicular to displacement.
Example: Carrying a school bag on a horizontal road — force is vertical, displacement is horizontal → Work done by you is zero.
2.3 Positive and Negative Work
Positive Work
When force and displacement are in the same direction.
Example: A person pushing a cart forward.
Negative Work
When force and displacement are in opposite directions.
Example: Friction force acting opposite to motion.
3. Energy
Energy is the capacity to do work.
Without energy, no work can be done. Everything that can do work possesses energy.
Forms of Energy
- Mechanical Energy
- Heat Energy
- Light Energy
- Chemical Energy
- Electrical Energy
- Nuclear Energy
- Sound Energy
4. Mechanical Energy
Mechanical energy is of two types:
- Kinetic Energy
- Potential Energy
5. Kinetic Energy (KE)
Kinetic energy is the energy possessed by an object due to its motion.
Example:
- Moving car
- Running person
- Flying airplane
Formula for Kinetic Energy
KE = (1/2)mv^2
Where:
m = Mass
v = Velocity
Important Points:
- KE increases with velocity.
- If velocity doubles, KE becomes four times.
- Heavier objects have more KE at same speed.
Derivation of Kinetic Energy (Conceptual)
When a force moves an object, work is done. This work is stored as kinetic energy. Using equations of motion and force concepts, we get KE = ½mv².
6. Potential Energy (PE)
Potential energy is the energy possessed by an object due to its position or shape.
Example:
- Water stored in dam
- Stretched rubber band
- Raised stone
Gravitational Potential Energy
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Where:
m = Mass
g = Acceleration due to gravity
h = Height
Important Points:
- PE increases with height.
- Heavier objects have more PE.
- At ground level, PE is considered zero.
7. Law of Conservation of Energy
Energy can neither be created nor destroyed. It can only change from one form to another.
Example:
When a stone falls:
- At top → Maximum PE, Minimum KE
- At bottom → Maximum KE, Minimum PE
Total Energy remains constant.
8. Transformation of Energy
Examples:
- Electric bulb → Electrical → Light + Heat
- Car engine → Chemical → Mechanical
- Hydroelectric plant → Potential → Electrical
- Solar panel → Solar → Electrical
9. Power
Power tells us how fast work is done.
If two people do same work but one does it faster, that person has more power.
Formula for Power
P = W/t
Where:
P = Power
W = Work
t = Time
SI Unit of Power
- Watt (W)
- 1 Watt = 1 Joule per second
1 kilowatt (kW) = 1000 Watts
10. Commercial Unit of Energy
Electrical energy is measured in kilowatt-hour (kWh).
1 kWh = 1 Unit of electricity
1 kWh = 3.6 × 10⁶ Joules
11. Work Done by Gravity
When an object falls:
- Gravity does positive work.
When we lift an object: - We do work against gravity.
12. Work Done by Friction
Friction always opposes motion.
Therefore:
- Friction does negative work.
- It converts mechanical energy into heat.
Example:
- Rubbing hands produces heat.
- Brakes stop vehicles.
13. Energy in Daily Life
Human Body
Food → Chemical Energy → Mechanical Energy
Windmill
Wind Energy → Mechanical → Electrical
Dam
Water PE → Mechanical → Electrical
14. Numerical Applications
Example 1
A 2 kg object is raised to 10 m.
PE = mgh
= 2 × 10 × 10
= 200 J
Example 2
A 5 kg object moving at 4 m/s.
KE = ½ × 5 × 4²
= 2.5 × 16
= 40 J
15. Differences Between KE and PE
| Kinetic Energy | Potential Energy |
|---|---|
| Due to motion | Due to position |
| Depends on velocity | Depends on height |
| Zero at rest | Can exist at rest |
16. Important Graph Concepts
Work-Displacement Graph
Area under force-displacement graph gives work done.
17. Energy Resources
Renewable:
- Solar
- Wind
- Hydropower
Non-renewable:
- Coal
- Petroleum
- Natural gas
18. Work Done in Different Situations
- Lifting load → Positive work
- Throwing ball upward → Negative work by gravity
- Falling ball → Positive work by gravity
- Holding object stationary → Zero work
19. Mechanical Energy = KE + PE
At any point:
Total Energy = KE + PE
If PE decreases, KE increases.
20. Importance of Work and Energy
- Helps understand machines.
- Important in transportation.
- Essential in electricity generation.
- Basis of all engineering fields.
21. Sample Concept Questions
- Why is no work done in circular motion?
Because displacement is perpendicular to force. - Why do heavy vehicles need more fuel?
They require more energy to move due to larger mass. - Why does a stretched rubber band possess energy?
Due to change in shape (elastic potential energy).
22. Real-Life Applications
- Roller coaster (Energy transformation)
- Pendulum motion
- Hydroelectric power plants
- Wind turbines
- Electric motors
23. Summary
- Work = Force × Displacement
- Unit of Work = Joule
- Energy = Capacity to do work
- KE = ½mv²
- PE = mgh
- Power = Work/Time
- Energy is conserved
Work and Energy form the foundation of physics and help explain motion, machines, electricity, and natural processes.
✅ CONCLUSION
The chapter “Work and Energy” is one of the most important chapters in Class 9 Physics. It connects motion, force, and energy. Understanding this topic helps in solving numerical problems and understanding real-life phenomena.
Energy is everywhere — in moving objects, stored fuels, sunlight, flowing water, and even inside our bodies. The law of conservation of energy shows that energy is never lost but only changes its form.
Thus, the study of work and energy builds the base for higher classes and advanced scientific learning.
✅ WORK AND ENERGY – MCQS
1. Work is done when:
A) Force is applied
B) Object moves
C) Force causes displacement
D) Object is heavy
Answer: C
2. SI unit of work is:
A) Newton
B) Watt
C) Joule
D) Pascal
Answer: C
3. 1 Joule equals:
A) 1 N × 1 m
B) 1 N ÷ 1 m
C) 1 kg × 1 m
D) 1 W × 1 s
Answer: A
4. If displacement is zero, work done is:
A) Positive
B) Negative
C) Zero
D) Infinite
Answer: C
5. Work done by friction is:
A) Positive
B) Negative
C) Zero
D) Infinite
Answer: B
6. Kinetic energy depends on:
A) Height
B) Mass and velocity
C) Force only
D) Time
Answer: B
7. Formula of kinetic energy is:
A) mgh
B) F × s
C) ½mv²
D) W/t
Answer: C
8. Potential energy depends on:
A) Speed
B) Height
C) Color
D) Shape
Answer: B
9. Formula of potential energy is:
A) ½mv²
B) mgh
C) W/t
D) F/m
Answer: B
10. Energy is:
A) Force
B) Power
C) Capacity to do work
D) Speed
Answer: C
11. SI unit of energy:
A) Joule
B) Newton
C) Watt
D) Pascal
Answer: A
12. Power is:
A) Work
B) Energy
C) Work done per unit time
D) Force
Answer: C
13. SI unit of power:
A) Joule
B) Newton
C) Watt
D) Volt
Answer: C
14. 1 kW equals:
A) 10 W
B) 100 W
C) 1000 W
D) 10000 W
Answer: C
15. Commercial unit of electrical energy:
A) Joule
B) kWh
C) Watt
D) Volt
Answer: B
16. 1 kWh equals:
A) 3.6 × 10⁶ J
B) 36 J
C) 360 J
D) 3.6 J
Answer: A
17. A moving car has:
A) Potential energy
B) Kinetic energy
C) No energy
D) Heat energy only
Answer: B
18. A raised stone has:
A) KE
B) PE
C) No energy
D) Sound energy
Answer: B
19. Law of conservation of energy states:
A) Energy can be created
B) Energy can be destroyed
C) Energy changes form
D) Energy disappears
Answer: C
20. When a ball falls:
A) PE increases
B) KE decreases
C) KE increases
D) Energy vanishes
Answer: C
21. At highest point, falling object has:
A) Maximum KE
B) Maximum PE
C) Zero energy
D) Infinite energy
Answer: B
22. At ground level:
A) PE maximum
B) KE minimum
C) PE minimum
D) Energy zero
Answer: C
23. If velocity doubles, KE becomes:
A) Double
B) Half
C) Four times
D) Same
Answer: C
24. Work done against gravity is:
A) Positive
B) Negative
C) Zero
D) Infinite
Answer: A
25. Friction converts mechanical energy into:
A) Light
B) Heat
C) Sound only
D) Electrical
Answer: B
26. Work is scalar quantity.
A) True
B) False
C) Sometimes
D) None
Answer: A
27. Energy is scalar quantity.
A) True
B) False
C) Vector
D) None
Answer: A
28. Power depends on:
A) Work and time
B) Mass
C) Height
D) Gravity
Answer: A
29. Heavy vehicles require more:
A) Speed
B) Energy
C) Height
D) Time
Answer: B
30. Stretched rubber band has:
A) KE
B) Gravitational PE
C) Elastic PE
D) No energy
Answer: C
- Work done in circular motion is: C) Zero
- A stationary object has KE: C) Zero
- A dam stores water with: B) PE
- Electric bulb converts electrical to: B) Light + Heat
- Windmill converts wind energy to: C) Electrical
- Food provides: C) Chemical energy
- Joule is named after: A) Scientist
- Work = Force ×: C) Displacement
- Unit of force: A) Newton
- Weight is a: C) Force
- More height means more: B) PE
- Faster object means more: A) KE
- Powerful machine works in: B) Less time
- Energy cannot be: C) Created or destroyed
- Moving air has: B) Wind energy
- Solar panel converts: B) Solar to electrical
- Lifting object increases: A) PE
- Falling object loses: B) PE
- KE at rest is: C) Zero
- Total mechanical energy = KE + PE: A) True
- Unit of KE: A) Joule
- Unit of PE: A) Joule
- Unit of Power: C) Watt
- Friction does: B) Negative work
- Gravity during fall does: A) Positive work
- Energy stored in battery: C) Chemical
- Energy in moving train: B) KE
- Work done if force perpendicular: C) Zero
- Human body converts food into: B) Mechanical energy
- ½mv² depends on: B) Velocity square
- Increasing mass increases: B) KE
- Dam stores energy as: B) PE
- Electric heater converts: B) Electrical to heat
- Power = Energy /: C) Time
- Watt is Joule per: B) Second
- 1000 W = 1: C) kW
- Total energy remains: A) Constant
- Work done is zero if object: C) Does not move
- Object at height has: B) PE
- Falling body converts PE to: B) KE
- Kinetic energy always: B) Positive
- Potential energy at ground: C) Zero
- Unit of commercial energy: B) kWh
- Mechanical energy includes: D) KE + PE
- Energy of flowing water: B) KE
- More speed means more: A) KE
- Work is done when: C) Displacement occurs
- Force without motion: C) No work
- Gravity is a: C) Force
- Energy stored in stretched spring: C) Elastic PE
- Energy change is called: B) Transformation
- Generator converts mechanical to: C) Electrical
- SI unit of energy named after: A) Joule
- Heat is a form of: B) Energy
- Sound energy travels through: B) Medium
- Light energy comes from: C) Sun
- Burning fuel produces: C) Chemical energy
- Power tells rate of: B) Doing work
- Energy possessed due to motion: A) KE
- Energy due to position: B) PE
Final 10
- Energy stored in raised hammer: B) PE
- Energy of moving bullet: A) KE
- Joule is equal to: A) N×m
- Faster work means more: B) Power
- Conservation law applies to: C) Energy
- Total mechanical energy remains: A) Constant
- SI unit of displacement: B) Meter
- Work done increases with: A) Force
- Electric energy measured in: B) kWh
- Energy makes work: C) Possible
✅ WORK AND ENERGY – VERY SHORT ANSWER QUESTIONS
1. What is work in Physics?
Work is done when force causes displacement.
2. What is the SI unit of work?
Joule (J).
3. Define 1 Joule.
Work done by 1 N force moving object 1 m.
4. What is energy?
Capacity to do work.
5. SI unit of energy?
Joule (J).
6. What is power?
Rate of doing work.
7. SI unit of power?
Watt (W).
8. Formula of work?
W = F × s.
9. Formula of kinetic energy?
KE = ½mv².
10. Formula of potential energy?
PE = mgh.
11. What is kinetic energy?
Energy due to motion.
12. What is potential energy?
Energy due to position.
13. When is work zero?
When displacement is zero.
14. What kind of quantity is work?
Scalar quantity.
15. What kind of quantity is energy?
Scalar quantity.
16. What kind of quantity is power?
Scalar quantity.
17. Commercial unit of electricity?
Kilowatt-hour (kWh).
18. 1 kWh equals how many joules?
3.6 × 10⁶ J.
19. What is mechanical energy?
Sum of KE and PE.
20. State conservation of energy.
Energy cannot be created or destroyed.
21. What happens to PE when object falls?
It decreases.
22. What happens to KE when object falls?
It increases.
23. Work done by friction is?
Negative work.
24. Work done by gravity during fall?
Positive work.
25. Unit of force?
Newton (N).
26. Does a stationary object have KE?
No.
27. Does a raised object have energy?
Yes, potential energy.
28. What energy does flowing water have?
Kinetic energy.
29. What energy is stored in dam water?
Potential energy.
30. What energy does a moving car have?
Kinetic energy.
31. What is elastic potential energy?
Energy stored due to shape change.
32. Example of elastic energy?
Stretched rubber band.
33. What is 1 watt?
1 joule per second.
34. Formula of power?
P = W/t.
35. What is displacement?
Shortest distance in specific direction.
36. If force is perpendicular to motion, work is?
Zero.
37. What energy does food contain?
Chemical energy.
38. What energy does battery store?
Chemical energy.
39. What energy does bulb produce?
Light and heat energy.
40. What is total mechanical energy?
KE + PE.
41. If velocity doubles, KE becomes?
Four times.
42. If height increases, PE?
Increases.
43. Who discovered Joule?
James Prescott Joule.
44. Work depends on what two factors?
Force and displacement.
45. What energy does wind possess?
Kinetic energy.
46. What is gravitational potential energy?
Energy due to height.
47. Energy change is called?
Energy transformation.
48. Unit of height?
Meter.
49. Unit of mass?
Kilogram.
50. Unit of time?
Second.
51. What energy does sun provide?
Solar energy.
52. What energy does heater produce?
Heat energy.
53. What energy is used in vehicles?
Chemical energy.
54. Work done increases with?
Force.
55. Work done increases with?
Displacement.
56. What is g value on Earth?
9.8 m/s² (approx. 10 m/s²).
57. Energy of flying bird?
Kinetic energy.
58. What is negative work?
Force opposite to motion.
59. What is positive work?
Force in direction of motion.
60. What energy does stretched spring have?
Elastic potential energy.
61. What energy does generator produce?
Electrical energy.
62. What energy does motor convert?
Electrical to mechanical.
63. Is energy conserved?
Yes.
64. What happens to total energy in fall?
Remains constant.
65. Does friction produce heat?
Yes.
66. What is 1000 watts called?
1 kilowatt.
67. What is 1 kW?
1000 watts.
68. Does heavy object have more PE?
Yes.
69. Does faster object have more KE?
Yes.
70. Can energy be destroyed?
No.
71. Is work vector or scalar?
Scalar.
72. Is power vector or scalar?
Scalar.
73. Energy stored at height?
Potential energy.
74. Energy of running man?
Kinetic energy.
75. Energy of compressed spring?
Elastic potential energy.
76. Energy in coal?
Chemical energy.
77. What is mechanical energy?
KE + PE.
78. Work done by lifting load?
Positive.
79. Work done by gravity upward?
Negative.
80. Work done by gravity downward?
Positive.
81. Unit of mechanical energy?
Joule.
82. Does holding bag do work?
No (in Physics).
83. What energy does fan use?
Electrical energy.
84. What energy does fan produce?
Mechanical energy.
85. What energy does hydropower plant use?
Potential energy of water.
86. What is rate of energy transfer?
Power.
87. What is SI unit of displacement?
Meter.
88. What is SI unit of velocity?
m/s.
89. What is SI unit of acceleration?
m/s².
90. What is SI unit of mass?
Kilogram.
91. What is work done if object doesn’t move?
Zero.
92. What energy does fire produce?
Heat and light energy.
93. What energy does moving train have?
Kinetic energy.
94. Energy stored in raised hammer?
Potential energy.
95. What energy does falling stone gain?
Kinetic energy.
96. Does KE depend on mass?
Yes.
97. Does PE depend on height?
Yes.
98. What is energy measured in?
Joule.
99. What is power measured in?
Watt.
100. Why is energy important?
It makes work possible.
✅ WORK AND ENERGY – SHORT ANSWER QUESTIONS
1. Define work in Physics.
In Physics, work is said to be done when a force applied on an object causes displacement in the direction of the force. If there is no displacement, no work is done. Both force and displacement are necessary conditions for work.
2. What are the conditions for work to be done?
Two conditions must be satisfied: first, a force must act on the object; second, the object must move in the direction of the force. If either force or displacement is absent, work is zero.
3. Define 1 Joule.
One joule is the work done when a force of one newton displaces an object by one meter in the direction of the force. It is the SI unit of work and energy.
4. Define energy.
Energy is the capacity or ability to do work. Any object that can do work possesses energy. Energy exists in various forms such as kinetic, potential, heat, light, and chemical energy.
5. What is kinetic energy?
Kinetic energy is the energy possessed by an object due to its motion. The faster the object moves or the heavier it is, the more kinetic energy it has. It is calculated using the formula KE = ½mv².
6. What is potential energy?
Potential energy is the energy possessed by an object due to its position or configuration. For example, a stone kept at a height has gravitational potential energy. It depends on mass, height, and gravity.
7. State the formula of kinetic energy and explain its terms.
The formula for kinetic energy is KE = ½mv², where m is mass and v is velocity. It shows that kinetic energy increases with mass and the square of velocity.
8. State the formula of potential energy and explain its terms.
The formula for gravitational potential energy is PE = mgh, where m is mass, g is acceleration due to gravity, and h is height. It shows that potential energy increases with height and mass.
9. What is mechanical energy?
Mechanical energy is the sum of kinetic energy and potential energy possessed by an object. It represents the total energy due to motion and position. Mechanical energy remains constant in absence of external forces.
10. State the law of conservation of energy.
The law states that energy can neither be created nor destroyed. It can only change from one form to another. The total energy of an isolated system always remains constant.
11. What is power?
Power is the rate at which work is done or energy is transferred. It measures how fast work is completed. The formula of power is P = W/t.
12. Define 1 watt.
One watt is the power of a device that does one joule of work in one second. It is the SI unit of power.
13. What is commercial unit of energy?
The commercial unit of electrical energy is kilowatt-hour (kWh). It is the energy consumed when a 1000-watt appliance runs for one hour.
14. What is negative work?
Negative work is done when the force acts opposite to the direction of displacement. For example, friction always does negative work as it opposes motion.
15. What is positive work?
Positive work is done when force and displacement are in the same direction. For example, pushing a cart forward involves positive work.
16. When is work zero?
Work is zero when displacement is zero, even if force is applied. Also, if force acts perpendicular to displacement, work done is zero.
17. Why does friction produce heat?
Friction opposes motion and converts mechanical energy into heat energy. This conversion increases temperature and reduces useful mechanical energy.
18. Explain energy transformation with example.
Energy transformation is the change of energy from one form to another. For example, in an electric bulb, electrical energy converts into light and heat energy.
19. Why does a falling object gain speed?
As an object falls, its potential energy decreases and converts into kinetic energy. This increase in kinetic energy increases its speed.
20. Why do heavy vehicles require more fuel?
Heavy vehicles have more mass and require more energy to move. Greater energy is needed to overcome inertia and friction, so they consume more fuel.
(Continuing in same format…)
21. What happens to energy when a ball is thrown upward?
When a ball is thrown upward, its kinetic energy gradually decreases while its potential energy increases. At the highest point, kinetic energy becomes zero and potential energy is maximum.
22. Why does a stretched rubber band possess energy?
A stretched rubber band stores energy due to change in shape. This stored energy is called elastic potential energy and can perform work when released.
23. Explain work done by gravity.
When an object falls downward, gravity does positive work because force and displacement are in same direction. When lifting upward, work done against gravity is positive by the person.
24. What is total mechanical energy of falling body?
The total mechanical energy (KE + PE) of a freely falling body remains constant if air resistance is neglected.
25. Why is work a scalar quantity?
Work has only magnitude and no direction. It is calculated as the product of force and displacement without considering direction, so it is scalar.
26. Why is energy a scalar quantity?
Energy has magnitude but no direction. It does not depend on direction; hence it is scalar.
27. Explain the relation between velocity and kinetic energy.
Kinetic energy is proportional to the square of velocity. If velocity doubles, kinetic energy becomes four times.
28. What is elastic potential energy?
Elastic potential energy is the energy stored in an object when it is stretched or compressed. Example: compressed spring.
29. What happens to potential energy at ground level?
At ground level, potential energy is considered zero because height is zero.
30. Explain the role of power in machines.
Power helps determine how fast a machine can do work. Machines with higher power can perform the same task in less
31. What is renewable energy?
Renewable energy comes from sources that are naturally replenished, such as sunlight, wind, and water. These sources do not get exhausted easily and are environmentally friendly.
32. What is non-renewable energy?
Non-renewable energy comes from sources like coal, petroleum, and natural gas. These resources are limited and take millions of years to form.
33. How does a hydroelectric plant generate electricity?
Water stored at height has potential energy. When it flows down, it turns turbines converting potential energy into mechanical energy and then electrical energy.
34. Why does a moving train have energy?
A moving train possesses kinetic energy due to its motion. The greater its mass and speed, the greater its kinetic energy.
35. Explain conservation of mechanical energy.
In absence of friction, the sum of kinetic and potential energy of a system remains constant during motion.
- Work depends on force and displacement.
- Energy is required to perform any activity.
- Wind energy is kinetic energy of moving air.
- Solar energy comes from the Sun.
- Electrical energy powers appliances.
- Falling water converts PE to KE.
- A raised hammer stores PE.
- Friction reduces mechanical energy.
- KE is zero at rest.
- PE increases with height.
- Power measures speed of work.
- 1 kW = 1000 W.
- Holding a bag does no work in Physics.
- Work done can be negative.
- Mechanical energy = KE + PE.
- Energy cannot be destroyed.
- Heat is a form of energy.
- Sound energy travels through medium.
- Light energy enables vision.
- Burning fuel releases chemical energy.
- KE depends on mass.
- PE depends on height.
- Energy transfer occurs in machines.
- Generator converts mechanical to electrical.
- Motor converts electrical to mechanical.
- More mass means more PE.
- More speed means more KE.
- Work done increases with force.
- Work done increases with displacement.
- Friction does negative work.
- Gravity does positive work in fall.
- Power is work per time.
- Joule is unit of energy.
- Watt is unit of power.
- kWh is commercial unit.
- Total energy remains constant.
- A spring stores elastic energy.
- Dam stores gravitational PE.
- Moving water has KE.
- Work requires motion.
- Energy makes work possible.
- Velocity squared affects KE.
- Height affects PE.
- Machines increase efficiency.
- Energy changes form constantly.
- Work done against gravity increases PE.
- Free fall converts PE to KE.
- Force without motion means no work.
- Electric heater converts electrical to heat.
- Solar panels convert solar to electrical.
- Fuel contains chemical energy.
- Heavy objects need more energy.
- Fast objects have more KE.
- Energy is measured in joules.
- Power is measured in watts.
- Energy is conserved in nature.
- KE is always positive.
- PE at ground is zero.
- Total mechanical energy constant without friction.
- Friction produces heat.
- Energy is needed for life processes.
- Windmill uses wind energy.
- Falling stone gains KE.
- Raised object has PE.
- Work and energy explain motion in physics.
✅ WORK AND ENERGY – LONG ANSWER QUESTIONS
1. Define work and explain its conditions with examples.
Work in Physics is said to be done when a force applied on an object causes displacement in the direction of the force. Two essential conditions must be satisfied: first, a force must act on the object; second, the object must move due to that force. If either condition is missing, no work is done. For example, pushing a wall does not result in work if the wall does not move. Similarly, holding a heavy bag while standing still does no work in Physics because there is no displacement. However, pushing a cart and moving it forward involves work because force and displacement are in the same direction. Work is calculated as the product of force and displacement, and its SI unit is joule.
2. Derive the expression for kinetic energy.
Kinetic energy is the energy possessed by an object due to its motion. To derive its expression, consider an object of mass “m” moving with velocity “v”. When a force is applied, work is done on the object. According to Newton’s second law, force equals mass multiplied by acceleration. Using equations of motion, we relate velocity and displacement. When the work done by the force equals force multiplied by displacement, and substituting the acceleration terms, we obtain the formula KE = ½mv². This shows that kinetic energy depends on mass and the square of velocity. If velocity doubles, kinetic energy becomes four times. Thus, heavier and faster objects possess greater kinetic energy.
3. Explain potential energy and its types.
Potential energy is the energy possessed by an object due to its position or configuration. The most common type is gravitational potential energy, which depends on height. A stone kept at a height has potential energy because it can fall and do work. Its formula is PE = mgh. Another type is elastic potential energy, stored in objects that are stretched or compressed, such as springs and rubber bands. Potential energy increases with height and mass. At ground level, it is considered zero. Potential energy plays an important role in dams, roller coasters, and many natural phenomena. It transforms into kinetic energy when the object moves.
4. State and explain the law of conservation of energy with example.
The law of conservation of energy states that energy can neither be created nor destroyed; it can only change from one form to another. The total energy of an isolated system always remains constant. For example, when a ball is thrown upward, it has maximum kinetic energy at the bottom. As it rises, kinetic energy decreases while potential energy increases. At the highest point, kinetic energy becomes zero and potential energy becomes maximum. When the ball falls back, potential energy converts into kinetic energy again. Throughout the motion, the total mechanical energy remains constant, proving the conservation law.
5. What is power? Explain its importance and unit.
Power is the rate at which work is done or energy is transferred. It tells us how quickly a task is completed. The formula of power is P = W/t, where W is work and t is time. The SI unit of power is watt. One watt is defined as the power of a device that does one joule of work in one second. Power is important because it helps compare machines. For example, two machines may do the same work, but the one that completes it faster has greater power. Electrical appliances like bulbs, fans, and heaters are rated in watts to indicate their power consumption.
6. Differentiate between kinetic energy and potential energy.
Kinetic energy is the energy possessed by an object due to its motion, while potential energy is the energy possessed due to its position or configuration. Kinetic energy depends on mass and velocity and is given by ½mv². Potential energy depends on mass, gravity, and height and is given by mgh. A moving car has kinetic energy, whereas a car parked on a hill has potential energy. Kinetic energy becomes zero when the object stops, but potential energy can exist even when the object is at rest. Both together form mechanical energy.
7. Explain mechanical energy and its conservation.
Mechanical energy is the sum of kinetic and potential energy of an object. It represents the total energy due to motion and position. During motion under gravity, potential energy converts into kinetic energy and vice versa. For example, in a freely falling object, potential energy decreases while kinetic energy increases. However, in the absence of friction or air resistance, the total mechanical energy remains constant. This principle is known as conservation of mechanical energy. It is widely used to study the motion of objects like pendulums, roller coasters, and satellites.
8. Explain work done by gravity in different situations.
Gravity exerts force on objects near Earth. When an object falls downward, gravity and displacement are in the same direction, so gravity does positive work. As a result, the object gains kinetic energy. When an object is lifted upward, gravity acts downward while displacement is upward. Hence, gravity does negative work. In this case, work is done against gravity to increase potential energy. Thus, gravity plays a major role in energy transformation between kinetic and potential energy.
9. What is commercial unit of energy? Why is it used?
The commercial unit of electrical energy is kilowatt-hour (kWh). One kilowatt-hour is the energy consumed when a 1000-watt appliance works for one hour. It equals 3.6 × 10⁶ joules. Since joule is a very small unit, it is not convenient for measuring large amounts of electrical energy used in homes and industries. Therefore, electricity bills are calculated in kilowatt-hours. This unit makes it easier to measure and calculate energy consumption over time.
10. Explain transformation of energy with daily life examples.
Energy transformation refers to the conversion of energy from one form to another. In daily life, many examples can be observed. An electric bulb converts electrical energy into light and heat energy. A car engine converts chemical energy of fuel into mechanical energy. A hydroelectric plant converts potential energy of water into electrical energy. In the human body, chemical energy from food converts into mechanical energy for movement. These examples show that energy constantly changes form but the total energy remains conserved.
11. Explain negative work with examples.
Negative work is done when the direction of force is opposite to the direction of displacement. A common example is friction, which always opposes motion. When a box slides on the floor, friction acts opposite to its movement and slows it down. Thus, friction does negative work and reduces kinetic energy. Another example is gravity when a ball is thrown upward. Gravity acts downward while displacement is upward, so gravity does negative work. Negative work reduces the energy of the object and can convert mechanical energy into heat.
12. Why is energy important in daily life?
Energy is essential for performing all activities. It powers machines, vehicles, appliances, and industries. Human life depends on energy obtained from food. Electricity provides light, heating, and cooling. Energy drives transportation systems and communication devices. Without energy, no work can be done. Modern civilization relies heavily on various forms of energy such as electrical, chemical, and solar energy. Therefore, efficient use and conservation of energy are very important for sustainable development.
13. Explain elastic potential energy.
Elastic potential energy is the energy stored in an object when it is stretched or compressed. When we stretch a rubber band or compress a spring, work is done on it. This work is stored as elastic potential energy. When released, the object returns to its original shape and the stored energy converts into kinetic energy. Elastic potential energy depends on the amount of stretch or compression. It is widely used in toys, bows and arrows, shock absorbers, and mechanical devices.
14. Explain work done when force is perpendicular to displacement.
When force is applied perpendicular to the direction of displacement, work done is zero. This is because work depends on the component of force in the direction of displacement. For example, when a person carries a bag horizontally, the force applied upward balances gravity, but displacement is horizontal. Since the force and displacement are perpendicular, no work is done in Physics. Similarly, in circular motion, centripetal force is perpendicular to motion and does no work.
15. Why does kinetic energy depend on square of velocity?
Kinetic energy is proportional to the square of velocity, as shown by the formula KE = ½mv². This means that if velocity doubles, kinetic energy becomes four times. This happens because work done in accelerating an object depends on velocity change, and mathematical derivation shows velocity squared term. This explains why high-speed vehicles have much greater energy and can cause more damage during collisions.
16. Explain work-energy theorem.
The work-energy theorem states that the work done on an object is equal to the change in its kinetic energy. When a force accelerates an object, the work done increases its kinetic energy. If negative work is done, kinetic energy decreases. This theorem connects force, motion, and energy concepts and helps solve many physics problems.
17. Explain energy conversion in hydroelectric power plant.
In a hydroelectric plant, water stored at height has gravitational potential energy. When released, water flows downward, converting potential energy into kinetic energy. This moving water rotates turbines, converting kinetic energy into mechanical energy. The generator then converts mechanical energy into electrical energy. Thus, energy changes from potential to kinetic to mechanical and finally to electrical form.
18. Explain relation between mass and energy in motion.
Kinetic energy depends directly on mass. For the same velocity, an object with greater mass has greater kinetic energy. For example, a truck moving at a certain speed has much more kinetic energy than a bicycle at the same speed. Therefore, heavier vehicles require more energy to start and stop. Similarly, potential energy also increases with mass.
19. Describe the role of friction in energy loss.
Friction opposes motion and converts mechanical energy into heat energy. This reduces the useful energy available for work. For example, brakes use friction to stop vehicles by converting kinetic energy into heat. Although friction is useful in some cases, it often leads to energy loss in machines. Therefore, lubricants are used to reduce friction and save energy.
20. Summarize the importance of work and energy in physics.
Work and energy are fundamental concepts in Physics. They help explain motion, machine operation, and natural phenomena. Energy transformation occurs in all physical processes. The law of conservation of energy ensures that total energy remains constant. Understanding work and energy is essential for solving numerical problems and studying advanced topics in Physics. These concepts form the foundation for engineering, electricity generation, transportation, and daily life applications.
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