Concepts of Work and Energy in Easy Way
Work:
When a force is applied on a body and the body gets displaced, work is said to be done.
or
Work is done when a force moves an object from one place to another.
Example:
If you push a box and it moves, you are doing work.
In simple words:
Work = Force × Distance
Examples of Work:
- Lifting a school bag from the floor.
- Pulling a toy with a string.
- Dragging a chair from one place to another.
- Kicking a football and it moves.
When no work is done:
If you push a wall and it does not move, no work is done because there is no movement.
Energy:
Energy is the ability to do work.
Various Form of Energy:
1. Mechanical Energy
Mechanical energy is the energy possessed by an object due to its motion or position. It helps objects move or perform work.
Example: A moving bicycle has mechanical energy.
2. Heat Energy
Heat energy is the energy that makes things hot or warm. It flows from a hotter object to a colder object.
Example: A hot cup of tea contains heat energy.
3. Light Energy
Light energy is the energy that enables us to see objects around us. It is produced by sources such as the Sun and electric bulbs.
Example: The Sun gives us light energy during the day.
4. Sound Energy
Sound energy is the energy produced when an object vibrates. It can be heard by our ears.
Example: A ringing bell produces sound energy.
5. Electrical Energy
Electrical energy is the energy produced by the flow of electric current. It is used to operate electrical devices.
Example: A fan works using electrical energy.
6. Chemical Energy
Chemical energy is the t energy stored in food, fuels, and batteries. It is released during chemical reactions.
Example: Food gives us chemical energy to do work.
7. Solar Energy
Solar energy is the energy obtained from the Sun. It is a clean and renewable source of energy.
Example: Solar panels use solar energy to produce electricity.
8. Nuclear Energy
Nuclear energy is the energy released from the nucleus of an atom. It can produce a large amount of power.
Example: Nuclear power plants generate electricity using nuclear energy.
9. Magnetic Energy
Magnetic energy is the energy associated with magnets and magnetic fields. It can attract or repel magnetic materials.
Explanation:
When a magnet pulls iron objects towards itself, it shows the presence of magnetic energy. This energy is used in devices such as electric motors, speakers, and magnetic cranes.
10. Muscular Energy:
Muscular energy is the energy produced by the muscles of our body. It helps us perform different physical activities.
Explanation:
When we walk, run, jump, lift objects, or ride a bicycle, our muscles use energy obtained from food. This energy is called muscular energy.
11. Kinetic Energy
Kinetic energy is the energy possessed by an object due to its motion. Any object that is moving has kinetic energy.
Explanation:
The faster an object moves, the more kinetic energy it has. If an object is at rest, it has no kinetic energy. Kinetic energy helps an object do work while it is moving.
12. Potential Energy
Definition:
Potential energy is the energy possessed by an object due to its position or condition. It is stored energy.
Explanation:
An object kept at a height stores energy because it can do work when it falls. Similarly, a stretched rubber band stores energy due to its stretched position. This stored energy is called potential energy.
Difference Between Kinetic Energy and Potential Energy
| Kinetic Energy | Potential Energy |
| Energy due to motion | Energy due to position or condition |
| Present only when the object is moving | Can be present even when the object is at rest |
| Example: Moving car | Example: Water stored in a dam |
A. Fill in the Blanks
- The SI unit of work is Joule.
- Coal and oil have chemical energy stored in them.
- Atomic reactors utilize nuclear energy.
- In an electric geyser, electrical energy is converted into heat energy.
- A generator converts mechanical energy into electrical energy.
- The energy due to a change in the shape of the object is called elastic potential energy.
- No work is done if there is no displacement.
- The energy of an object due to its motion is called kinetic energy.
- We get heat energy from the burning of fuels like coal, kerosene and petrol.
- Green plants, while preparing food convert solar energy into chemical energy.
- Energy possessed by muscles of human beings and animals is called muscular energy.
- Gravitational potential energy possessed by an object is directly proportional to its height above the surface of the earth.
B. Choose the Correct Option
- The capacity to do work is called:
(b) energy ✅ - The amount of work done depends upon:
(d) both (a) and (b) ✅ - Which correctly represents energy conversion in hydropower plants?
(d) Potential energy → Kinetic energy → Electric energy ✅ - Which one is not true?
(d) Energy can be destroyed ✅ - Which is another form of 1 Joule?
(a) 1 Nm ✅ - Which form of energy does flowing water possess?
(d) Kinetic energy ✅ - A table fan converts electrical energy into:
(d) Mechanical energy ✅
- A ball rolling on a smooth surface possesses:
(a) Kinetic energy ✅ - A stretched rubber band possesses:
(b) Elastic potential energy ✅ - Thermal power plant:
(d) Converts heat energy into electrical energy ✅ - The energy possessed by an object due to its position is called:
(c) Potential energy ✅ - The gravitational potential energy possessed by an object does not depend upon its:
(c) Speed ✅
C. Match the Column-A with Column-B
| Column-A | Column-B |
| 1. Water stored in a dam | (c) Potential energy |
| 2. Vibrating membrane of a drum | (e) Sound energy |
| 3. Energy of human beings | (d) Muscular energy |
| 4. Moving object | (a) Kinetic energy |
| 5. Energy produced on burning of wood | (b) Heat energy |
Answers:
1 → (c)
2 → (e)
3 → (d)
4 → (a)
5 → (b)
D. Write T for True and F for False Statements
- The water of flowing river has potential energy. → F
- Energy transformation occurs in many activities that we do in our day-to-day life. → T
- Work done is independent of the magnitude of the force. → F
- Energy does not occur in various forms. → F
- The SI unit of energy is same as that of work. → T
- An electromagnet converts electrical energy into magnetic energy. → T
- Mechanical energy is produced on burning a fuel. → F
- When a matchstick is rubbed against a matchbox, heat and light are produced at the expense of kinetic energy. → T
- An arrow on a stretched bow possesses elastic potential energy. → T
- An object can work more than the energy it possesses. → F
- Kinetic energy is inversely proportional to the square of speed of that object. → F
- Energy can neither be created nor be destroyed. → T
E. Short Answer Type Questions
1. Define work.
Answer:
When a force is applied on a body and the body gets displaced, work is said to be done.
Example: Pushing a trolley and making it move.
2. What is energy?
Answer:
Energy is the capacity to do work.
Example: A moving bicycle has energy because it can do work.
3. Is it possible that a force is acting on an object but still the work done is zero? Explain giving one example.
Answer:
Yes, it is possible. If the object does not move, the work done is zero.
Example: A person pushing a wall that does not move.
4. How does the kinetic energy of a moving object depend on its (i) speed, and (ii) mass?
Answer:
(i) Kinetic energy increases with speed.
(ii) Kinetic energy also increases with mass.
Example: A truck has more kinetic energy than a bicycle moving at the same speed.
5. Name five appliances or machines which use an electric motor.
Answer:
- Electric fan
- Mixer grinder
- Washing machine
- Water pump
- Vacuum cleaner
6. What is mechanical energy? Give two examples.
Answer:
Mechanical energy is the sum of kinetic energy and potential energy.
Examples:
- Moving car
- Swinging pendulum
7. Write the uses of heat energy.
Answer:
Heat energy is used for:
- Cooking food
- Boiling water
- Ironing clothes
- Keeping us warm
8. What do you mean by the law of conservation of energy?
Answer:
The law of conservation of energy states that energy can neither be created nor destroyed. It can only change from one form to another.
Example: In a bulb, electrical energy changes into light and heat energy.
9. Write the energy transformations which take place in an electric bulb (or electric lamp).
Answer:
Electrical Energy → Light Energy + Heat Energy
Example: When a bulb glows, it gives light and also becomes warm.
10. What are the factors on which the amount of work done depends? How are they related to work?
Answer:
The amount of work done depends on:
- Force applied
- Displacement of the object
Relation:
W=F×d
Where:
- W = Work done
- F = Force
- d = Displacement
11. Differentiate between:
(a) Gravitational Potential Energy and Elastic Potential Energy
| Gravitational Potential Energy | Elastic Potential Energy |
| Energy due to height or position. | Energy due to stretching or compression. |
| Example: Water in a dam. | Example: Stretched rubber band. |
(b) Light Energy and Heat Energy
| Light Energy | Heat Energy |
| Helps us to see objects. | Makes objects hot. |
| Example: Sunlight. | Example: Fire. |
(c) Kinetic Energy and Potential Energy
| Kinetic Energy | Potential Energy |
| Energy due to motion. | Energy due to position or shape. |
| Example: Moving car. | Example: Water stored in a dam. |
F. Long Answer Type Questions
1. Define the term “work”. Write the formula for work done and explain the symbols.
Answer:
Work is done when a force causes an object to move in the direction of the force.
Formula:
W=F×d
Where:
- W = Work done
- F = Force applied
- d = Displacement
Example: If a force of 10 N moves an object by 2 m, then work done = 20 J.
2. Why has water stored in a dam potential energy?
Answer:
Water stored in a dam is at a height above the ground. Therefore, it possesses gravitational potential energy.
Example: When the water falls, it can rotate turbines and produce electricity.
3(a). Define the term “energy” of an object. What is the SI unit of energy?
Answer:
Energy is the capacity of an object to do work.
SI Unit of Energy: Joule (J)
Example: Electrical energy is measured in joules.
3(b). What are the various forms of energy?
Answer:
The various forms of energy are:
- Mechanical energy
- Heat energy
- Light energy
- Sound energy
- Electrical energy
- Chemical energy
- Magnetic energy
- Nuclear energy
- Solar energy
- Muscular energy
3(c). Two bodies having equal masses are moving with speeds v and 2v. Find the ratio of their kinetic energies.
Answer:
Kinetic Energy ∝ Speed²
Ratio = v² : (2v)²
= v² : 4v²
= 1 : 4
Answer: 1 : 4
4. Why does a fast-moving car have more kinetic energy than one moving slowly?
Answer:
A fast-moving car has greater speed. Since kinetic energy depends on speed, the faster car has more kinetic energy.
Example: A racing car has more kinetic energy than a slowly moving car.
5. Explain why no work is done when a man tries to push a wall.
Answer:
The wall does not move even though force is applied. Since there is no displacement, no work is done.
Example: Pushing a strong wall that remains in the same position.
6. What do you understand by the term “transformation of energy”? Explain with an example.
Answer:
Transformation of energy means changing energy from one form to another.
Example: In an electric fan:
Electrical Energy → Mechanical Energy
7. What is conservation of energy? Explain with the help of an example.
Answer:
Conservation of energy means energy can neither be created nor destroyed. It can only change from one form to another.
Example: In a hydroelectric power plant:
Potential Energy → Kinetic Energy → Electrical Energy
The total amount of energy remains the same.
8. Explain how the total energy of a swinging pendulum remains conserved.
Answer:
At the highest position, the pendulum has maximum potential energy and minimum kinetic energy.
As it moves downward, potential energy changes into kinetic energy.
At the lowest position, kinetic energy becomes maximum and potential energy becomes minimum.
As it moves upward again, kinetic energy changes back into potential energy.
Thus, the total energy remains constant throughout the motion.
Example: A swing moving to and fro in a playground.