Revision Term 1 Tg Science Class 8

# Revision Term 1: TG Science Class 8

Welcome to this comprehensive review of the key scientific principles covered in the first term of your Class 8 science curriculum. This guide is designed to reinforce your understanding of fundamental concepts in physics, chemistry, and biology, ensuring you are well-prepared for your examinations and future scientific studies. We will delve into the fascinating worlds of forces and pressure, explore the properties of materials, and unravel the mysteries of sound and light. Each section is crafted to provide a thorough review, complete with detailed explanations, practical examples, and links to external resources for further exploration.

## Unit 1: Forces and Pressure

This unit introduces the fundamental concepts of force and pressure, which are essential for understanding how objects interact with each other and their environment. We will explore the different types of forces, their effects, and how they are measured. We will also examine the concept of pressure and its applications in our daily lives. A key focus will be on the force of friction, its various forms, and its significance in both enabling and resisting motion.

### Understanding Forces

A force is a push or a pull that can cause an object to change its state of motion or shape. Forces are all around us, governing everything from the orbit of planets to the movement of a bicycle. To fully grasp the concept of force, it is important to understand its various types and effects.

#### Types of Forces

Forces can be broadly classified into two categories: **contact forces** and **non-contact forces**.

– **Contact forces** are those that act between two objects that are in direct physical contact with each other. Examples include:
– **Frictional Force**: The force that opposes motion between two surfaces in contact.
– **Normal Force**: The support force exerted by a surface on an object resting on it.
– **Tension Force**: The force transmitted through a string, rope, or wire when it is pulled tight by forces acting from opposite ends.
– **Applied Force**: A force that is applied to an object by a person or another object.

– **Non-contact forces** are those that act between two objects that are not in direct physical contact. Examples include:
– **Gravitational Force**: The force of attraction between any two objects with mass. This is the force that keeps us on the ground and governs the motion of celestial bodies. [1]
– **Electrostatic Force**: The force of attraction or repulsion between electrically charged objects.
– **Magnetic Force**: The force exerted by magnets on each other and on certain metals.

#### Effects of Forces

A force can have several effects on an object:

– It can change the state of motion of an object, i.e., make a stationary object move or stop a moving object.
– It can change the speed of a moving object.
– It can change the direction of motion of an object.
– It can change the shape or size of an object.

To learn more about the fundamental concepts of forces, you can watch this informative video:

**Video: Forces and their types** – [Watch on YouTube](https://www.youtube.com/watch?v=M8RFL2yyWAI)

### Pressure

Pressure is defined as the force acting perpendicularly on a unit area of a surface. It is calculated using the formula:

> Pressure = Force / Area

The SI unit of pressure is the Pascal (Pa), which is equal to one Newton per square meter (N/m²). Pressure is a scalar quantity, meaning it has magnitude but no direction.

#### Applications of Pressure in Daily Life

The concept of pressure has numerous applications in our daily lives:

– **Sharp Objects**: A sharp knife has a very small surface area at its cutting edge, which allows it to exert a large pressure with a small force, making it easy to cut through objects.
– **Wide Straps**: School bags have wide straps to distribute the weight of the bag over a larger area, reducing the pressure on the shoulders.
– **Building Foundations**: The foundations of buildings are made wide to reduce the pressure on the ground, preventing the building from sinking.
– **Hydraulic Systems**: Hydraulic systems, such as those used in car brakes and lifts, use the principle of pressure transmission in liquids (Pascal’s principle) to multiply force.

For a deeper understanding of pressure, watch this video:

**Video: What is Pressure?** – [Watch on YouTube](https://www.youtube.com/watch?v=gzODOJ-xzGM)

### Friction

Friction is a contact force that opposes the relative motion or tendency of relative motion between two surfaces in contact. It is a necessary evil in our lives, as it has both advantages and disadvantages.

#### Types of Friction

There are four main types of friction:

1. **Static Friction**: The frictional force that exists between two surfaces when there is no relative motion between them. It is a self-adjusting force that increases with the applied force until it reaches a maximum value, known as the limiting friction.
2. **Sliding Friction (Kinetic Friction)**: The frictional force that exists between two surfaces when they are in relative motion. Sliding friction is generally less than static friction.
3. **Rolling Friction**: The frictional force that exists when one object rolls over the surface of another object. Rolling friction is much smaller than sliding friction, which is why it is easier to roll an object than to slide it.
4. **Fluid Friction (Drag)**: The frictional force exerted by fluids (liquids and gases) on objects moving through them. The magnitude of fluid friction depends on the speed of the object, its shape, and the nature of the fluid. [2]

#### Factors Affecting Friction

The force of friction depends on two main factors:

1. **The nature of the surfaces in contact**: Rough surfaces have more irregularities, which interlock with each other, resulting in greater friction. Smooth surfaces have fewer irregularities and therefore less friction.
2. **The normal force pressing the surfaces together**: The greater the normal force, the greater the friction. This is why it is harder to push a heavy box than a light one.

#### Advantages and Disadvantages of Friction

**Advantages:**

– Friction enables us to walk, run, and write.
– It helps in the transmission of power in machines through belts and gears.
– Brakes of vehicles work due to friction.
– It allows us to light a matchstick.

**Disadvantages:**

– Friction opposes motion, which leads to a wastage of energy.
– It causes wear and tear of moving parts of machinery.
– It produces heat, which can damage machinery.

To explore the concept of friction in more detail, watch this video:

**Video: Friction and its types** – [Watch on YouTube](https://www.youtube.com/watch?v=x9duHoieGHk)

## Unit 2: Materials

In this unit, we will explore the fascinating world of materials, focusing on synthetic fibers, plastics, metals, and non-metals. Understanding the properties and uses of different materials is crucial in our technologically advanced society. We will examine how these materials are made, their impact on the environment, and their applications in our daily lives.

### Synthetic Fibers and Plastics

Synthetic fibers and plastics are polymers, which are large molecules made up of repeating smaller units called monomers. These materials have become an integral part of our lives due to their versatility, durability, and low cost.

#### Synthetic Fibers

Synthetic fibers are man-made fibers produced from chemical substances. They have several advantages over natural fibers, such as being stronger, more durable, and more resistant to wrinkles and moths. Some common synthetic fibers include:

– **Rayon**: Often called artificial silk, rayon is made from cellulose obtained from wood pulp. It is soft, comfortable, and absorbent, but it is not as strong as other synthetic fibers.
– **Nylon**: The first fully synthetic fiber, nylon is known for its strength, elasticity, and abrasion resistance. It is used in a wide range of products, including clothing, ropes, and carpets.
– **Polyester**: A very popular synthetic fiber, polyester is strong, wrinkle-resistant, and quick-drying. It is often blended with natural fibers like cotton to improve their properties.
– **Acrylic**: Known for its wool-like feel, acrylic is soft, warm, and lightweight. It is resistant to sunlight and chemicals, making it suitable for outdoor use.

#### Plastics

Plastics are synthetic polymers that can be molded into various shapes. They are classified into two main types:

– **Thermoplastics**: These plastics can be repeatedly softened by heating and hardened by cooling. Examples include polyethylene (used in plastic bags), PVC (used in pipes), and PET (used in bottles).
– **Thermosetting Plastics**: These plastics, once molded, cannot be softened by heating. They are hard and rigid. Examples include Bakelite (used in electrical switches) and melamine (used in dinnerware).

#### Plastics and the Environment

While plastics are incredibly useful, their widespread use has led to significant environmental problems. Most plastics are non-biodegradable, meaning they do not decompose naturally. This leads to the accumulation of plastic waste in landfills and oceans, harming wildlife and ecosystems. [3] It is crucial to reduce, reuse, and recycle plastics to mitigate their environmental impact.

To learn more about synthetic fibers and plastics, watch this video:

**Video: Synthetic Fibers and Plastics** – [Watch on YouTube](https://www.youtube.com/watch?v=KsnxeJQ9aio)

### Metals and Non-Metals

Elements can be broadly classified into metals and non-metals based on their physical and chemical properties. Understanding these properties helps us to choose the right material for a specific application.

#### Physical Properties of Metals and Non-Metals

| Property | Metals | Non-Metals |
| :———— | :——————————————- | :—————————————– |
| **Lustre** | Have a shiny surface (metallic lustre) | Do not have a shiny surface (except iodine) |
| **Hardness** | Generally hard (except sodium and potassium) | Generally soft (except diamond) |
| **Malleability** | Can be beaten into thin sheets | Are brittle (break into pieces when hammered) |
| **Ductility** | Can be drawn into wires | Are not ductile |
| **Conductivity** | Good conductors of heat and electricity | Poor conductors of heat and electricity (except graphite) |
| **Sonority** | Produce a ringing sound when struck | Are not sonorous |

#### Chemical Properties of Metals and Non-Metals

– **Reaction with Oxygen**: Metals react with oxygen to form basic oxides, while non-metals react with oxygen to form acidic or neutral oxides.
– **Reaction with Water**: Some metals react with water to produce metal hydroxides and hydrogen gas. Non-metals generally do not react with water.
– **Reaction with Acids**: Most metals react with dilute acids to produce a salt and hydrogen gas. Non-metals do not react with dilute acids.
– **Reaction with Bases**: Some metals react with bases to produce hydrogen gas. The reactions of non-metals with bases are complex.

#### Uses of Metals and Non-Metals

Metals and non-metals have a wide range of applications:

– **Metals**: Used in construction (iron), electrical wiring (copper, aluminum), jewelry (gold, silver), and machinery (various alloys).
– **Non-Metals**: Essential for life (oxygen, carbon, nitrogen), used in fertilizers (phosphorus, nitrogen), and as fuels (carbon).

For a detailed explanation of the properties of metals and non-metals, watch this video:

**Video: Metals and Non-Metals** – [Watch on YouTube](https://www.youtube.com/watch?v=b1SAI6Vygpo)

## Unit 3: Sound and Light

This unit explores the fascinating phenomena of sound and light. We will investigate how sound is produced, how it travels, and its various properties. We will also delve into the nature of light, its behavior when it interacts with different objects, and the principles of reflection.

### Sound

Sound is a form of energy that is produced by vibrating objects and can be heard when it reaches our ears. It travels in the form of waves, which are disturbances that transfer energy from one point to another.

#### Production and Propagation of Sound

Sound is produced when an object vibrates, causing the particles of the medium around it to vibrate as well. These vibrations are passed on from one particle to the next, creating a sound wave. Sound requires a medium (solid, liquid, or gas) to travel and cannot travel through a vacuum. The speed of sound is greatest in solids, less in liquids, and least in gases.

#### Characteristics of Sound Waves

– **Amplitude**: The maximum displacement of the particles of the medium from their mean position. The amplitude of a sound wave determines its loudness. A larger amplitude corresponds to a louder sound.
– **Frequency**: The number of vibrations per second. The frequency of a sound wave determines its pitch. A higher frequency corresponds to a higher pitch.
– **Time Period**: The time taken to complete one vibration. It is the reciprocal of frequency.

#### Reflection of Sound

Like light, sound can also be reflected off surfaces. The reflection of sound is called an echo. The laws of reflection of sound are the same as those for light. Reverberation is the persistence of sound in a large hall due to repeated reflections from the walls, ceiling, and floor.

To learn more about the properties of sound, watch this video:

**Video: Sound Tutorial** – [Watch on YouTube](https://www.youtube.com/watch?v=9c2nKGNnbMc)

### Light

Light is a form of energy that enables us to see the world around us. It travels in straight lines and can be reflected, refracted, and dispersed.

#### Reflection of Light

Reflection is the bouncing back of light when it strikes a surface. The laws of reflection are:

1. The angle of incidence is equal to the angle of reflection.
2. The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.

#### Types of Reflection

– **Regular Reflection**: Occurs when a parallel beam of light strikes a smooth and polished surface, and the reflected rays are also parallel. This type of reflection forms images.
– **Irregular or Diffuse Reflection**: Occurs when a parallel beam of light strikes a rough or irregular surface, and the reflected rays are scattered in different directions. This type of reflection does not form images but makes objects visible.

#### Mirrors

A mirror is a polished surface that reflects most of the light falling on it. There are two main types of mirrors:

– **Plane Mirrors**: Have a flat reflecting surface. The image formed by a plane mirror is virtual, erect, of the same size as the object, and laterally inverted.
– **Spherical Mirrors**: Have a curved reflecting surface. They can be either concave or convex.
– **Concave Mirrors**: Have a reflecting surface that is curved inwards. They can form both real and virtual images.
– **Convex Mirrors**: Have a reflecting surface that is curved outwards. They always form virtual, erect, and diminished images.

For a visual explanation of sound waves, watch this video from Khan Academy:

**Video: Sound waves** – [Watch on YouTube](https://www.youtube.com/watch?v=9u1PjsBfrmg)

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## References

[1] National Aeronautics and Space Administration. (n.d.). *What Is Gravity?* NASA Space Place. Retrieved from https://spaceplace.nasa.gov/what-is-gravity/en/

[2] National Center for Education Statistics. (n.d.). *Friction*. The NCES Kids’ Zone. Retrieved from https://nces.ed.gov/nceskids/help/user_guide/graph/variables/friction.asp

[3] United Nations Environment Programme. (n.d.). *Our planet is choking on plastic*. UNEP. Retrieved from https://www.unep.org/interactive/beat-plastic-pollution/

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## Key Concepts Summary

To consolidate your understanding of the topics covered in this revision guide, let us summarize the key concepts from each unit.

### Forces and Pressure: Key Takeaways

Understanding forces and pressure is fundamental to comprehending how the physical world operates. Forces are responsible for all motion and changes in motion. They can be categorized as contact forces, which require physical touch, and non-contact forces, which act at a distance. The most common non-contact force is gravity, which attracts all objects with mass towards each other. Pressure, on the other hand, is the force distributed over an area. A smaller area results in higher pressure for the same force, which explains why sharp objects cut easily and why building foundations are wide.

Friction is a special type of contact force that opposes motion. While it can be a hindrance by causing energy loss and wear, it is also essential for many everyday activities. Without friction, we would not be able to walk, drive, or even hold objects. The magnitude of friction depends on the nature of the surfaces in contact and the force pressing them together. Understanding how to reduce friction through lubrication and the use of ball bearings is crucial in engineering and machinery design.

### Materials: Key Takeaways

The study of materials is essential for understanding the world around us and for developing new technologies. Synthetic fibers and plastics are polymers that have revolutionized many industries due to their versatility and low cost. However, the widespread use of plastics has created significant environmental challenges due to their non-biodegradable nature. It is imperative that we adopt sustainable practices, such as reducing plastic consumption, reusing plastic products, and recycling plastic waste, to protect our planet.

Metals and non-metals exhibit distinct physical and chemical properties that determine their uses. Metals are generally hard, lustrous, malleable, ductile, and good conductors of heat and electricity. They are used extensively in construction, electrical wiring, and manufacturing. Non-metals, on the other hand, are generally soft, non-lustrous, brittle, and poor conductors. They are essential for life and are used in various applications, including fertilizers, fuels, and pharmaceuticals. The periodic table organizes elements based on their properties, providing a valuable tool for predicting the behavior of different materials.

### Sound and Light: Key Takeaways

Sound and light are forms of energy that allow us to perceive the world. Sound is produced by vibrations and travels as waves through a medium. The characteristics of sound waves, such as amplitude and frequency, determine the loudness and pitch of the sound we hear. The reflection of sound is responsible for echoes and reverberation, which have important applications in acoustics and communication.

Light is a form of electromagnetic radiation that travels in straight lines. It can be reflected, refracted, and dispersed. The reflection of light from mirrors allows us to see images. Plane mirrors produce virtual, erect, and laterally inverted images, while spherical mirrors can produce both real and virtual images depending on the position of the object. Understanding the behavior of light is crucial in many fields, including optics, photography, and astronomy.

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## Practical Applications and Real-World Examples

The concepts covered in this revision guide are not just theoretical; they have numerous practical applications in our daily lives and in various fields of science and technology.

### Forces in Action

Consider the simple act of riding a bicycle. Multiple forces are at play: the force you apply to the pedals, the frictional force between the tires and the road that propels you forward, the air resistance that opposes your motion, and the gravitational force that keeps you on the ground. Understanding these forces allows engineers to design more efficient bicycles and helps athletes optimize their performance.

In the field of aerospace engineering, understanding gravitational force is crucial for launching satellites and spacecraft. Engineers must calculate the precise amount of force needed to overcome Earth’s gravity and place objects into orbit. Similarly, understanding air resistance is essential for designing aircraft that can fly efficiently.

### Pressure in Engineering and Medicine

The concept of pressure is fundamental in hydraulic systems, which are used in a wide range of applications, from car brakes to heavy machinery. Hydraulic systems use the principle that pressure applied to a confined fluid is transmitted equally in all directions (Pascal’s principle). This allows a small force to be multiplied into a much larger force, enabling us to lift heavy objects with ease.

In medicine, understanding pressure is crucial for measuring blood pressure, which is an important indicator of cardiovascular health. Blood pressure is the force exerted by blood against the walls of blood vessels. High blood pressure can lead to serious health problems, including heart disease and stroke.

### Materials in Modern Technology

Synthetic fibers and plastics are ubiquitous in modern technology. From the clothes we wear to the smartphones we use, these materials play a vital role. The development of new polymers with specific properties, such as high strength, flexibility, or conductivity, is driving innovation in many fields, including electronics, medicine, and transportation.

Metals are essential for modern infrastructure and technology. Steel, an alloy of iron, is used in the construction of buildings, bridges, and vehicles. Copper is used extensively in electrical wiring due to its excellent conductivity. Aluminum is used in aircraft and packaging due to its light weight and corrosion resistance. The development of new alloys with improved properties is an ongoing area of research in materials science.

### Sound and Light in Communication and Entertainment

Sound and light are the basis of modern communication and entertainment technologies. Sound waves are used in telephones, radios, and speakers to transmit information and music. The principles of acoustics are used to design concert halls and recording studios to optimize sound quality.

Light is used in fiber optic cables to transmit data at high speeds over long distances. Lasers, which produce highly focused beams of light, are used in a wide range of applications, including surgery, manufacturing, and data storage. The study of optics has led to the development of cameras, telescopes, and microscopes, which have revolutionized our ability to observe and understand the world.

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## Conclusion

This comprehensive revision guide has provided you with a thorough review of the key scientific principles covered in the first term of your Class 8 science curriculum. We have explored the fundamental concepts of forces and pressure, delved into the properties and uses of different materials, and unraveled the mysteries of sound and light. By understanding these concepts, you are well-equipped to excel in your examinations and to appreciate the scientific principles that govern the world around you.

Remember that science is not just about memorizing facts; it is about understanding the underlying principles and applying them to solve problems. Continue to explore, question, and experiment, and you will develop a deep and lasting appreciation for the wonders of science. Good luck with your studies, and may your curiosity continue to guide you on your scientific journey.