Course Content
Class 8th Science
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Class 8th Math
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Class 8 Social Science History
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Class 8 Social Science Geography
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Class 8 Social Science Civics: Social and Political Life – III
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Class 8th Englisn Honeydew Summary
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Class 8 English Honeydew Poem
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Class 8 English It So Happened
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Online Class For 8th Standard Students (CBSE) (English Medium)

## Introduction

In science, a push or pull of an object is known as force. The interaction between two objects arises force. Force has both magnitude and direction. The strength of a force is expressed in magnitude. Force brings about a change in the direction or state of motion of a body. In class 8 Science chapter 11, various characteristics of force along with the types of force are discussed.

### Push

A push is a force exerted away from the body, e.g: Hitting a snooker ball, kicking a football.

### Magnetic force

The force of attraction or repulsion between two magnetic bodies due to their poles is known as a magnetic force.

## What Is It?

### Pull

A pull is a force exerted towards the body, e.g: drawing a bucket of water from a well, playing tug of war.

### Force

• A force is a push or a pull.
• The interaction between objects that can change the state of the objects.

## Playing With It

### Net force

• The resultant of all the forces acting on a body is known as net force.
• The acceleration of the body is along the direction of the net force.

### Frictional force

• The force that opposes the relative motion between two surfaces.
• Acts between the surface of the two bodies in contact.
• Type of contact force.

## What Can Force Do?

### Vector

– Vector quantities are expressed in magnitude as well as the direction of the object. E.g: Velocity, displacement, weight, momentum, force, acceleration etc.
– Vectors are used to find the resultant component acting on a body.
– When multiple forces act on a body, they can be resolved into one component known as the net force acting on the object.
Example:

Vectors are also useful when the force acts at an angle to the horizontal.

### Application of Force

– A force is an effort that changes the state of an object at rest or in motion.
– It can change an object’s direction and velocity.
– Force can also change the shape of an object.

### State of Motion

The state of motion of an object is defined by its velocity – the speed with a direction. Thus, inertia could be redefined as follows:
Inertia = tendency of an object to resist changes in its velocity.
An object at rest has zero velocity – and (in the absence of an unbalanced force) will remain with a zero velocity; it will not change its state of motion (i.e., velocity). An object in motion with a velocity of 2 m/s, East; will (in the absence of an unbalanced force) remain in motion with a velocity of 2 m/s, East; it will not change its state of motion (i.e., velocity). Objects resist changes in their velocity.

## Types of Forces

### Contact

A touch or contact is required to do most of our everyday activities. E.g Lifting, pulling, kicking, pushing etc.

### Contact forces

Forces that require a touch or contact to be applied are known as contact forces. E.g: Muscular forces, frictional forces

### Muscular force

The force applied by the effort of our muscles e.g lifting a heavy box, pulling a bucket of water, pedalling a cycle.

### Non-contact forces

• Forces that do not need a contact, or that have their influence without a touch.
• Example: magnetic force, electrostatic force, gravitational force.

### Gravitational force

• The attractive force that a body experience towards the centre of the earth is called the force of gravity due to earth.
• Property of the universe, every object attracts or exerts a force on every other object.

### Electrostatic force

The force of attraction or repulsion experienced by a charged body from another charged body in the same vicinity is known as Electrostatic Force.

### Nuclear forces

• The nuclear force acts between all the particles in the nucleus. i.e., between two neutrons, between two protons and between a neutron and a proton.
• It is an attractive force in all cases.
• It is the force that keeps the nucleus intact by overcoming the enormous repulsive force between positive protons.

## Thrust and Pressure

### Pressure

• Force acting per unit area is known as pressure.
• P=force/area
• SI unit is Pascal.

### Distribution of pressure

• Force acting on a smaller area applies more pressure than the same force acting on a larger area.
• Examples: Porters place a round cloth on their heads to increase surface area and reduce pressure.
• A sharp knife cuts better as more pressure is exerted over a smaller area.

### Pressure in fluids

The pressure exerted by a fluid(gases or liquids) in a container is transmitted undiminished in all direction on the walls of the container.

### Upthrust

The upward force exerted by a fluid on an object is known as upthrust or buoyant force.

## Atmospheric Pressure

### Gaseous pressure

Gases exert the same pressure on the walls of the container in all directions.

### Atmospheric pressure

• Our atmosphere extends to several kilometres above sea level. The weight of the air acts as pressure known as atmospheric pressure.
• The pressure in our body balances the atmospheric pressure and that is why we do not feel it.

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