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# Force, work and energy

## Force

From morning till night, you do different kinds of activities like walking, running, playing games, writing, etc. At the same time, you see vehicles like car, scooter, plane, etc. moving from one place to another. Do you know how these things move? They all need force.

Nothing moves on its own. Everything needs a push or pull to make it move.

A push or pull that can change the environment of an object is called force.

## Effects of force

Force is used to move objects. Push a table. What happens! It moves from its place. Now, pull it back. What do you see? It again moves from its place. So, a push or pull (force) can move an object.

## Force is used to stop a moving object

Have you ever played football ? While playing with a ball what do you do to stop a ball ? You apply force in the opposite direction to its motion. The force applied can stop the moving ball.

## Force is used to change the speed of an object

When we kick a slow-moving ball, it runs with a great speed or when a child pushes the pedals harder, it moves faster. So, we can say a force can change the speed of an object

## Force is used to change the direction of a moving body

While playing cricket, when the batsman hits a ball with his bat, the ball moves in other direction. By applying force with a bat, the direction of the ball is changed.

## Types of force

In nature, there are two types of forces acting on various objects.

1. Gravitational force.
2. Frictional force.

## Gravitational force

The force that pulls the objects downwards the center of the Earth is called gravitational force or gravity. All the things which are thrown in air come back to the Earth because the Earth attracts everything towards its center.

Have you ever seen that you throw a ball and it does not come down ? No.

A fruit or a leaf from a tree falls down towards the Earth. We are able to walk on earth because of this force. In space, there is no gravitational force of Earth, that is why people float in air.

## Frictional force

Whenever two surfaces are in contact, they rub against each other and try to stop the movement of the other. This is called frictional force. Frictional force helps in stopping the moving object. It is always applied in the direction opposite to the motion of an object.

If there is no friction , then moving objects can never stop.

Smooth surfaces have less friction while rough surfaces have more friction. If you walk on a smooth surface, you slip because the force of friction to stop is less. Now, can you tell, why do you slip when you step on a banana skin ?

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Soles of shoes and slippers wear out because of the friction between the ground and sole. We are able to walk because of friction.

Which will cause more friction-- a carpet or granite surface ? Why ?

## Work

In science, the work 'work' means a push or pull which moves an object through a distance.

When we push a chair it moves from its place. In this case, we say that the work is done.

Now, push a wall.

Does it move ? No. When the things don't move we say that no work is done.

The amount of work depends on two things:

1. The amount of force applied on it.
2. The distance over which it is applied.

We use different types of machines to do our work faster and make it simpler. It is very easy to cut vegetables with a knife. Tools which help to make our work simpler, faster and easier are called simple machines e.g., tongs, knife, screwdriver, scissors, etc. are examples of simple machines.

Big machines like cranes, etc. are made up of two or three simple machines. They are called complex machines.

## Energy

Now, you know that work is done, when force is applied. But for applying this force, we need energy. Work cannot be done without energy. When work is done, energy is used. Energy is the ability to do work.

From where do we get this energy ? The human body gets its energy from food.

Vehicles such as car, bus, etc get energy from fuels such as petrol, diesel, etc. The main source of energy is the sun. Plants use sun's energy to make food. Animals and human beings eat these plants and get energy to do work.

So, the ultimate source of energy is sun. Without sun, life is not possible on the Earth.

## Kinds of energy

according to the law of science, energy can neither be created nor destroyed. it is converted from one form to another. Energy has many forms like heat energy, light energy, muscular energy, electrical energy, etc.

### 1. Potential energy:

Energy stored in an object due to its high position is called potential energy, e.g., a bicycle at the top of a mountain possesses potential energy.

### 2. Kinetic energy:

Energy due to the motion of an object is called kinetic energy. The moving water in a river has kinetic energy.

Potential energy and kinetic energy together are known as mechanical energy.

### 3. Heat energy:

Heat energy is obtained by burning wood, coal and other fuels. The sun also gives us heat energy. It is used for cooking food, heating water and warming rooms etc.

### 4. Muscular energy:

The energy stored in the muscles of human beings and animals is called muscular energy.

When we lift an object, we use muscular energy of our arms.

### 5. Chemical energy:

Energy stored in chemicals is called chemical energy. e.g., fire-crackers, batteries, etc. have chemical energy.

### 6. Electrical energy:

Energy produced from electricity is called electrical energy. It is used to light houses and to drive machines. We can use fans, coolers, T.V., etc. due to electrical energy.

### 7. Magnetic energy:

A magnet has magnetic force which can push or pull metals.

### 8. Wind energy:

Moving air also has energy. Windmills can be moved with the help of wind energy. It is used for pumping out water and also for generating electricity.

### 9. Water energy:

Flowing water also has energy. It can be used to produce electricity. Electricity produced by the energy of flowing water is called hydroelectricity.

### 10. Solar energy:

The energy of the sun is called solar energy. It is a never-ending source of energy. Now a days, solar cooker, solar heater and solar batteries are used.

## Source of energy:

You already know about different forms of energy. We can say that the sun is the ultimate source of energy. Besides sun, wind, moving water, coal, petrol, wood, kerosene, etc. are also source of energy.

## In the nutshell

• A push or pull on an object is called force.
• Energy is the ability to do work.
• Work is said to be done when an object moves through a distance with the help of force.
• Force can change the movement, direction and speed of an object.
• Energy can neither be created nor destroyed, it can be transformed from one form to another.
• Wood, water, sun, fuel, chemicals, etc. are different sources of energy.
• Heat energy, light energy, wind energy, electrical energy, water energy, kinetic energy, potential energy, muscular energy, etc. are various forms of energy.