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Science means knowledge. An organized knowledge of everything that exists or occurs around us is science. In science, people observe things around them and try experiments. Those people who find out new facts are called Scientists. People have tried to find out how things work for thousand of years. A vast store of knowledge is gained due to the efforts of great scientists of the past. We have used this knowledge in building machines, airplanes, computers, etc. Our knowledge is also used to find out the nature of matter, the universe, the earth, man and the other living and non-living things. And thus we have different streams of science like medicine, biology, chemistry, physics, etc.

Since science offers us an exciting challenge, people are choosing science as their career. There are many people who have been given awards like the Noble Prize for their outstanding discovery.

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Professional men and women who by making new discoveries and carrying out investigation earn their living in various fields are known as Scientists. They invent new ways of using resources and also investigate the universe around us. Sometimes they work as a team with other workers and few other scientists, where in each member contributes his or her own knowledge. Scientists value their work as it benefits society.

There are scientists who have become famous for their spectacular discoveries and inventions. There are also millions of scientists who by their hard work have contributed to the scientific knowledge. Today's world basically depends on computers, mails, telephones, electricity, modern medical equipment's and other scientific discoveries.

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Everything that we see around us like, mountains, trees, ice, clouds, humans, animals, furnitures, are all made up of matter. Glass, water and air through which we can see are also made up of matter. Under the subject chemistry we study the structure of non-living beings whereas, study of the structure of living beings is covered under the subject of biology. Most of the matter in the universe is non-living for e.g. the rocks and the soil, earth and water. All the plants, animals, birds, human beings are the forms of living matter. Matter in the universe can be converted from one state to another but cannot be created nor can be destroyed.

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In Greek, the word atom means one that cannot be cut. All the matter in the universe is made up of very small particles and thus anything we hear, see, feel and smell are made up of such particles. We cannot see these particles through our eyes as they are very tiny. British scientist John Dalton who gave the name atom thought that atom cannot be cut or broken into smaller pieces. Thus he suggested every element is made up of different atoms, and thus was this called as Dalton's Atomic Theory.

It is now known that atoms are also made up of even smaller particles. Nucleus is in the center of the atom, which in turn, is made up of protons and neutrons. Particles called electrons keep orbiting around in different layers, around this nucleus. As they are much heavier than electrons, nucleus make up most of the atoms mass. The number of protons give the atomic number. Each electron has negative electric charge and each proton has a positive electric charge, whereas neutrons are neutral i.e. they have no charge. In an atom, the number of protons and electrons are always the same.

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Group of atoms joined together is called a Molecule. A molecule may contain many different kinds of atoms or some kind of atoms. An oxygen molecule consists of two similar atoms i.e. two oxygen atoms joined together. The molecules of carbon dioxide and water, etc contain different types of atoms:- carbon & oxygen and hydrogen & oxygen respectively. The substances whose molecules contain different types of atoms are called compounds.

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All forms of matter were made up of just four elements, according to ancient Greek philosopher Aristotle. These four elements were air, water, fire and earth. In the universe, most of the substances are formed of different elements. An element is a substance that consists of atoms of one kind only. For e.g. - Aluminum is a metal as aluminum is made up of aluminum atoms only. Likewise silver is made up of silver atoms only. So the silver is also an element.

Water consists of two types of elements - hydrogen and oxygen and so it is a compound. In the universe, most of the things are compounds. Hydrogen and Helium are the most common elements in the universe. Oxygen is the most abundant element in the Earth's Crust followed by silicon and aluminum. The most common elements in the human body are carbon, hydrogen and oxygen.

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Push or pull is called a force. A push or a pull acts on an object and makes it move. There are several types of forces, which act in nature. The blowing wind exerts a force and pushes the things. The earth exerts a force of gravity and pulls everything towards it. This force of gravity gives things their weight. Machines and animals also make forces.

Machines are used to generate large forces, whereas animal force is used to pull carts, luggage and to get many works done. Four things may happen when a force is applied to any object.
1. The object may start to move.
2. Its speed may change, if the object is already moving.
3. The direction of the moving object may change.
4. The size and shape of the object may change by the force.

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The force that makes everything fall towards the earth is the force of gravity of the earth. For e.g. a ball thrown into the air, it comes back to the ground. Everything falls towards the earth when left free in the air. Even the sun and the moon have the force of gravity and it is this force of gravity that attracts all the planets towards the sun and holds them in their orbits. The force of gravitational attraction between two objects depends upon the distance between them and the mass of objects.

The greater the distance between two objects, the smaller the force of attraction between them. The greater the mass of the object, the greater the force of gravity. Since the moon is much smaller and has less mass than the earth, the gravity on the moon is less than that on the earth. The value of the earth's force of gravity is 9.8 meters per sec.