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# Paragraph on “What is Motion?” complete paragraph for Class 9, Class 10, Class 11 and Class 12

What is Motion?

How astronomers came to understand the structure of the solar system. They arrived at their conclusions by observing the planets and measuring their distances from the Earth and the speed of their orbits. Their findings were not completely accurate because these astronomers did not understand the laws of motion. In fact, these were not fully under-stood until this century, when Einstein, continuing the work of Isaac Newton, formulated his theory of relativity.

A body is in motion when it is moving. Its speed is the distance it travels divided by the time it takes to complete that distance. In physics, the word velocity describes speed in a particular direction. A force is anything that produces an effect. A mechanical force produces motion.

The Greeks were the first people to try and understand motion. Aristotle believed that force was necessary to explain motion. He thought, for example, that a stick could not roll across a table without some force behind it.

Galileo Galilei was the first to question Aristotle’s theory nearly two thousand years later. Both he, and Isaac Newton who based much of his thought on the work of Galileo, realised that it was the forces that oppose motion that need to be explained. Newton formulated his ideas in his three laws of motion .

In his first law, he stated that a body will continue to move at a constant velocity once it has been set in motion unless it is acted upon by some force. A spaceship for example, will glide infinitely in space once it has been set in motion, because there are no forces to act against it. A stick will roll to a halt on a table because the friction caused by the surface of the table will set up an opposing force.

In his second law, Newton stated that the changes in velocity in a body, that is acceleration and deceleration, are proportional to the force acting upon it. His third law states that for every force there is an equal force opposing it. This means that if you put a book on a table, there is the downward force of the book on the table. If this were the only force, the book would continue to move down-wards. The fact that this does not happen is because there is an equal reaction in the opposite direction.

All Newton’s laws were based on the ob-server being stationary. More recently, it was realised that if an observer is moving, his motion must be taken into account. This means that when you measure the movement of the other planets in the solar system, the motion of the Earth must be accounted for.