Radiophysics. Халюшева Г.Р. - 10 стр.

         2 Unit 2
         2.1 Text 2
                                            Newton’s three laws of moion
          Isaak Newton was born in 1642 in England to a poor farmer’s family. Isaak attended a country school until he was
eleven. At the age of twelve he was sent to a town school, seven miles away from his home, to continue his education. The
boy made good progress and worked hard for his goal – Cambridge. His application for entrance into Cambridge University
was accepted in 1661.
          Soon his tutors found that he showed unusual knowledge of subjects that were to form the topics of further lectures.
He had mastered those subjects independently because he was interested in them. The result was that he was soon excused
from attending certain courses, which provided him with more time for making experiments.
          Newton’s interests were centered on mathematics and mechanics and on studying different phenomena of Nature.
Newton was forty-two years old, however, when he started putting down on paper his ideas which were the result of more
than twenty yeas of scientific thinking. It is indeed surprising that it took Newton only eighteen months to produce his
wonderful work – the Principia, that was later on called the greatest product of a single human mind.
          Of the many things that Newton accomplished in the Principia, the major ones, namely the three laws of
mechanics, are briefly touched upon below.
          First Law. Every body continues in its state of rest or uniform motion in a straight line except in so far as it may be
compelled by impressed force to change that state.
          The first law introduces the idea of inertia and is often called the law of inertia. Inertia is defined as that property
by virtue of which a body resists changes in motion.
          Suppose that you are riding in a bus going 20 miles per hour. The bus stops suddenly. It is no longer doing 20 miles
per hour. But you are. Unless you seize a handrail, you will keep moving due to your being a “body” in motion. Your
having experienced a thrust is a demonstration of inertia.
          On the Earth’s surface, however, it is difficult to demonstrate fully the law of motion because air–resistance and the
tremendous forces of gravity prevent an object from travelling at constant speed in a straight line. But one of the first proofs
of the first law is found in the movement of the heavenly bodies which meet practically no friction in their travelling
through space.
          Modern artificial satellites obey Newton’s first law. Their being set on proper orbits, completely free from the
Earth’s atmosphere and air friction, is of great importance. It is known that some early satellites were burned up on account
of their having been set on wrong orbits. Successful flights of modern spaceships have proved the validity of Newton’s first
law in actual practice.
          Second Law. Any change in motion of a body is in proportion to the force pressing on it and takes place in the
direction of the straight line in which the pressing force acts.
          The second law gives a valuable means for measuring forces. The mathematical relationships provided by the
second law allow scientists to measure the force of gravitation at any point of the Earth’s surface. The ability of making
such calculations is of great value in planning the orbit of an artificial satellite. The availability of electronic computers
provides a reliable means for making the necessary calculations with great speed and accuracy.
          Third Law. For every action exerted on a body, there is an equal and opposite reaction. Another way of stating the
third law is this: “Whenever one body exerts a force on another, the second body exerts an equal and opposite force on the
first body.”
          For example, when you press a stone with your finger, your finger is also pressed back by the stone.
          When you fire a rifle, the forward thrust of the bullet is matched by a backward thrust or “kick” against your
shoulder.
          Nowhere else today, perhaps, is Newton’s third law of motion of such great importance as in the field of jet
propulsion and rocket flights.
          In the case of jet-propelled plane’s flying the thrust of gases issuing from the jet engine reacts against the engine
itself and causes a forward thrust. It is not true, as it was thought before, that the rearward gases push against the air; if it
were so, Newton’s third law would not be true. But jet engines are air-breathers and the hot gases burned in them feed on
the air supply they take from the atmosphere.
          Rockets on the other hand, carry their fuel along with them and are able of travelling in outer space where there is
no air and hence, no air resistance. In the near-perfect vacuum of space Newton’s third law operates ideally. The powerful
thrust inside the rocket’s engine results in an equal and opposite thrust forward of the rocket itself because there is no air-
friction in outer space. Hence, the rockets' travelling with fantastic speeds of thousands of miles per hour is possible.