The mystery of the moving ball

Isacc Newton (1642-1727) was the greatest scientist who ever lived and his work forms the basis on which all physics stands

Isacc Newton (1642-1727) was the greatest scientist who ever lived and his work forms the basis on which all physics stands. In 1687 he published his Philosophiae Naturalis Principica Mathematica (Mathematical Principles of Natural Philosophy), the most influential scientific work ever published.

In it, he outlined three laws of motion and the law of universal gravitation - laws that explain how everything in the universe moves around. This basic physics is superbly explained by Isaac Asimov in Understanding Physics (Barnes and Noble, 1993).

Newton's first law of motion may be stated: A body remains at rest or, if already in motion, remains in uniform motion at constant speed in a straight line, unless it is acted on by an unbalanced external force. The first law is sometimes called the principle of inertia because of the tendency of motion or rest to maintain itself unless forced to change.

The first law clashes with our intuition. We know that if we roll a ball along the ground it will gradually slow down and stop. This is because of the force of friction between the inevitable unevenness of the ground and of the surface of the ball. If everything was perfectly smooth and there was no air resistance, the ball would roll in a straight line at constant velocity forever. It was Newton's genius to see beyond the familiar world to the friction-free world of the first law.

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A force can be defined as "that which imposes a change in the speed of a body, or its direction of motion, or both". A change in velocity is an acceleration. A force therefore accelerates a body. For a given body, one can measure the size of a force by observing the acceleration it causes.

Place a football on the ground and kick it. After 10 seconds, measure its velocity - let us say it is three metres per second (3m/sec). Dividing by 10 seconds, one can say the force produced an acceleration of 0.3m/sec/sec. Let us say that a lighter kick produced an acceleration of 0.15m/sec/sec. One can conclude that the force of the second kick is half that of the first kick.

Now kick a bowling ball of the same diameter as the football with the force of the first kick described above. This kick will produce an acceleration far less than achieved with the football because the bowling ball mass is greater. Newton used the word mass to indicate the amount of inertia possessed by a body.

His second law of motion states: the acceleration produced by a particular force acting on a body is directly proportional to the magnitude of the force and inversely proportional to the mass of the body.To exert a force you must have some material thing to exert the force on. The force connects two bodies. When a ball falls to the floor, it exerts a force on the ground, but, since it rebounds, the floor also exerts a force on the ball. There seems to be two forces of equal size but opposite direction.

NEWTON said that this is always the case and stated the third law of motion: whenever one body exerts a force on a second body the second body exerts an equal and opposite force on the first body.

A rocket taking off demonstrates Newton's third law. The fuel burns into a hot, expanding gas that exerts a force against the inside walls of the engine. The walls exert an equal and opposite force against the gases. The gases are accelerated downwards and the rocket is accelerated upwards.Newton also deduced the law of universal gravitation which states that two bodies are attracted to each other by a force of gravity proportional to the product of their masses and inversely proportional to the square of the distance between them.

He reasoned that the same force that causes an apple to fall from a tree also holds the moon in orbit around the earth. The gravitational pull of the earth on the moon constantly changes the direction of travel of the moon, which is an acceleration. So, the moon is constantly accelerating towards earth but never gets any closer because it keeps changing direction.

Newton used his laws of gravitation and of motion to deduce the three laws of planetary motion previously established by Johannes Kepler (1571-1630) based on observations of how the planets rotate around the sun. Newton theoretically calculated the period of revolution of the moon around the earth. His result closely matched the period of revolution measured by observation. Newton had shown that his laws hold in the heavens as on earth - they are universal laws.

As a child, Newton felt abandoned by his mother. This deeply scarred him and he never developed a capacity to form normal social intimacies. He remained a lonely, paranoid and driven figure throughout his life.

A servant described Newton as follows: "I never knew him to take any recreation or pastime either in riding out to take the air, or walking, bowling or any other exercise whatever, thinking all hours lost that were not spent in his studies, to which he kept so close that he seldom left his chamber unless to lecture at term time."

In his later years, a friend asked Newton how it was that he, a single man, had been able to make such spectacular advances. Newton replied: "By thinking of nothing else."

William Reville is associate professor of biochemistry and director of microscopy at UCC