Physics

# An Explanation of Newtons three Laws of Motion

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In the first half of the sixteenth century, Nicolas Copernicus's heliocentric revelations modified mans understanding of the cosmos, taking the Earth out of the center of the universe and making it just one of 6 planets (those know in the day) it obit of the Sun. Copernicus meticulously describe his theory and geometric proofs of it in his book On the Revolutions of the Heavenly Orbs, but would not allow publication of the book until he was on his deathbed.

Those who read Copernicus' book and subscribed to his theory, began to wonder what force or power held the planets in their orbits. One such Copernican convert was Johannes Kepler, who in his own book Harmonies of the World refers to the existence of a mysterious force exerted by the sun on the planets and by planets on their moons. While Kepler himself would never understand Gravity he was the first to realize its existence.

Isaac Newton was born in 1643, a hundred years after the death of Copernicus and 13 years after Kepler died, but during his education would read the books these men had written as well as others investigating the new heliocentric reality. Newton took particular interest in the mysterious force Kepler had pointed to and focused his attention on solving the enigma. Experimenting with gravity here on Earth, Newton gained a unique understanding of the properties of gravitational attraction and in the process discovered a few other things about the motion of objects, claiming that he was able to see further than others by standing on the shoulders of giants.

In 1876, Sir Isaac Newton published his own book called Principia, and in it described his universal law of gravity along with his three laws of motion. Newton's law of universal gravity would later be modified by Albert Einstein's own theory of General Relativity, but his three laws of motion remain intact and true, and are fundamental precepts of modern physics.

FIRST LAW:

A body continues in a state of rest or uniform motion unless acted upon by an external force.

Newton's 1st law of motion is today more commonly referred to as the law of inertia. It basically states, that if an object is standing still, it will continue to stand still until some external force is applied to cause it to move. Conversely, if an object is in motion, it will continue to travel in the same direction and at the same velocity until some other external force acts upon it to change its direction or velocity.

SECOND LAW:

Acceleration of a body is proportional to the force acting upon it and in line with that force.

Newton's 2nd law of motion is perhaps the most difficult to grasp for first year physics students, primarily because it involves several variables, the mass of an object m, its acceleration in feet per second a, and the net force applied to achieve it F, or F=ma. However, if you consider that the mass of an object is a constant [is not going to change], then force and velocity become directly proportional, and understanding Newton's 2nd law becomes a bit easier . If you increase the force applied to an object, you also increase its velocity. This can be demonstrated with a simple illustration.

Consider the lowly baseball being hit by a bat. In this case, assume that the ball is standing still and not thrown by a pitcher (as on a pedestal like that used in peewee ball). The bat has a fixed mass and is swung by the batter, causing it to move at a certain velocity [that being the highest at the end of the bat.] The force the bat will apply to the ball can thus be calculated by multiplying the bats mass (weight) by it's velocity at the point where it contacts the ball. Now, if you were to divide that force by the mass of the ball, you could predict with certainty, according to Newton's second law of motion, the exact velocity at which the ball will travel, at least at the first instant. Gravity and the friction cause by the air are two forces that will immediately act upon the ball changing both its velocity and trajectory. But if you could do this experiment in the vacuum of space, beyond any other gravitational forces, the ball would maintain its velocity forever.

In physics today, Newton's second law of motion is also referred to as the law of linear momentum.

THIRD LAW:

For every reaction, there is an equal and opposite reaction.

The 3rd law of motion is perhaps the easiest to comprehend and personally experience as well. For example, if you and someone of about the same weight were sitting in chairs equipped with wheels under them on a dance floor, and were to push off of each other, both exerting the same amount of force, you would both be propelled at the same velocity in opposite directions. Furthermore, given that the forces of gravity and friction applied to both of you would also be the same, you should both travel the same distance before coming to a stop. If however, one of you was twenty pounds heavier, the lighter person would travel twice as far in one direction than the heavier person would travel in the other.

There can be no doubt that Sir Isaac Newton was a brilliant man. His laws of motion remain a part of the basic foundation of modern physics, and a thousand years from now, should our species survive that long, his will still be a name remembered. And an understanding of his laws of motion, will then too, be fundamental to an understanding of the basic natural forces at work in the universe. If anyone were ever to build a Mt. Rushmore of those who have most advanced human scientific understanding, a sculpture of Sir Isaac Newton would surely merit inclusion, and an engraving of his three laws of motion beneath it would offer fitting tribute to a man considered by many as the father of modern physics.

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