Wesley Dowler

Scientific advances often promote the continuation of an existing model, bringing a more in-depth understanding of the theory or new evidence to support the current paradigm. Sometimes, however, advances are incompatible with the existing model, producing major changes within a discipline to better account for unexpected observations. This paper will explore how both Galileo and Newton dramatically changed Aristotle’s concept of motion and how these changes allowed Newton to explain the orbit of the moon.

According to Thomas Kuhn, science advances through three stages: the pre-paradigm phase, normal science, and revolutionary science. In the first phase, there is no generally accepted view to interpret experimental results; various incompatible theories compete to become the accepted framework. Once one theory has widespread consensus, the normal phase begins, where scientific understanding progresses in conjunction with the commonly accepted model. As this stage continues, various anomalies are discovered and used to refine the overarching model. Occasionally, an observation is incompatible with the current understanding and a new paradigm will emerge, offering an innovative perspective to the discipline. This new model can better explain both the new and old observations, allowing a new phase of normal science to begin (Kuhn). A striking example of this revolutionary shift occurred in physics, when Galileo Galilei dismantled the traditional understanding set by Aristotle. A few years later, Sir Isaac Newton applied this new model to the entire universe and developed his three universal laws of motion, providing an explanation for planetary movement.

Aristotle built the foundation of physics in the fourth century BCE, using observations and philosophy to develop governing rules for the physical world. His conclusions, which often produced accurate predictions, were generally regarded as true until the beginning of the Renaissance. For instance, he believed all things eventually return to their natural place and come to rest (stop moving). In the case of soil, it will fall to the ground, which is considered its natural place; in the case of fire, it will rise toward the sky, moving toward its natural place. In Physics, he suggests the rate of this movement increases as the weight of the object increases. According to Aristotle, “We see that bodies which have a greater impulse either of weight or of lightness, if they are alike in other respects, move faster over an equal space, and in the ratio which their magnitudes bear to each other” (75). In other words, a heavier rock will fall to the earth faster than a lighter rock, if they are similar in all other properties.

Another important theory for Aristotle was the nature of the planets and stars, including their composition and movements. At the time, people thought the world was comprised of four elements: earth, water, air, and fire. To explain the heavenly spheres and bodies, he developed a fifth element called aether. This heavenly substance gave the celestial orbs properties not found on earth, allowing them to remain suspended in the sky and rotate around the earth; the laws of physics in space were assumed to be completely unique from the laws of physics found in everyday life on earth. In What the Middle Ages Inherited from Aristotle, Edward Grant describes this model: “In this scheme, Aristotle assumed that each physical orb had its own immaterial mover, which, although completely immobile, was eternally able to cause its assigned orb to move effortlessly around the earth with uniform, circular motion. These ‘immovable,’ or ‘unmoved,’ movers were unique in the world because they were capable of causing motion without themselves being in motion” (67). Aristotle’s views of the universe placed the earth at the center of a solar system with perfect spheres moving around it in perfect circles, an understanding that continued to influence astronomy for many centuries.

As time progressed, many aspects of Aristotle’s philosophy began to contradict observations of the physical world, especially as instrumentation afforded measurements with greater accuracy. Many examples can be found in the works of Galileo, who faced opposition from the Catholic Church. At the time, science and philosophy were intimately intertwined and married to the church, which supported the Aristotelian view of the universe. When Galileo developed an efficient telescope, he was able to see that the celestial objects were not perfect spheres as originally thought and that planetary movement is better explained if the planets revolve around the sun (as opposed to the planets and sun revolving around the earth). Although Galileo was unable determine why the stars did not shift in this model (the instrumentation of his time was unable to measure this shift), he continued to publicize this view, even making fun of the pope in his writings. Eventually, the church condemned his views as heretical, which many see as the point where science and philosophy broke apart (Brom).

In addition to his observations of the solar system, Galileo also disagreed with many of Aristotle’s views on motion. For instance, he found if a ball is sent down a greased ramp, it would roll much farther than a ball sent down a ramp without grease. Through these experiments, “Galileo realized there is no real difference between an object moving at a steady rate and one that is not moving at all – both objects are unaffected by forces” (Hart-Davis et al. 87). In other words, as the friction of the ramp was removed, it became clear the natural state of the ball was not to be at rest as Aristotle thought, but rather to continue at a constant speed (a concept known as inertia). He also noticed various balls differing only in their mass would reach the end of the ramp at the same time. From this, he found the acceleration of an object was independent of mass – another objection to Aristotle’s views. Furthermore, Galileo examined how objects move through the air: they follow a curve, continuously falling toward the earth as they move from their origin. In one thought experiment, a cannonball is fired horizontally, while another cannonball is dropped at the same instant from an identical height. Surprisingly, each cannonball will hit the earth at the same time, because even as the ball from the cannon is moving along a horizontal vector (path), it never stops moving downward at the same acceleration as the dropped cannonball.

In the summer of 1665, the great plague was devastating many portions of the western world. At Cambridge University, the situation became so dire that students were dismissed from their studies to return home. Isaac Newton, who had recently received a degree, returned to his family’s farm in the county of Lincolnshire, but instead of taking a break from his academic endeavors, he began to study the fundamental questions of his time (Krull and Kulikov).

Galileo’s experiments had brought numerous insights into how objects fall toward the earth, but his calculations were relatively limited. For instance, he was able to calculate the average speed of an object by dividing the total distance by the total time, but he was unable to calculate the exact speed of the object at a particular instance (which continuously changed due to the acceleration of gravity). Newton developed a new branch of mathematics to calculate the way things change over time. This framework, known as calculus, allowed him to calculate the instantaneous velocity of a moving object. For the first time in history, “it was possible to calculate quantities that are constantly changing, like the speed of a falling apple at any particular moment” (Newton’s Dark Secrets).

Newton also developed an explanation for the orbit of the moon during this summer. With the invention of the telescope, people began to see the moon as ordinary matter as opposed to a perfect sphere, but they still assumed it had different physical properties because nobody could explain why it didn’t fall to the earth. As Newton considered Galileo’s cannon experiment, where the cannonball curved down to the earth in a flat field, he began to consider what would happen if the cannonball was launched at a much faster velocity. If the speed of the ball was great enough, it would go beyond the field, and the round shape of the earth would become a factor; as Edward Dolnick explained in a recent interview, “as it falls, that bullet curves down toward the earth in just the same way as the earth is curving away from it.” Newton realized the moon was not immune to physical laws found on earth, but rather it is endlessly falling around the earth, allowing it to remain in orbit! After this discovery, Newton would to use calculus to provide a mathematical proof for the elliptical motion of the planets, further dismantling the model initiated by Aristotle and bringing the birth of modern physics.

The model of gravity proposed by Newton has allowed engineers to bring satellites into orbit, continuously falling around the earth’s curve, much like the moon. In addition, calculus has provided insight into nearly every branch of science, from chemistry to economics (Gregersen 15). As scientific understanding continues to advance, the world we inhabit will also undergo important transformations in technology directly related to these scientific developments. In modern times, quantum mechanics has indicated the Newtonian view may provide an inaccurate picture of the molecular world, suggesting a new revolution on the horizon. Regardless, Newton’s three laws of motion as presented in Principia continue to define the modern view of the macroscopic world and illustrate the importance of scientific progression in understanding natural wonders.

Works Cited

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