Newton’s three laws of motion were the backbone of classical physics, which were subsequently replaced by Albert Einsteins discoveries concerning Theory of Relativity. But till recently, Newton’s three laws of motion played a pivotal role in aiding our understanding of common geo-physical phenomenon. All three laws concern the action or influence of one physical object own another. These laws are usually applied to physical entities that can be idealized as particles, in the sense that the magnitude of its structure and size can be disregarded as its motion alone is considered. A classic case of this approximation is the consideration of whole planetary bodies as orbiting particles is allowed under this framework.
The first law, also usually refered to as the ‘law of inertia’, states that “Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it”. (Newton, 1729, p.19) This leads to two separate inferences. First, “An object that is at rest will stay at rest unless an unbalanced force acts upon it; second, an object that is in motion will not change its velocity unless an unbalanced force acts upon it.” (Newton, 1729, p.19) This law should be qualified with the statement that they are valid only within an inertial reference frame. Two reference frames that are in relative motion with each other can also be considered as suitable insofar as the principle of Newtonian relativity is identified and ascertained.
The second law states that “The relationship between an object’s mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.” (Thomson, 1867, p.12) This is the most significant of the three laws due to the fact that it permits quantitative calculations of moving objects/particles.For example, it helps measure how velocities of objects change when external forces are applied to it. This law is also comparable with Aristotle’s theories on dynamics. The key distinction though, is that Newton maintained that a force causes only a change in velocity (either acceleration or decceleration) whereas Aristotle proposed a constancy in velocity of the object. In simpler terms, this law could be stated as “Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object)”. (www.teachertech.rice.edu) This means that heavier objects require greater force to be dislodged than lighter objects.
The third law of motion given by Newton is perhaps the most quoted of the three laws, having firlmly assimilated itself as part of popular culture. it states that, “To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions.” (Newton, 1729, p.19) Here, Newton actually means momentum, when he talks about motion. This law implies that all forces are a result of interactions between various objects in the physical world. As a consequence, there is nothing called unidirectional force that is excluded from the influence of any other object. Newton’s third law finds broad range of application. For example the law of conservation of momentum is a direct derivative of this law. But it also has its limitations, which has made necessary other sophisticated theories like quantum mechanics to explain force fields and other phenomenon.
However, all of Newton’s laws are verified by experiments conducted over a span of two centuries. And they serve as useful utilities in practical everyday applications, where the speeds of encountered objects are nowhere near the speed of light. These three laws, along with the law of universal gravitation and the techniques of calculus, offered humankind an unprecedented opportunity to explore and understand natural occurances in the physical world, till they were replaced by more advanced offerings from Albert Einstein in the twentieth century.
Newton, Isaac, “Mathematical Principles of Natural Philosophy”, 1729 English translation based on 3rd Latin edition (1726), volume 1, containing Book 1, especially at the section Axioms or Laws of Motion starting page 19.
Thomson, W (Lord Kelvin), and Tait, P G, (1867), Treatise on natural philosophy, volume 1, especially at Section 242, Newton’s laws of motion.
Newton’s Three Laws of Motion, retrieved from <http://teachertech.rice.edu/Participants/louviere/Newton/law2.html> on 25th June, 2011.