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A Guide to Motion and Energy in Classical Mechanics

written by: Aggeliki K. • edited by: Lamar Stonecypher • updated: 9/29/2011

What is motion and what is energy? These fundamental questions are difficult to answer. The first attempt to give some explanations was made through classical mechanics. Here you will find a collection of articles with everything you need to know about the physical laws of motion and energy.

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    An object is moving when its position is changing with respect to a reference point. This is the scientific definition of movement. As far as speed is concerned, it is defined as the distance the object travels in a period of time. Many confuse the terms speed and velocity, though. Velocity is actually the speed of an object in a specific direction. The next term that is important to refer to when speaking about motion is acceleration. It is defined as the change in the velocity of an object over a period of time. These basic terms are fundamental to classical mechanics, but are also very familiar to each individual, since their use is so common in our everyday life:

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    Newton's Laws

    A force is a push or a pull that causes an object to undergo a change in speed, direction, or shape. The use of forces is fundamental in physics and, along with some mathematical insight, Sir Isaac Newton managed to formulate three laws of motion that were first published on July 5, 1687 in his work "Philosophiæ Naturalis Principia Mathematica." These laws describe how an acting force affects the motion of a body and are actually the basis of classical mechanics.
    In just a few words, the first law introduces inertia, the property of matter to resist any change in its state of motion. The second law directly connects the acceleration of a body to the acting net force, and finally the third law relates the two mutual forces of action and reaction between two bodies.
    The fact that Newton's laws were not improved on for nearly three hundred years is remarkable! Newton also discovered the law of gravitation by making empirical observations. Although it has been superseded by the general relativity of Einstein, it continues to be valid when we are dealing with low mass objects.
    Discover how Newton contributed to modern science by visiting the articles below:
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    Although we use the term in our everyday lives, it is very difficult to determine what energy is. Scientifically, it is defined as the capacity to do work. One of its most amazing properties is that it can neither be created nor destroyed, but only transferred from one body to another or changed from one form to another. These transformations occur through transfer of heat, work, or radiation, and the forms that energy takes are basically two: kinetic and potential. All other types of energy- chemical, electric, geothermal, solar, etc.- belong to one of the two forms mentioned. This is a simple and elegant way not only to describe this complex and hard-to-define concept, but also to measure it to a satisfactory degree.
    Another property is that matter can be changed into energy and vice versa, a fact that was mathematically expressed by the famous Einstein equation. This property however is not recognized by classic mechanics, where energy and mass are two completely different things.
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    More About Forces, Momentum and Energy

    The links below contain additional information on forces and the principle of conservation of momentum. This last principle, together with the conservation of energy, can be used to describe a wide range of phenomena including the phenomenon of collision ("elastic" where energy is conserved and "inelastic" where energy is not conserved):
    As mentioned before, Newton's laws of motion are the most fundamental in classical mechanics. Although these laws are accurate enough at very small scales and low speeds, there are certain circumstances in which they are considered to be inappropriate for describing the occurring phenomena. Extremely high speeds (near the speed of light) and very strong gravitational fields demand the use of special relativity and general relativity correspondingly. On the other hand, the laws of conservation of energy, momentum, and even angular momentum, are universal and can be applied to both light and matter, in classical and non-classical physics.