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Concepts in Classical and Modern Physics

Physics is the study of the properties and interactions of matter and energy. Classical physics refers to the concepts developed prior to 1900 and is applicable to everyday experience. It is applied in modern engineering and technology from predicting weather to launching a rocket or building a bridge. Modern physics refers to the collection of concepts of quantum physics and relativity developed in the first half of the 1900’s. These concepts embody the study of tiny (subatomic) particles or lightening fast speeds. They find applications in technologies such as atomic energy or semiconductors.

Aristotle may be considered one of the earliest physicists. He popularized the notion that Earth was the center around which the universe revolves. In his view rocks released from a height came to rest on the Earth because they were made primarily of earth. Smoke rose because it contained fire (the stuff of the Sun and the heavens). Heavy objects fell faster than lighter objects. Matter was continuous, and earth, water, air and fire fused to transmute into all the substances we see around us.

Aristotle’s ideas eventually lost popularity to ideas formulated by Dalton, Galileo, Copernicus, and Newton. Aristotle’s geocentric model of the universe was replaced by the heliocentric model of Copernicus. We eventually learned objects left entirely on their own would travel in a straight line at constant speed, planets were held in orbit by gravity, and objects in a vacuum fell with the same acceleration. Aristotle’s notion of continuous matter and four elements gave way to atomic theory. By the 1900’s we were aware of many elements. We believed that matter was made of particles, and if a molecule of water was broken apart we would be left with atoms of hydrogen and oxygen – which we believed could not be further broken down. During the dawn of the 1900’s, atomic theory progressed with the discovery of subatomic particles led by the efforts of Thomson, Chadwick, and Rutherford.

There was also competition between continuous and particulate models of light. Prior to 1900, James Clerk Maxwell had argued with success that light, electricity and magnetism were like continuous fluids that traveled in the form of waves. However, his equations did not fit experimental data from light given off by hot objects. Modern physics was born in 1900 when Planck showed that the experimental data could be accurately described by assuming that energy was absorbed or transmitted in discrete bundles or “quanta.“ In 1905 Einstein further provided support for particles of energy. The basic tenet of quantum physics is that matter and energy are quantized. In other words, matter and energy exist and interact as discrete particles.

Quantum physics is one branch of modern physics. The other branch is relativity, which deals with how

observers see events. Einstein introduced the special theory of relativity in 1905. One tenet of the special theory of relativity is that all systems that appear to an observer to be stationary or to be moving in a straight line with constant speed, obey the same laws of physics. Suppose two children are sitting in the back seat of a parked car playing catch. The familiar motion of the ball is described by the laws of physics. Suppose a second pair of children is playing catch in a car traveling along a straight smooth highway at 50 km/h, and a third pair is in a car parked on an Earth-like planet in a galaxy traveling away from the Earth at 80% of the speed of light. If an observer in one of these three cars could also see the balls in the other cars, all the balls would exhibit exactly the same behavior as in his own car. If all three cars were to speed up in the same direction by 50 km/h in 5 seconds, the observer in one of the cars would see the ball curve through the air in the same way in all the cars.

The second tenet of the special theory of relativity is that properties of electromagnetic radiation are the same regardless of the speed of their source. Microwaves are a form of electromagnetic radiation. A microwave oven on Earth would be observed (if possible) by the Earth bound person to work the same way as a microwave oven on an Earth-like planet in a galaxy traveling away from the Earth at 80% of the speed of light.

These tenets do not seem to have great significance when discussing common events; however the results are surprising when applied to subatomic particles and objects moving near the speed of light. Because of its large mass, we can be relatively certain of the momentum (velocity and mass) and location of a car. However, when quantum physics is applied to miniscule particles such as electrons, we find we can not precisely determine both the location and the momentum of the particles at the same time.  These particles can only be said to have varying probabilities of being in different places, which gives them the ability to show up on either side of seemingly impenetrable barriers.  Further, we are used to unvarying standards for measuring space and time.  Car parts are manufactured in different cities and are fit together in one assembly plant to be fabricated into the final product.  This is possible because in our usual experience an inch is an inch and a centimeter is a centimeter for everyone, and the parts all arrive at the plant at just the right time because all our clocks and calendars are synchronized.   However, when relativity is applied to objects approaching the speed of light, time and distance become elastic. Distance shrinks in the direction the object is traveling, time slows down, and events that are simultaneous to a slow moving observer are no longer simultaneous.

Despite the divergence of modern physics from Aristotle’s beliefs, physicists agree with the most important aspect of his world view.  Aristotle had faith that nature was understandable. He also had faith in a grand, overarching plan for the physical universe and that this plan would be verified through experimentation. This is the physicist's quest.

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