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Forces of Nature

WebLecture: Observation and Error

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Forces of Nature Weblecture

Introduction to the Study of Physics

Reading

Science and Observation

Scientists seek to know the secrets of the universe. For some, like St. Augustine, this is a sacred duty: in discovering the laws of nature, we catch a glimpse of the mind of God the Creator. For some, the thrill lies in discovering a new way of seeing things that opens our minds to endless possibilities (do note that these are not mutually exclusive!). In any case, scientists must make some basic assumptions about nature, whether they think it is divinely ordained or not:

Without these assumptions, scientific investigation would be a waste of time: we would never be able to depend on any set of observations to give us useful information about the current or future state of things.

Since we do assume cause-and-effect relationships, and patterns of behavior, scientists are constantly on the lookout for phenomena that are

In order to find these relationships, scientists conduct observations, which may be experiments (when they can control sufficient situations), field observations (when they can't isolate all the objects or put them in a lab, or when putting them in a lab will affect the outcome), or historical surveys (when the only thing available is lots of data over long periods of time). They attempt to classify similar objects (using various and sometimes contradictory criteria that result in different classification systmes emphasizing different characteristics). Analysis of the data from any of these methods may reveal either the periodicity (which indicates that factor X depends on a regularly changing factor Y), or a proportional dependency (if X goes up, Y goes up, Y goes down, Y doubles, Y squares, Y goes down by the inverse square....).

Looking for these categories and relationships is what scientists (and you too) do every day. Finding a new one may mean fame, fortune, and a Nobel prize for a professional scientist, but understanding and recognizing similar relationships can be immensely satisfying even if your livelihood doesn't depend on it.

And for those of us who are so inclined...such discoveries do indeed give us insight into the mind of the Creator God we believe designed and still cares for His creation.

Measurement and Error

One might say that measurement is what turns a casual look into scientific observation. Measurements allow us to quantify and compare, and the instruments we use for measuring help us achieve some objectivity in reporting a phenomenon.

There are really only a few kinds things that we can measure directly. Each of these base measurements has a unit system that is widely recognized and used in the scientific community.

  1. Distance: meters
  2. Time: seconds
  3. Mass: kilograms
  4. Temperature: Kelvin
  5. Amount of a substance: mole
  6. Electric current*: ampere
  7. Candela: luminous intensity

All other quantities are derived or expressed in combinations of these units. Density is a ratio of mass to volume, where volume is simply three distances combined in three dimensions: gram/cubic centimeter or kg/m3. Velocity is distance/time: meters/second. In calculating any derived quantity, it is critical to pay attention not only to the numbers or quantities but to the units themselves.

One of the units above is a counting unit: a mole, like "one dozen", is a specific number of distinct whole objects (atoms, molecules, cars, stars). It is possible to be exact when we are counting: we can count 12 whole cookies, no more, no less, with accuracy and precision.

All of the other units describe measures in a continuous dimension. We measure these quantities by comparing them to some scale, and the error in our measurements depends on the division of units on the scale. We use two concepts to evaluate our measurements:

It is possible for a set of data to be precise without being accurate if our equipment is mis-calibrated, so it is necessary to have some idea of the range of values we should normally expect when making a particular observation.

The Subject Matter of Physics

A Brief History of Natural Philosophy

We only have time to touch briefly on a few points in establishing the scope of our study of physics.

In the classical and later in the medieval world view, the study of natural phenomena was a single, all-encompassing area of investigation called natural philosophy, which undertook to explain all the characteristics and behaviors of matter in all its forms. Aristotle's theory, which formed and guided Western European attempts to explain these phenomena from about 300 B.C. until around 1500 A.D., divided terrestrial matter into four types: Earth, Fire, Water, and Air, each with its own pair of primary characteristics (wet, dry, hot, cold) and each with its own natural motion (up or down). Celestial matter was of a different kind altogether, not subject to corruption or generation, perpetually moving in perfect circles. Living matter possessed the ability to animate terrestrial matter beyond its natural motions, but all plants and animals still partook of the four elements in their living forms of black and yellow bile, blood and phlegm. The balance and interaction of these fluids dictated both the physical and mental health of individuals. Thus Aristotelian natural philosophy provided a unified world view where all matter, celestial and terrestrial, living and non-living, moved according to their various natures, unless constant force or power was use to impel them in a different direction.

The Renaissance challenged Aristotle's world view on several fronts: Vesalius disputed the physiology of the humors, Copernicus challenged the central place of the Earth in the universe, and ultimately, Newton and Gilbert redefined the concept of a natural force within matter itself. Matter, whether part of living or non-living objects, whether celestial or terrestrial, whether Earth or Water, Gold or Mercury, possessed a force that depended only upon the amount of matter involved. The study of types of matter and their interactions became its own specialization: chemistry. The study of the heavens, despite their common matter with earth, became astronomy. The study living things became biology. What was left — the study of matter and its response to its own characteristic forces — became physics.

The study of physics today covers these major areas, which are often divided and organized in different ways to suit the pedagogy of the particular textbook author:

  1. Kinematics: the study of motion, including momentum and kinetic energy
  2. Dynamics: the action of forces on moving bodies
  3. Thermodynamics: the study of energy flow, work, and conservation in systems over time.
  4. Special applications of mechanics:
    • Distortion and flow in solids and liquid substances.
    • Periodic motion (waves)
    • Static Equilibrium (engineering)
  5. Forces and force fields:
    • Gravity
    • Electrical charge, fields, and currents
    • Magnetic fields
  6. Electromagnetic waves, light, and optics
  7. Relativity: motion, energy, and forces at high speeds ( more than 10% the speed of light)
  8. Quantum mechanics: atomic structure, motion and energy
  9. Nuclear reactions (often also studied in chemistry!): fission and fusion

Physicists consider their science fundamental to all other scientific specializations. The concepts of atomic structure, electrical charge, and thermodynamics underly chemical explanations of molecular bonding and the ideal gas law, as well as the behavior of acids and bases. The laws of nuclear fusion explain energy generation in stars; the laws of gravity and relativity explain the structure and behavior of solar systems and galaxies. In turn, chemical concepts based on fundamental physical ideas explain cell structure and metabolic reactions, energy exchange and transformation in living organisms.

You might think that physics should be the first subject that we study in high school, but it is often the last, because it is considered "advanced". By its very nature, physics deals with concepts that are abstract and often not directly observable, or even easily comprehended through analogies. Many of its more detailed descriptions rely on advanced mathematical techniques. Recognizing these challenges will help us identify when we are running into trouble understanding what is going on. Precision is key to any scientific study, and it begins with observation.

Characteristics of Matter — for Physicists

For our purposes, we are interested in only a few characteristics of the matter we will study in this course.

For Discussion: