Physics 2020 – Introduction to Principles of Physics
Lecture 1 Notes
Defining “Physics”
What is Physics? Take a moment to think about this and write down what you think Physics is.
Now, according to Merriam-Webster, physics is “a science that deals with matter and energy and the
way they act on each other in heat, light, electricity, and sound”.
According to Google, physics is “the branch of science concerned with the nature and properties of
matter and energy. The subject matter of physics, distinguished from that of chemistry and biology,
includes mechanics, heat, light and other radiation, sound, electricity, magnetism, and the structure of
atoms.”
We read at Physics.org: The dictionary definition of physics is “the study of matter, energy, and the
interaction between them”, but what that really means is that physics is about asking fundamental
questions and trying to answer them by observing and experimenting.
Put another way, physics is asking 'nature' a good question (experiment) and 'listening' carefully for the
answer (observing).
The language of physics is mathematics. Reading further at Physics.org: Many apparently
complicated things in nature can be understood in terms of relatively simple mathematical
relationships. Physicists try to uncover these relationships through observing, creating mathematical
models, and testing them by doing experiments.
The last phrase is crucial. Physics is an experimental science and thus, as Richard Feynman bluntly put
it: "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't
agree with experiment, it's wrong."
It was once thought, long ago, that if one took a small rock and big rock to the top of a tower and
dropped them at the same time, the big (heavier) rock would hit the ground before the small (lighter)
rock. While this sounds quite reasonable, experiment actually shows that objects of different weight hit
the ground at the same time; they fall at the same rate. One famous example of such an experiment
was performed in 1971 by Apollo 15 astronaut David Scott on the Moon. Astronaut Scott dropped a
hammer and a falcon feather (their lunar lander spacecraft was named Falcon) at the same time and
both hit the surface of the Moon at the same time. So the ancient theory of gravity which held that
heavier objects fall faster than light objects is, no matter how reasonable, wrong since it does not agree
with experiment.
Defining “Scientific Theory”
What exactly do we mean by theory or better, scientific theory? A scientific theory is, according to the
“Conceptual Physics” textbook, “a synthesis of a large body of information that encompasses welltested and verified hypothesis about certain aspects of the natural world” and a hypothesis is “an
educated guess, a reasonable explanation of an observation or experimental result that is not fully
accepted as factual until tested over and over again by experiment.
Putting this together, a valid scientific theory
•
must be able to make predictions
•
must take into account all relevant experimental data
•
must have the possibility of being disproven
This last point is crucial; a theory that cannot be disproven, in principle, is not a scientific theory. Can
you think of some examples of non-scientific theories that cannot be disproven in principle?
Note that since a scientific theory must have the possibility of being disproven, a scientific theory can
never be proven by experiment! That is to say, a scientific theory is not proven no matter how many
experiments are done confirming the theory's predictions.
However, a scientific theory that has has passed and continues to pass numerous and exhaustive
experimental tests are considered correct (until proven otherwise).
Defining “Scientific Law”
Lastly, there is the concept of scientific law which “Conceptual Physics” defines as “a general
hypothesis or statement about the relationship of natural quantities that has been tested over and over
again and has not been contradicted. Also known as a principle.”
For example, as we'll learn later in the course, we have the law of Conservation of Energy which you
may have heard expressed as energy cannot be created or destroyed but can only change form. Such a
statement is not a theory but rather a principle that makes a (very deep) statement about the nature of
reality as we understand it.