Let's think about what physics is and what topics are covered in an introductory physics course.
What is physics?
To be honest, it’s really difficult to define exactly what physics is. For one, physics keeps changing as we progress and make new discoveries. New theories don't just bring new answers. They also create new questions that might not have even made sense when viewed from within the previous theory of physics. This makes physics exciting and interesting, but it also forces attempts at defining physics into generalizations about what physics has been rather than what it might be at some point in the future.
That said, definitions are useful. So, if it’s a definition you want, it’s a definition you’ll get. For the most part, physicists are trying to do the following:
Precisely define the most fundamental measurable quantities in the universe (e.g., velocity, electric field, kinetic energy). The effort to find the most fundamental description of the universe is a quest that has historically always been a big part of physics, as can be seen in the comic image below.
See Also1.1: Physics- An IntroductionPhysics | Definition, Types, Topics, Importance, & Facts1.1: The Basics of PhysicsThere are some measurable quantities that are not considered fundamental, like the population of Ireland or the value of the stock exchange. Even though those quantities are measurable, and theoretically might be describable using an unthinkably complex application of the fundamental laws of physics, we would not typically consider theories about such things to be physics. Exactly where the line is drawn between fundamental and not fundamental is a little vague and shifts with history.
A rough guide is that if you were to reply with the word “why?” over and over to whatever explanation a professor were giving you, the last and best scientific answer he or she would give you is most likely to be a law of physics, assuming they don't throw you out of their office before that point.
Find relationships between those fundamental measured quantities (e.g., Newton’s Laws, conservation of energy, special relativity). These patterns and correlations are expressed using words, equations, graphs, charts, diagrams, models, and any other means that allow us to express a relationship in a way that we as humans can better understand and use.
Believe it or not, the principles, graphs, laws, and equations used in physics were not created to make explaining the universe more difficult; they are there to make it easier to explain the universe and precisely illustrate the patterns found within it. This might serve as little solace to a physics student struggling with a seemingly random and unrelated set of equations and definitions, but try to remember that underneath all of the intimidating and sometimes foreign-looking mathematical symbols, there is usually a simple conceptual idea. The math simply makes it possible to formulate a conceptual idea in a very precise and concise manner.
Image credit: Adventures Inside the Atom, 1948 General Electric, George Roussos (public domain)
OK, so boiling physics down to only two things is admittedly a bit of a gross simplification and glosses over some of the finer points of what physicists do and how they do it. But trying to describe a complex universe with simple and useful clarifying laws is what physics is all about. So maybe trying to describe the complex activity of what physicists do with a simple and clarifying definition isn’t such a bad idea after all.
What will I learn by studying physics on Khan Academy?
In physics, we want to explain why objects move around the way they do. However, it would be hard to explain motion if we didn't know how to describe motion. So first, in the topics One-dimensional motion and Two-dimensional motion, we'll learn how to precisely describe the motion of objects and predict their motion for some special cases.
With the ability to precisely describe motion under our belt, we'll learn in Forces and Newton's Laws how the concept of force allows us to explain why objects change their motion.
We'll continue mastering and expanding our ability to deal with motion by showing that conservation laws are an alternative way to explain the motion of an object. These conservation laws give constraints on how the motion of a system can change. Conservation of energy will be learned in Work and energy, and conservation of momentum will be learned in Impacts and linear momentum.
Up to that point we'll have mostly considered objects that are not changing their rotational motion, so in Moments, torque, and angular momentum we'll learn how to describe and explain rotational motion and pick up a new conservation law along the way—conservation of angular momentum.
After this point, we'll deploy what we learned about motion, forces, and conservation laws to analyze how to deal with a variety of new forces and phenomena. We'll learn how to deal with liquids and gases in Fluids and Thermal physics. Then in Electricity and Magnetism we'll learn about two new forces—the electric force and the magnetic force. In Circuits we'll see how electric forces cause current to flow. In Optics we'll investigate the ways in which electromagnetic waves (i.e., light) can bend and reflect. Once we learn about light, we get to learn Einstein's theory of Special relativity. And that's just to name a few.
By the end you should have a nice understanding of introductory physics and the mathematical tools physicists use to describe and explain the universe. But no summary can describe all the interesting and powerful aspects of physics. The best way to find out is to jump in and see for yourself.
