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### Course: Physics library>Unit 1

Lesson 1: Introduction to physics

# What is physics?

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:
1. 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.
2. 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.
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.

## Want to join the conversation?

• What is G-Force
• A G force is when you feel your same weight against you. for example a roller coaster is 2 Gs,which means you will feel 2x your weight which ever direction the roller coaster is going.
• Does the study of physics alter what we discover in mathematics?
• For sure. Newton invented calculus to solve the specific problem of calculating planetary motion. Bessel functions were invented to solve the spherical harmonics of electron orbital wavefunctions.
• anyone on here just to try to get ahead before school starts up again .
• Yes. Lol.
• I just watched Oppenheimer, it sparked a chain reaction to understand physics.
• This wasn't mentioned in the text, but what is dark matter? Why is there so much of it, but it is unobtainable?
• dark matter is matter that doesn't emit or absorb light. We can't directly measure it.
• Can Newton’s 1st and 2nd law of motion both be active at the same time?
• Sure. Newton's first law states that an object at rest or moving with a constant velocity will stay at rest or stay moving with a constant velocity as long as no force acts upon it. Newton's second law states that the net force on an object is equal to the mass of the object times the acceleration of the object. So if an object is moving with constant velocity and no force is acted upon it (1st law), you can calculate with Newton's second law that its acceleration is zero. In this sense both laws are active in the same time.
• why is there no gravity in space? and why do we have gravity?
• There actually is gravity in space: the whole reason the Earth orbits around the sun is because the sun is a 'heavy' object that makes an impression on the fabric of spacetime, like your body would on a trampoline. The sun's sheer mass makes the earth and all of the other planets orbit around it because the gravity is so strong on this cosmic scale that the Earth will never actually stop spinning and be incinerated by the sun: what will likely happen first is the sun will become a red giant near the end of its life and swallow up Mercury, Venus, and Earth.
The reason we have gravity is because gravity is a property of matter, anything that has mass and takes up space. Fundamentally, all matter attracts other matter, and the bigger the object, the stronger gravitational pull it has.