## 2.1 Introduction to Isaac Newton

Isaac Newton is probably one of the smartest people of all time. Aside from discovering the foundations of physics, he was also the first person to describe the force of gravity. He designed the first practical reflecting telescope and explained how colours work based on the phenomenon of white light splitting into a rainbow after passing through a prism. He has been credited with inventing ridge-edged coins (to fight counterfeiting) and the cat-flap door (seriously), and was an influential religious philosopher. But my favourite story about Newton is the following.

Around 1666, Newton locked himself in his room for a while and, basically, invented calculus.[1. If any math historians are currently reading this, please forgive the impreciseness in this paragraph.] Calculus is a set of concepts and techniques, completely new to the usual addition-subtraction-multiplication kind of math, which allowed people to finally use numbers to describe changes — like the change of position (velocity) or the change of velocity (acceleration). But despite the enormous importance of this invention, for some reason, Newton didn’t tell anyone about it for years afterwards. He mentioned some of the basics in an annotation to a footnote somewhere, and actually used calculus in his major physics works, but never published the original paper on calculus itself. A few years later, a man named Gottfried Wilhelm Leibniz also invented calculus, completely independently of Newton’s work. Newton got fairly upset about this, accusing Leibniz of plagiarizing from, well, the papers that he had failed to show anybody.

## 1.3 Falling Objects

Critical Questions:

• How does something move when it’s falling through the air?
• What exactly happens when you throw a ball up and it falls back down?

Before we leave this chapter behind and start getting into the real meat of physics, I’d like to discuss one more topic: falling objects. This will complete our picture of simple motion and set the stage for the chapter on Newton’s Laws.

Imagine that you hold a ball in your hand. Picture one that you feel familiar with — a baseball, or a tennis ball, or your dog’s little red plastic chew ball (better yet, pick up an actual object and use that instead). Now imagine holding that ball out in front of you and letting go.

If you’ve read along carefully so far, you should feel quite confident in describing the ball’s motion: it starts with no velocity, then accelerates downwards. Easy.

## 1.2 Speed and Acceleration

Critical Questions:

• What happens in a car when you push on the gas pedal or step on the brake?

As a physics teacher, I blame a lot of my problems on cars. People spend hours every day in cars of one kind or another, and they’ve developed strong ideas about the relationships between the gas pedal, the engine, and the car’s movement. I will soon try to convince you to think differently about those relationships, but for now I’m going to make use of what you already know.

Speed is the easy one. If you want to know how fast a car is going, just glance at its speedometer. Unfortunately for high school physics students, however, it is quite easy to complicate even such a simple concept as speed. This practice goes all the way back to pre-Socratic Greece, when the philosopher Zeno asked how a flying arrow could both occupy a space and yet also be in motion, a question which eventually forced mathematicians to give up and invent calculus. Nevertheless, for the purposes of this site, “speed” (or “velocity”) means nothing more than how fast a thing is travelling.

We’re going to have to be a bit more careful about acceleration, though. Not only is the physics definition of acceleration slightly different from the everyday one, it also represents our first tricky concept — one that you might find difficult to wrap your head around.

## 1.1 Introduction to Motion

It may seem strange to begin a site about the mysteries of physics with a chapter on motion. After all, moving is something we do every day, all of the time. It doesn’t usually seem very mysterious. You may think that you are sitting still right now, staring at your computer, possibly with your legs propped up on the corner of your desk or a small, obedient child.

But that is not the case. In fact, just beneath you there is a fantastically large piece of rock which is spinning around in space. Being on the surface of this hunk of rock, you are moving with it — at speeds of up to 1,674 kilometers per hour (or 1,040 miles per hour). That is a few hundred kilometers per hour faster than the speed of sound.

And the planet does more than just spin — it’s also orbiting the sun. It may take a while to complete one full orbit (one year), but in that time it has gone a long way — almost a billion kilometers. We’re moving around the sun at a speed of over one hundred thousand kilometers per hour.

## Feynman on Explaining Physics

Here’s a video of physicist Richard Feynman describing why it can be problematic to use physics to “explain” phenomena. It takes him a while to get to the point he’s trying to make, but I recommend that you stick around until the end. The goal of physics is to answer “why” at deeper and deeper … Read more

## 0.2 Thinking Like A Physicist

Critical Questions:

• What does it take to really understand physics?

The crucial problem in teaching physics is that most people have a certain understanding of physical principles before they ever approach the subject in a classroom or book. The reason this is a problem rather than a benefit is that the average person’s understanding of physics is wrong.

To be more specific, there are two kinds of “understanding” one can have about physics. The first is the one that babies slowly gain as they teeter precariously on their pudgy little legs and try to manipulate solid objects with their hands and, occasionally, mouths. This is baby science in action: after a few hundreds trials, even an infant’s brain knows that if you push an object to the right, it will generally move in that direction.

By the time we’ve grown up, this understanding has solidified into that intuitive, unconscious awareness of the relationship between cause and effect which allows us to catch baseballs, flip pancakes, or juggle chainsaws.

## 0.1 Welcome

One day, when I was in high school, my chemistry teacher began a lesson about subatomic particles.

“You know those electrons and protons you’ve been hearing about the last few years?” she asked.

“Yes,” we students dutifully replied.

“You know how all of your teachers have always said that they’re the smallest things we know of?”

“And you know how you’ve always been told that they’re the fundamental building blocks of matter — that it’s impossible to break them apart into component bits?”

“Of course!” we scoffed — although nervously, because already we could sense that perhaps we had been misled.

“Well,” said my teacher, “it turns out that protons are in fact made up of even smaller particles called ‘quarks’. And that’s what we’re going to learn about today.”

Needless to say, I was outraged. I felt like a kid hearing that Santa Claus doesn’t really exist. What were they going to tell me next? What other lies had I been swallowing in school? Was two plus two ever really four? Were sentences that ended with a preposition really that big of a problem?