2.3 Newton’s First Law, Part 1: Imaginary Physics Problem Land

Critical Questions:

  • Why do we tend to get Newton’s Laws wrong in our heads?
  • Why do physics problems always involve “frictionless slopes” and “massless ropes”?

Imagine an object. Think of something with a bit of weight to it, like a small wooden treasure chest or a fresh loaf of good, dense olive bread. Or, once again, you can go ahead and try this simple experiment in real life.

Pain_ciabatta_aux_olives_noires
Afterwards, eat the bread.
Now imagine that you start to push this object. You keep pushing until it’s got a good speed going, and then you let go. What happens next?

You have watched this exact scenario play out an uncounted number of times in your life, and so you probably have a pretty good idea of the results. The object will move forward for a little while, and then eventually it will slow down and stop.

After observing this behaviour over and over again, most people develop an unconscious version of Newton’s First Law in their own heads. Their version goes something like this: Everything, no matter how fast it’s going, will eventually slow down and stop. And, surprise surprise, this is wrong.

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Stuff a-Brewin’

In case you haven’t noticed, I’ve been posting chapter updates twice a week since the site began. But this week I’m tutoring like a maniac as final exams approach. So if you’re sitting there waiting desperately to find out the connection between forces and Isaac Newton, you’re just going to have to be patient.

The good news is that rather than resting on my laurels, I’ve got some fun and exciting plans for the site that will soon be announced formally. Real-life Pop Physics events! Interviews with actual physicists! Cute dog videos!

Please stay tuned.

2.2 Forces

Critical Questions:

  • What makes things move?
  • What’s happening to the molecules in my hand when I reach up and touch something solid, like a door?

As a teacher, I’ve always found it easy to define the term force. Ready? It is a push or a pull. Identifying forces in real life, though, can a bit more tricky.

Of course there are the obvious ones. Push open a door, and you’ve applied a force to it. Throw a ball, ditto. Pull yourself up a rope, lift a fork laden with pasta to your mouth, or punch someone in the kidney: all forces.

Hadouken - Street Fighter
The Hadouken special attack: definitely a force.

But forces can, of course, get a lot more complicated. For example: if you push down on the gas pedal, that’s an obvious force, but what force pushes the car forward? Of course there’s an engine and the engine spins the wheels, but what is actually pushing the car? Not the wheels, exactly — we’ll have to get back to that one.

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TEDx Talk: How to Save the Earth from Asteroids

Here’s a TEDx Talk by astronomer Phil Plait (who also blogs for Discover Magazine at Bad Astronomy) about some of the asteroids that have already collided with Earth and what we can do if another big one heads our way.

He briefly mentions the fact that blowing the thing up with a nuclear bomb (as in the movie Armageddon) would probably be useless. This was expanded on in a video featuring Neil deGrasse Tyson,1 wherein he explained that if you blow up an asteroid the size of Texas that’s heading towards the Earth, chances are you’ll just end up with two half-Texas-sized asteroids heading towards the Earth.

  1. I think; but even if it wasn’t him, it’s a good default answer.

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.

Sir Isaac Newton 1702
He was also the main aesthetic inspiration for the 'hair' bands of the late 1970s.
Around 1666, Newton locked himself in his room for a while and, basically, invented calculus.1 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.

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  1. If any math historians are currently reading this, please forgive the impreciseness in this paragraph.

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.

Dog fetching ball
Warning: the presence of an actual dog may affect the outcome of this experiment.

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.

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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.

The Batmobile
Fig. 1: The Batmobile, which doesn't break the laws of physics, but certainly bends them.
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.

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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.

Mir from Space
Unless you are reading this in space, in which case: awesome.

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.

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