2.4
Newton’s First Law, Part 2: Inertia

Critical Questions:

• How do things move when they’re floating through space?
• What is inertia?
• Why can’t you move your sailboat by pointing a fan at the sail?

Let’s head off into Imaginary Physics Problem Land (see the previous post) in order to understand the basics of Newton’s laws before we go deeper down the rabbit hole.

When discussing Newton’s First Law, the specific IPPL we’ll use is one in which we ignore not only friction, but also all other forces that might interfere. So imagine yourself floating in space, in a spacesuit, with that solid object that you had at the beginning of the previous section.

Space is useful to us here for a few reasons:

1. It has no air, so we don’t have to worry about air resistance.
2. It has no gravity, because if you’re far enough away from any planets or stars then you don’t feel any gravitational pull.
3. It has no flat surfaces like tables, so we don’t have to worry about friction.[1]

Now take your object in hand (or space-glove), start pushing it until it’s going at some observable velocity, and then let go. What happens next?

In my mind, this scene always plays out like a Hollywood movie. The object floats serenely by the camera; it might spin gently around as it travels, and there might be some classical music playing in the background to emphasize the scene’s tranquility, but other than that, its motion is constant.

More specifically, it moves at a constant speed in a constant direction — it does not accelerate at all… unless it bumps into something, like the outside of a spaceship, or comes too close to a planet and crashes down onto the surface. But without anything to get in its way, the object will just keep on moving at a constant velocity forever.

Ok, I made a bit of a leap there in saying “forever”. Perhaps you weren’t thinking about eternity. Perhaps you thought that the object would still eventually slow down, even if it didn’t run into anything. But the truth is that if nothing ever got in its way, this thing would keep moving at that constant velocity forever.

To investigate the other half of Newton’s First Law, we need to do one more quick experiment. Go back into space and hold the object, except this time just let go of it without giving it any kind of push. This actually happened in Stanley Kubrick’s film 2001: A Space Odyssey, by the way, when a pen was left floating inside a space ship and an attendant later came by to grab it out of mid-air.[2] Although this scene occurs in the world of science fiction, it also happens to be accurate: if something is not moving, it will stay stationary until something else causes it to move.

The helmet is more for aerodynamics than anything else.

And so we’ve finally arrived at Newton’s First Law. It is a two-parter: the first part says that if an object is not moving, it will remain stationary unless an external force causes it to do so; the second part says that if an object is moving, it will continue to move at the same speed and in the same direction until an external force acts on it.

If, at this point, you are not feeling the kind of dumbstruck awe that those statements might have evoked when they were first published, you may be forgiven. But in describing these behaviours, Newton pinpointed an important fundamental fact about the motion of matter, and it was only by understanding the first law that the second was made possible.

This property of matter — that it tends to keep doing whatever it is doing — is called “inertia”. Some people have trouble with that term, because they tend to think of it as a quantity: they might think that something which is moving quickly might have more inertia than a slow-moving object (the quantity they’re thinking of here is momentum). In fact, inertia is not a quantity at all but a property of all matter. Everything[3] ‘has’ inertia, which simply means that it is made of matter and its motion will not change unless a force causes it to change.

So if we return to our everyday understanding of motion, which says that moving things tend to slow down, we can now see exactly why we were mistaken. Slide something across a frictionless table in a room with no air resistance, and it’ll go on forever (assuming you have an infinitely long table in an infinitely long room). Add in friction, and it’ll slow down until it stops.

There’s one last thing I’d like to point out about Newton’s First Law, and that’s my use of the word “external” to describe the types of forces that can change an object’s motion. An external force is, simply, a force that comes from outside of whatever is moving.

The easiest way to explain this is using the example of a fan on a sailboat. If Wile E. Coyote put wheels on a sailboat in order to catch the roadrunner, but then found that he couldn’t move due to a lack of wind, he might stand on the deck and point an electric fan at the sail in order to fill it and get himself rolling.

This might look plausible in a world where cartoon animals stand motionless in mid-air for a moment before realizing they’re about to fall, allowing them time to hold up a sign saying “Uh-oh”. In real life, however, the force provided by the fan is an internal force, because if the boat were to move, it would take the fan along with it.

(Edit: Mythbusters disproved this, meaning that Newton’s Laws and in fact, all of modern science are incorrect! No, just kidding, but this isn’t exactly an internal force. Read more and watch the video here.)

Try it yourself: stand up right now and push on your back with one of your arms. Did you fall forward? If so, you might be drunk, and you are in no condition to read a website about physics. In order to move his sailboat, that rascally coyote would need a wind source that stays outside of the boat — in other words, an external force.

I think we all know what it's like to realize you're standing on nothing above an improbably high cliff in the Arizona desert.

This should make some intuitive sense. After all, there are all kinds of forces acting inside of any moving object. Throw a ball through the air, and the intermolecular electromagnetic forces holding all those particles together certainly won’t affect your throw.

But in order to understand precisely why only external forces can affect an object’s motion, we’ll need to understand Newton’s Third Law, which is only one chapter away.[4]

In the meantime, let’s take a look at the big one: Newton’s Second Law.

Big Ideas

• Newton’s First Law says that if an object is not moving, then it will remain stationary unless an external force causes it to do so, and if an object is moving, it will continue to move at the same speed and in the same direction until an external force acts on it.
• An external force is a force provided by something outside of the moving object (or set of objects).
1. Actually, all three of these are not quite correct, and yes, one tends to say that a lot when one is talking about physics (because the point of physics is to continually update and correct previous ideas). As we’ll discuss later, although the force of gravity gets weaker the farther you are from an object, it still acts at infinite distances, so there is no spot in the universe which experiences no gravity at all, unless everything everywhere is exerting exactly the right amount of gravitational pull to cancel everything else out. Also, things like interstellar gas and dust can cause friction, and in an absolute vacuum you’ve got particles appearing and disappearing due to quantum fluctuations. But even in real life, these effects are minimal and can often be ignored.
2. This was in the days before computer graphics could create an entire character, complete with googly eyes and some kind of Jamaican accent. The effect, in this case, was achieved by affixing the pen to a piece of glass, which was held in front of the camera and spun slowly around.
3. Everything made of matter, that is. Things that aren’t made of matter (e.g. light) obey other rules, which we’ll talk about soon.
4. Here’s a hint: all internal forces are always cancelled out by other internal forces.
2.4 Newton’s First Law, Part 2: Inertia   (from Chapter 2: Newton's Laws)