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.

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.

Okay, it’s not exactly wrong, per se. In fact, once you gain a complete understanding of the ways in which this version of Newton’s First Law is (kind of) correct, you’ll be able to sound more intelligent the next time someone tries to sell you a perpetual motion device, which, believe it or not, actually still happens from time to time.

But in terms of fundamental physics, that version is wrong. The reason this version of the law seems right to us is because of a force called friction. Friction is everywhere, always busily holding things in place or interfering with their motion by slowing them down. Friction is a force that occurs whenever two surfaces are rubbing against each other, and it is impossible in any practical sense to avoid it. When you slide a solid object across a table, the reason it slows down is because the force of friction is slowing it down for you.

I’ll explain friction in a bit more detail very soon, but in the meantime, I’d like to introduce a new idea. It is one which physicists have become so used to that they often forget to mention it when teaching the subject to neophytes. I call it Imaginary Physics Problem Land, or IPPL (pronounced “nipple”, but without the n). IPPL is a fantastical place in which any aspect of real life that would confuse or complicate a hypothetical situation can be whisked away using a few magic words. Textbooks invoke IPPL when they say something like, “Friction can be ignored.”

They do this because, as I just mentioned, friction likes to barge in on nice, simple physics problems and make the math more complicated or obscure the fundamental concepts at work. Another way to set your problem within IPPL is to say that it involves something like a “massless, inextensible rope.” You can learn a lot about classical mechanics using these kinds of ropes, even though they do not exist in real life.

Indiana Jones whip
Indy’s whip defies the laws of physics too, but I’m fairly sure it’s all fiction.

As you can imagine, IPPL causes a lot of angst for the intelligent, thoughtful student. Once all of these imaginary conditions have been placed on physics problems, the entire subject begins to seem like a purely fanciful exercise — like a lot of people sitting around in lab coats drinking scotch and asking what would happen if the entire planet suddenly turned into a giant mongoose.

But the most important thing to understand is that the conclusions that can be drawn from thinking about IPPL are still perfectly valid when applied to the real world, as long as you remember to add friction or the rope’s mass back in when you’re done. In the beginning, we ignore certain things in order to simplify the conceptual and mathematical parts of the problem, but we always put as many things back in afterwards once we’ve got them figured out.

In fact, if you look at the history of physics, it follows exactly this pattern. When Newton was working, he didn’t know about things like general relativity or quantum thermodynamics, and so his equations leave them out. The things we’ve learned since then haven’t proven him wrong, exactly, but they have led to more and more precise calculations by considering more of the factors that had previously been unaccounted for.

Big Ideas

  • In order to understand the fundamentals of physics, we often consider things that happen in IPPL (Imaginary Physics Problem Land), where we can ignore factors such as friction, gravity, or the masses of certain objects.

Next: 2.4 – Newton’s First Law, Part 2: Inertia

Previous: 2.2 – Forces