I’m So Totally Over Newton’s Laws of Motion
I saw it again. This time it was an introduction to an interesting online look at introductory physics. I was excited—but then, I wasn’t so excited. Right there in the first video they talked about how they were going to go over Newton’s Three Laws. We can do better than this. We don’t need to be stuck with the traditions of the past if we want students to understand physics.
What Are Newton’s Three Laws of Motion?
Just to be clear, let me state the Three Laws of Motion. You will find these in just about every introductory physics textbook (but not all—there are some pretty awesome textbooks out there). This is essentially the description from most books:
- Newton’s First Law: An object in motion stays in motion unless acted on by a force. An object at rest, stays at rest unless acted on by a force.
- Newton’s Second Law: The magnitude of an object’s acceleration is proportional to the net force and inversely proportional to the mass of the object.
- Newton’s Third Law: For every force there is an equal and opposite force. (I’ve already complained about the way most books talk about this one)
Yes, some books do it a little different—but that’s the main idea.
Newton’s First Law Is Really About Aristotle
I’m not sure this is exactly true, but it seems true enough. Remember that before Galileo and Newton, people looked to Aristotle for ideas about physics. Yes, it’s true that Aristotle wasn’t a scientist since he didn’t really do any experiments. However, that didn’t stop him from become a huge influence on the way people think about physics.
You can read Aristotle’s works if you like, but let me summarize some of his ideas about force. Really, to be fair we can call this Aristotle’s Law of Motion (there is only one law for him).
Aristotle’s Law of Motion: The natural state for an object is to be at rest. If you don’t push on an object, it will stop moving.
If now look back at Newton’s First Law, you can see that it is essentially a direct reply to Aristotle. Aristotle says that the natural state is at rest, Newton says that the natural state of an object is to not change its motion. Many introductory physics students see the problem with Newton’s First Law. Isn’t it just the same thing as Newton’s Second Law? Yes, but if you think of it in terms of Aristotle maybe it makes sense.
What is a Scientific Law?
It’s clear that people confuse scientific law with legal laws. I don’t think it would be crazy to say that many would say the process that a bill becomes a law (in government) is similar to the way a theory becomes a law in science. This is not true, but that’s what they think (and this is one of the reasons I suggest that we just stop using words like law in science).
Since Newton’s ideas are Laws, does that mean that they are true? No—there is no truth in science, there are just models. Some models work better than others, and some models are wrong but still useful. Let’s write down Newton’s Second Law in its common form as an equation:
Although this is a very useful model, it doesn’t always work. If you take a proton moving at half the speed of light and push on it with a force, you cannot use this to find the new velocity of the proton—but it’s still a great model. So, maybe we shouldn’t call it a Law.
Also, I’m not a big fan of naming scientific principles after people. It reminds me too much of the silly gas laws in chemistry. Open an intro chemistry book and you will probably find something like this:
Aren’t they really just one Ideal Gas Law (or I would even say Ideal Gas Principle). Maybe I’m just bitter that I don’t have a law named after me.
What Should We Do Instead of Newton’s Laws of Motion?
Science is all about models. If there is one thing I’ve tried to be consistent about—it’s that we build models in science. These models could be conceptual, physical, or mathematical. But they are still models. So, how do we model the effect forces have on matter? This can be used in place of Newton’s Laws of Motion—the Momentum Principle. It states the following:
Here, the vector p is the momentum of an object (mass multiplied by velocity). In the case of constant forces, you can replace the time derivatives with just the change in momentum divided by the change in time. It works just as well as Newton’s Second Law.
Still, you need to address the nature of force that says it is an interaction between two objects such that one object pushes with the same magnitude that the other object pushes back (this is the same as Newton’s Third Law).
Do I think that we should ban Newton’s Laws? No. There is still a place to talk about the historical development of the interaction between forces and matter and Newton played a large role here (but so did Aristotle and Galileo). Should there be a chapter titled “Newton’s Second Law”? No. I don’t think that is very helpful to students.
Here is the thing that should be banned (but I still see this)—test questions that say something like this:
Which of Newton’s Laws (First, Second or Third) says that an object will move in a straight line at a constant speed without a net force?
This is a terrible question for the following reasons:
- Does it really matter which law is First, Second, and Third?
- Technically, both the First and Second Law would be correct answers.
- It misses the main point about forces and motion and instead gives some type of recall-based question.
I just think we can do better. Just because most physics textbooks (but not all) have been very explicit about Newton’s Laws of Motion, this doesn’t mean that is the best way for students to learn.