# Newton's First Law Extended?

Doug1943
In material introducing Newton's First Law to students, we sometimes see a supplementary statement to this effect: in addition to changing the velocity of an object, forces can distort its shape.

But surely here two different situations are being conflated: (1) the effect of a single force on an object, and (2) the effect of two different forces whose vector sum is zero on the same object .

Doesn't this 'extension ' of the First Law introduce confusion, exactly where we should be trying to induce students to abstract away non-essentials as much as possible?

What's going on with the 'extended' situation -- shape distortion -- needs to be addressed when taking up the Third Law, and is in fact rather subtle.

What do others think?

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In material introducing Newton's First Law to students, we sometimes see a supplementary statement to this effect: in addition to changing the velocity of an object, forces can distort its shape.

But surely here two different situations are being conflated: (1) the effect of a single force on an object, and (2) the effect of two different forces whose vector sum is zero on the same object .
Imagine a jelly floating in zero g. Poke it with your finger. There is only one force acting on it. Do you think it will move rigidly?

Dale
Doug1943
Of course not. Let's extend the image: two cannon balls floating in zero g, connected by a loose chain. I poke one ... what happens? Or, a long steel bar. I give a push to one end. What happens?

Before we can try to analyze such situations, we need to abstract away from these complexities, and consider a point-mass, acted on by a single force. Once we have understood that, we can move on to complex objects.

My question is: is it not pedagogically unsound to introduce the First Law -- which for most students, as it was for the whole human race including its best thinkers for a long time, counter-intuitive -- by stating it, and then stating this 'amendment' or addition, which is really about a different situation?

It's like trying to teach youngsters the times tables by saying, "three times four is twelve ... most of the time. But if you're dealing with a dozen eggs and one hatches, it might be eleven. Or if someone adds an egg, then it's thirteen." Not that we don't want to make a distinction between abstract mathematics, and real life, but we shouldn't mix them up.

PeroK
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I think that you will find smart (or smart-ass) students responding as I did.

I do see your point that you need to be able to walk before you can run. However, I personally like to point out that there are caveats to applying laws in a spherical-cows-in-a-vacuum kind of way. It forstalls the smart asses, and reassures the genuinely smart ones that we're deliberately setting aside complicated cases for now.

Physics is not abstract mathematics. Acknowledging that the world is more complex than you have the maths to deal with right now doesn't seem wrong to me. I don't know, of course, if the treatments you are referring to are acknowledging complexity or simply trying to teach complexity by rote.

davenn
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Summary: In material introducing Newton's First Law to students, we sometimes see a supplementary statement to this effect: in addition to changing the velocity of an object, forces can distort its shape. Is this a good idea?
I don't think that would be a good idea; can you give an example where this is done?

Doug1943
I think it's nothing as grand as that, unfortunately.
And yes, it's a real problem. Kids are all too willing to accept any old improbable crazy or outright meaningless idea you tell them, if it's with the authority of The Teacher. They don't expect it to apply to the real world -- school-facts are a sort of separate universe -- kind of like formal religion's ethics, dutifully memorized and repeated but nothing to do with the way you actually behave.
I try to get my tutees to do Thought Experiments -- imagine the Earth gets smaller and smaller, while you get larger and larger ... until you have exchanged sizes ... who falls towards whom now? When did this changeover occur? So maybe there was no changeover point ... maybe you were both pulling on each other, but with different strengths ...
... and this is how I approach the First Law... an object is floating in space ... not moving ... it starts to move ... it must have been pushed or pulled [not that this follows logically -- I just assert it, as a reasonable supposition] ...

But when I get to them, they're 14 or so, and their thinking is pretty well fixed by then. So I don't think I'm creating any new Einsteins, able to escape from traditional conceptual frameworks.

As for physics not being mathematics. Well ... I avoid the "is" word when I can. And I always recall a talk I heard once on the 'unreasonable effectiveness of mathematics' in physical science, by a famous guy whose name I forget.

A tricky question. Of course it's not the same as pure mathematics, and it's observational, not purely deductive -- or whatever mathematics is -- but there is such a close interweaving ... I just tell my students to forget these words ... we're studying the universe and how it works, full stop. We're Natural Philosophers.

Staff Emeritus
A force can distort a moving or a static object. That has nothing to do with the 1st law, and I'm puzzled why it is even mentioned in the 1st law context.

If you want to start with the simplest case, consider only point particles that have no shape.

Doug1943
I don't think that would be a good idea; can you give an example where this is done?
Here's an example of what I mean: "A force causes an object to undergo a specific change. Unbalanced forces cause changes in speed, shape or direction" ... [ From: https://www.bbc.com/bitesize/guides/zyydmp3/revision/2 ] I've seen this reference to 'shape' several times. [I live in the UK and am mainly familiar with UK science teaching.]

Or look in Schaum's Outlines, Beginning Physics I, page 87: "A force has two basic effects on an object (1) It can change the motion of the object, which the subject of Newton's famous seocnd law ... (2) It can distort the shape of an object such as by stretching, compressing, or twisting the object."

To me, this is like teaching kids that "gravity is the Earth pulling you down". Ok ... not 'wrong'. But by their teenage years, we ought to be teaching them that "gravity is a word we use to refer to the fact that every bit of matter in the universe pulls on every other bit of matter". And of course this will be a surprise to them, so we then elaborate, and explain why we believe this.

So, as for forces causing an object to "change shape" ... sure, a net force accelerates an object until a resisting force exactly matches it -- at which point there is no net force. This is just the pure First Law -- no net force, no change in velocity. Net force, change in velocity. So an 'unbalanced' force causes a change in speed or direction ... the reference to a change in shape is confusing.

Or so I think.

pbuk
Doug1943
A force can distort a moving or a static object. That has nothing to do with the 1st law, and I'm puzzled why it is even mentioned in the 1st law context.

If you want to start with the simplest case, consider only point particles that have no shape.
Yes, agreed.

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Here's an example of what I mean: "A force causes an object to undergo a specific change. Unbalanced forces cause changes in speed, shape or direction" ... [ From: https://www.bbc.com/bitesize/guides/zyydmp3/revision/2 ] I've seen this reference to 'shape' several times. [I live in the UK and am mainly familiar with UK science teaching.]

Or look in Schaum's Outlines, Beginning Physics I, page 87: "A force has two basic effects on an object (1) It can change the motion of the object, which the subject of Newton's famous seocnd law ... (2) It can distort the shape of an object such as by stretching, compressing, or twisting the object."

To me, this is like teaching kids that "gravity is the Earth pulling you down". Ok ... not 'wrong'. But by their teenage years, we ought to be teaching them that "gravity is a word we use to refer to the fact that every bit of matter in the universe pulls on every other bit of matter". And of course this will be a surprise to them, so we then elaborate, and explain why we believe this.

So, as for forces causing an object to "change shape" ... sure, a net force accelerates an object until a resisting force exactly matches it -- at which point there is no net force. This is just the pure First Law -- no net force, no change in velocity. Net force, change in velocity. So an 'unbalanced' force causes a change in speed or direction ... the reference to a change in shape is confusing.

Or so I think.

It's a tricky one. Change of shape represents an acceleration of the centre of mass or not, depending on whether the external forces are balanced.

Most kids would have the notion of a squashing force, so I think it is probably important to tackle this in some way and explain how it relates to the force in Newton's laws.

Doug, perhaps you can explain it in terms of how the force is applied. Suppose you attach a weight to the bottom end of a spring, trying not to stretch the spring, and then release the weight. The weight will start moving with damped vibrations, approximately simple harmonic and eventually come to rest with the string being stretched. I think this is a great illustration of Newton's laws and the fact that forces can cause distortions. To further illustrate this you could slowly lower the weight by hand trying reduce any vibrations such that the resultant force remains (close to) zero. Perhaps you could give your students a spring or rubber band and get them to try it for themselves.

Doug1943
Yes, that sounds like an interesting way to address this issue.

What I was really trying to get at was this: everyone experiences the world. Everyone knows that 'things fall down' (and not up) ... except when they float ... and that heavier things fall faster than lighter ones. They know that the Earth pulls them down, they don't pull the Earth up. They know that forces can squash things. They already know these things before they get to physics class. And until about 500 years ago, they knew that the sun goes around the earth, not the Earth around the sun ... and the only reason they believe the latter now is that they have been told by Authorities that it's true, and have probably seen lots of films in which planets orbiting the sun are taken for granted.

Although I don't think most of them have even thought much about it, it probably wouldn't take too much suggestion on my part to get them to articulate that when you throw something, or shoot an arrow or a bullet, you give it energy which it burns up as it goes along until it has used up all its energy, at which point it stops moving forward.

Then they get to physics class -- or one of my tutoring sessions -- and they learn that almost all of the things they knew -- are wrong. Or rather, are over-simplifications that have erased bits of reality, mainly because those bits of reality don't manifest themselves in a way that's perceptible to us just using our natural senses

I try to do this via a combination of empirical evidence plus thought experiments -- the wonderful hammer-and-feather video of Armstrong on the moon, getting them to consider why things glide further on a low-friction surface, telling them about Humphrey and the giant lead spheres, explaining why if they could build a tower to the height of the International Space Station, they would see the astronaughts at zero g inside the station, but they would still weigh about 90% of what they do on the Earth's surface ... (and how their weight would increase as they climbed down to the surface, and then start decreasing as they descended below the surface ... plus considering how things act in very low g environments ... or in free fall -- if they took a bathroom scales with them while skydiving, could they weigh themselves while free-falling to earth? ...

I'm trying to get them to think in a certain way -- the way I think good scientists think -- [I'm not a scientist myself]... abstracting away from everything except the essence of what we're trying to understand.

So I find the original unadorned Newton's First Law is just what I want. And then... their science syllabus -- some of them -- add this bit about changing/distorting shape. Which is a different situation. Or rather, the same one -- a force ('unbalanced') is applied and something accelerates .. maybe just a few micro-metres -- until there is no unbalanced force any more ... at which point it stops accelerating. And I have to say, sure, a net force, acceleration ... no net force, no acceleration.

And yes, other things happen as well. In addition to a force sometimes changing an object's shape by distorting it ... well, apply a sharp force to a detonater or the primer of a bullet and we will see a real shape change. Apply a force to the surface of a liquid, and yet another thing happens, involving the Third Law which we carefully kept out of our initial discussion of the First Law.

But according to me ... we need to abstract away from all these other things, and first understand the pure First Law -- a net force applied to a point mass and what happens to that mass -- but of course for a fourteen-year-old, 'net force' and 'point mass' are new, exotic ideas. As is the concept of trying to abstract away from the blooming buzzing confusion of the real world and penetrate down to fundamentals.

Whether the acceleration is me sitting on a cushion until the force up equals my force down, with zero net force ... or a rocket motor burning for 100 seconds and then stopping, at which point acceleration -- given no other net forces -- ceases ... it's just Newton's First Law -- to change an object's velocity, you need a net force. If you apply a net force you change an object's velocity.

And then they say, " ... but it says here ... 'OR you change its shape'...see .. sometimes you change its velocity, but other times you just change its shape" ... and I then apply a net force to what hair I have left and accelerate it out of my head, or accelerate my face into the desk, and try to explain ... "Look. ... somebody miseducated by a Teachers' Training College who didn't actually know much physics did this and ..."

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Summary: In material introducing Newton's First Law to students, we sometimes see a supplementary statement to this effect: in addition to changing the velocity of an object, forces can distort its shape. Is this a good idea?

What's being conflated here is the appropriateness of the particle model. Overwhelmingly, Newton's Laws are introduced using the particle model. When we model objects as particles those objects cannot have their shape changed. So all an instructor has to do is point out that in the beginning we shall be using the particle model.

When I introduce the 1st Law I go through a lesson that "tricks" them into thinking an object is not moving because the camera used to film it is co-moving with the object. The entire point of the 1st Law is that there is no way to distinguish between a state of rest and a state of uniform rectilinear motion. In other words, that all inertial reference frames are equivalent.

By the time I get to this lesson, though, I've already covered the 2nd Law, and shown that a net force of zero results in an acceleration of zero. Which means the object is either at rest or moving in a straight line with a steady speed.

PeroK
Doug1943
Ah ... nice idea. I wonder if there is a YouTube video of something like that?

I know that the three Laws are interlinked ... some people like to see Number 1 as a special case of Number 2 ... and in both cases, we are neglecting Number 3 at first. And of course the relativeness of motion -- (and thus of anything with velocity as a component in its definition) is crucial.

I think part of the problem is that young people are not really introduced to 'how to think' early on in their schooling. There almost seems to be a bias against logic and analysis -- my favorite example is when they're taught that "There are four kinds of triangles -- scalene, isoceles, equilateral, and right."

I know in various places they've tried to introduce 'critical thinking', but I believe that means "critical of the existing social order, and in the same way your teacher is critical of it" ... and in the UK at least there is something called "P4C" -- Philosophy for Children -- but I have the impression that it has just become lessons in inclusiveness -- although I could be wrong.

I think it would be interesting to try to get kids used to the idea of 'thought experiments' at an early an age as possible -- there is probably some stage where such abstract thinking becomes possible.

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I also live in the UK, but I have never seen this. BBC Bitesize is not wholly reliable I have found. I suspect that some of the material has been edited by someone who doesn't actually understand the material. Here is an example I just found:

If a person has a weight of 600 N, how much will they weigh on the Moon?
You said:
600 N
Incorrect
Explanation:
Weight is mass multiplied by gravitational field strength. A weight of 600 N is equivalent to a mass of 60 kg. So 60 kg x 1.6 N/kg is 96 Newtons (N). 600 N is their weight on Earth.

Oh, so you only get an essential piece of information after you have answered the question.

I wouldn't mind betting that this question started off as 'If a person has a weight of 600 N at the surface of the Earth, how much will they weigh on the Moon?'

So much for BBC Bitesize :(

Gold Member
some people like to see Number 1 as a special case of Number 2
That's a fairly common introductory physics textbook error. The 1st Law is no mere consequence of the 2nd Law. It has survived to this day and has become the Principle of Relativity. That is, the assertion that inertial reference frames are equivalent.

Doug1943
That's very interesting. .. I looked at the other threads related to Newton's First Law, and saw this discussed ... but didn't feel I understood the whole idea of an 'inertial reference frame'. Have to dig deeper.

Doug1943
I also live in the UK, but I have never seen this. BBC Bitesize is not wholly reliable I have found. I suspect that some of the material has been edited by someone who doesn't actually understand the material. Here is an example I just found:

Oh, so you only get an essential piece of information after you have answered the question.

I wouldn't mind betting that this question started off as 'If a person has a weight of 600 N at the surface of the Earth, how much will they weigh on the Moon?'

So much for BBC Bitesize :(
Oh well ... the thought was meant kindly. In fact, they ought to run all of these science lessons, especially the online ones that young people are likely to encounter, and on their own, past a group of physicists before sending them out into the world. In principle, things like Bitesize are wonderful -- free, accessible to that large portion of young people who are no longer able, or at least willing, to read more than three sentences in a row ... but then, you've got to get it right.

On 'weight' vs 'mass'. I don't believe the students who encounter this distinction see it as anything but yet another arcane, meaningless thing to memorize and then promptly forget. In particular I believe that saying "X weighs 980 Newtons" is meaningless to them, although they will dutifully write it down.

There needs to be more effort to explain why this distinction is obscured by the accident of language. I recall when first reading Robert Heinlein's sci-fi novels, that his characters used 'mass' as a verb -- "She massed just under 50 kg" -- and being puzzled then. But of course it makes complete sense. (And it also wouldn't have the dual transitive-intransitive sense that 'weigh' does.)