Newton's first law?

[pjhirv]

Some physics textbook writer told me that Newton's first law applies only on bodies that feel no interactions at all. He said that if a body is on rest or moves in constant velocity, there is no external force acting on it. But I have heard another form of the law that says the net force acting on a body must be zero. This means there is interactions involved after all. So which one is correct?

 

[PeroK]

Some physics textbook writer told me that Newton's first law applies only on bodies that feel no interactions at all. He said that if a body is on rest or moves in constant velocity, there is no external force acting on it. But I have heard another form of the law that says the net force acting on a body must be zero. This means there is interactions involved after all. So which one is correct?

It makes no difference. I guess, if you were starting at the beginning, then the body would have no forces acting on it. After you've established that a net force of zero is physically equivalent (for a point particle) to no forces at all, then you see that it doesn't matter.

Personally, I prefer a more modern interpretation of Newton's first law, which is that there exist inertial reference frames. So, the second and third laws apply in a reference frame where the first law applies!

 

[.Scott]

If you want the "real" answer (which you probably don't), then I refer you to Newton's Principia
and suggest that you start reading at page 79. ... Hey, I'm being nice ... I could have directed you to the original Latin version.

More practically, when Newton's Laws of motion are discussed, they are discussed within the context of "Newtonian Physics" which has evolved over the decades - and continues to evolve even though it has been superseded by Einstein's works.

So, there is plenty of room for restatements of "Newton's" original law's.

 

[Nugatory]

..... if a body is on rest or moves in constant velocity, there is no external force acting on it.
...... another form of the law that says the net force acting on a body must be zero.

People routinely use the phrases "no force", "zero force", "no net force", "zero net force" interchangeably so those are, within the sloppy tolerances of natural language, just different ways of saying the same thing.
The interesting physics is not in which one is most accurate (if we want accuracy we look to the math instead), but in understanding why the differences don't matter.

 

[jbriggs444]

One way of understanding Newton's first law is as a statement that inertial frames exist. That there is such a thing as a frame of reference where non-interacting objects move along straight line trajectories at constant speed. Or simply stay at rest.

Then Newton's second law can be understood as a quantitative definition of "force".

With "force" defined we can apply Newton's second law and see that objects subject to zero net force move in straight line trajectories at constant speed. Just like non-interacting objects.

Newton's third law can be seen as asserting conservation of momentum.

I consider discussions about what Newton's laws "really" say to be largely a waste of time. There are a variety of ways to describe the same theoretical framework.

 

[Herman Trivilino]

Some physics textbook writer told me that Newton's first law applies only on bodies that feel no interactions at all.

In newtonian physics, the 1st Law describes bodies that are not accelerating.

He said that if a body is on rest or moves in constant velocity, there is no external force acting on it.

There is no net external force acting on it. There could, for example, be two forces of equal magnitude acting on the body in opposite directions. They would sum to zero, just as if there were no external force.

But I have heard another form of the law that says the net force acting on a body must be zero. This means there is interactions involved after all.

The net external force must be zero.

So which one is correct?

When worded correctly, they are both right.

But the real issue here is that Newton wrote those laws in the 1600's, and he wrote for his contemporaries, not for students, and certainly not for students in the 21st century. He was setting down one of the three laws that form the basis of his thesis on motion. The law came from Galileo, who earlier in that century wrote about what he called his Law of Inertia. It's the notion that there is no way to distinguish between being at rest and moving in a straight line at a steady speed. It's interesting to read Galileo's description of being in the hold of a ship, found in a Dialogue Concerning the Two Chief World Systems.

Since there is no way, even in principle, to distinguish between being at rest and moving in a straight line at a steady speed, the two states are equivalent. In the early 1900's physicists began to see that this principle applies to all the laws of physics. Einstein used it as the first of his two postulates in his 1905 theory of relativity. It stands proud in the hierarchy of physics and is called The Principle of Relativity.

This is the true meaning of Newton's 1st Law.

 

[PeroK]

Since there is no way, even in principle, to distinguish between being at rest and moving in a straight line at a steady speed, the two states are equivalent. In the early 1900's physicists began to see that this principle applies to all the laws of physics. Einstein used it as the first of his two postulates in his 1905 theory of relativity. It stands proud in the hierarchy of physics and is called The Principle of Relativity.

This is the true meaning of Newton's 1st Law.

In the 1905 paper, Einstein says "Let us take a system of co-ordinates in which the equations of Newtonian
mechanics hold good."

This emphasises that Newton's Laws do not hold in all reference frames. And that the first law can be taken as introducing the concept of an inertial reference frame.

 

[Orodruin]

Personally, I prefer a more modern interpretation of Newton's first law, which is that there exist inertial reference frames. So, the second and third laws apply in a reference frame where the first law applies!

For things moving free or at rest,
Observe what the first law does best.
It defines a key frame,
Inertial by name,
Where the second law then is expressed.

Source: https://www.physics.harvard.edu/undergrad/limericks

 

[pjhirv]

If you want the "real" answer (which you probably don't), then I refer you to Newton's Principia
and suggest that of "Newton's" original law's.

In Principia Newton is giving examples of the movement of planets and comets right after the first law. And those things aren't isolated from their surroundings, because the do feel for example gravitational interaction of other planets or the sun etc. So does this mean Newton is really talking about the net force after all?

 

[PeroK]

In Principia Newton is giving examples of the movement of planets and comets right after the first law. And those things aren't isolated from their surroundings, because the do feel for example gravitational interaction of other planets or the sun etc. So does this mean Newton is really talking about the net force after all?

The motion of the planets relies on the gravitational force to keep them in an elliptical orbit. The point that Newton is making is that they do not naturally slow down. Like objects on Earth would naturally slow down due to the unavoidable forces of resistance.

Prior to Newton, most people assumed that objects required a continuous force to keep them moving. And that the planets were kept in their orbits by the hand of God.

Newton's first law turned that around. The details are less important than the observation that no force is required to keep the planets from slowing down.

 

[pjhirv]

I found different physics textbook quotes about Newton's first law. And it seems that they are very different.

Physics for Scientists and Engineers 6th ed. (College Text) - Serway and Jewett:
"5.1 Newton’s First Law
Newton’s first law does not say
what happens for an object with
zero net force, that is, multiple
forces that cancel; it says what
happens in the absence of a force.
This is a subtle but important difference
that allows us to define
force as that which causes a
change in the motion. The description
of an object under the
effect of forces that balance is
covered by Newton’s second law."

Young - University Physics with Modern Physics 12e:
"Newton's first law of motion: A body acted on by no net force moves with
constant velocity (which may be zero) and zero acceleration."

Physics for Scientists and Engineers, Randall D. Knight:
"Newton's first law: An object that is at rest will remain at rest, or an object that is moving will continue move in straight line with constant velocity, if and only if the net force acting on the object is zero."

So Young and Randall says the net force is zero. And Serway says that net force has got nothing to do with the first law.

 

[PeroK]

So Young and Randall says the net force is zero. And Serway says that net force has got nothing to do with the first law.

See post #2 where I said:

It makes no difference. I guess, if you were starting at the beginning, then the body would have no forces acting on it. After you've established that a net force of zero is physically equivalent (for a point particle) to no forces at all, then you see that it doesn't matter.

It doesn't matter whether you start with "Young and Randall" or start with "Serway". You end up with the same theory of Newtonian mechanics. Sometimes in physics you can start from slightly different points of view and end up with the same theory.

 

[A.T.]

I found different physics textbook quotes about Newton's first law. And it seems that they are very different.

How are they "very different", if the difference has zero impact on the quantitative predictions?

 

[PeroK]

How are they "very different", if the difference has zero impact on the quantitative predictions?

I agree. They are subtly different, I would say.

 

[pjhirv]

I must explain that I'm a teacher myself, so that's why I want to be so extremely sure about the form of the law

Thank you everyone to share your thoughts on this topic. I have some great new ideas reading your answers. Please, if you have anything else to say about Newton's first law and the way it is written in text books, I would love hear it!

 

[.Scott]

In Principia Newton is giving examples of the movement of planets and comets right after the first law. And those things aren't isolated from their surroundings, because the do feel for example gravitational interaction of other planets or the sun etc. So does this mean Newton is really talking about the net force after all?

OK. So you really are interested in what Newton said. But please don't use what he said as anything that contradicts modern statements about Newtonian Physics. Any differences in the words will be due to the differing circumstances between the settings and audiences of his 17th century and ours in the 21st century.

It's even affected by differences in available technology:

Some things, found out after the rest, I chose to insert in places less suitable, rather than change the number of the propositions and citations.

More to the point, Isaac makes this request of you:

I heartily beg that what I have here done may be read with candour; and that the defects in a subject so difficult be not so much reprehended as kindly supplied, and investigated by new endeavors of my readers.

 

[weirdoguy]

I think that before reading Newton's Principia one should properly learn Newtonian mechanics from modern sources. Principia should be of historical interest only. Especially if you're a teacher.

 

[pjhirv]

Interesting article about Newton's first law:
https://liberalarts.vt.edu/news/art...den-truth-about-isaac-newton-s-300-year-.html

 

[PeroK]

Interesting article about Newton's first law:
https://liberalarts.vt.edu/news/art...den-truth-about-isaac-newton-s-300-year-.html

See my post #2 about a more modern interpretation. In an inertial frame of reference an object obeys Newton's first law.

The danger, as Newton himself warned about, is to take the Principia as gospel. That's what the philosopher in that article is doing. Assuming that every word and phrase in the Principia is crucial and physics rests on a precise exegesis.

It's the ideas and mathematics behind Newton's laws that are crucial. Not the Latin text.

 

[weirdoguy]

That's what the philosopher in that article is doing.

That's why philosophers should leave physics alone, unless they are physicists themselves.

 

[pjhirv]

See my post #2 about a more modern interpretation. In an inertial frame of reference an object obeys Newton's first law.

The danger, as Newton himself warned about, is to take the Principia as gospel. That's what the philosopher in that article is doing. Assuming that every word and phrase in the Principia is crucial and physics rests on a precise exegesis.

It's the ideas and mathematics behind Newton's laws that are crucial. Not the Latin text.

I agree that Principia shouldn't be red like gospel. That is very true, but unfortunately lots of people still do it. And sometimes those people are the ones who make physics textbooks, and that way they get to influence millions of people with their own interpretation about the "gospel".

What I like about this articles philosopher, is that he makes point that sometimes crucial information may be even lost in translation. So it definitely takes away the power of gospel, and people would start to realise that the brilliant ideas are more important than the written text.

The article reveals that many people have found the textbook form of Newton's first law problematic. But haven't had the courage to question the word of "gospel" and asked more specific questions about it.

 

[weirdoguy]

And sometimes those people are the ones who make physics textbooks

I don't think any physicist would write a textbook based solely on Principia. I don't even think that many physicists read Principia. Do you have any particular book in mind?

 

[pjhirv]

I don't think any physicist would write a textbook based solely on Principia. I don't even think that many physicists read Principia. Do you have any particular book in mind?

I didn't mean that they actually read the Principia. But maybe someone hundreds years before them did read it, and made own interpretations. Those interpretations has lived their own life, when those have been told for future students on and on until this day.

 

[PeroK]

I didn't mean that they actually read the Principia. But maybe someone hundreds years before them did read it, and made own interpretations. Those interpretations has lived their own life, when those have been told for future students on and on until this day.

It's just not that important, IMO. I have little or no recollection of learning Newton's laws. It's not like a catechism. The things I remember are more advanced. Like Dr Marshall proving Newton's shell theorem.

 

[pjhirv]

So Young and Randall says the net force is zero. And Serway says that net force has got nothing to do with the first law.

Here's a good example of two different interpretations. Other ones defines the law with zero net force, and the other one says the object is totally isolated and there's no interactions at all.

The article suggest that the latter interpretation is still alive just because Principia was wrongly translated 300 years ago. So that really is a threat, if historical text are red like gospels.

 

[weirdoguy]

Great, but physicists don't read historical texts like gospels. And they are not afraid to challenge textbooks, mainly because during studies we read TONS of books and we see pretty quickly that there are a lot of inaccuracies even in the best ones. You can't became a physicist of you are not critical of what you read.
Anyway, I learned in high-school the form of 1st law as "intertial frame of reference exist" and I teach that to my students.

 

[Herman Trivilino]

I found different physics textbook quotes about Newton's first law. And it seems that they are very different. [...]

So Young and Randall says the net force is zero. And Serway says that net force has got nothing to do with the first law.

Instead of textbooks, go to the source. Newton's First Law as stated in the Principia:

"Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it."

So what you are reading in textbooks are author's interpretations of that statement. What is it you're trying to get at? Do you want your students to learn newtonian mechanics or the meaning of 17th century statements? You may want them to learn both. You and they each have a finite amount of time and effort to spend. The more of it you expend on one topic, the less you can spend on another. You want them to learn both. Fine. You will do so at the expense of some other topic in the course, or at the expense of either one.

In my experience as an introductory physics instructor, I learned that I can never get a decent fraction of the students to understand the nuances you are laboring over. They really don't care about different interpretations of 17th century statements, unless you plan to test them on that. And I don't know of any good way to test for an understanding of that. But I do know how to test for an understanding of newtonian mechanics, and that is anyway what I want them to learn. I taught introductory physics. If I were to teach a course on the philosophy of physics, or the history of physics, then that would be a different set of circumstances and expectations.

Here's an example of the weeds you can get into by trying to teach an understanding of statements made in the Principia. Before the above statement of the 1st Law, we find the following Definition IV:

"An impressed force is an action exerted upon a body in order to change its state, either of rest, or of uniform motion in a right line."

You see the circularity? Do you really want to expend precious resources in an effort to get students to understand why Newton did this?

What I did was start out by asking if any student can recite the 1st Law. At least one will be able to parrot the blurb they were taught in their previous science courses. I then explain to them that what I'm really trying to do is to get you to understand the meaning of that blurb. I then go on to talk about the things I stated in the latter part of Post #6. I follow that up with examples of situations familiar to students where they've encountered an inability to determine the difference between being at rest or moving in a straight line at a steady speed.

Then, when teaching the 2nd Law, which is usually not in that same session, I spiral back to this idea by showing a series of examples where we calculate the acceleration of a cart with forces acting on it, approaching and finally ending with an example where the net force on the cart is zero. Students are then exposed to the result that the acceleration is zero, meaning that the cart is either at rest or moving in a straight line at a steady speed. So the conclusion is that the 2nd Law makes no distinction between these two states, and is therefore consistent with the 1st Law.

 

[pjhirv]

Instead of textbooks, go to the source. Newton's First Law as stated in the Principia:

"Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it."

So what you are reading in textbooks are author's interpretations of that statement. What is it you're trying to get at? Do you want your students to learn newtonian mechanics or the meaning of 17th century statements? You may want them to learn both. You and they each have a finite amount of time and effort to spend. The more of it you expend on one topic, the less you can spend on another. You want them to learn both. Fine. You will do so at the expense of some other topic in the course, or at the expense of either one.

In my experience as an introductory physics instructor, I learned that I can never get a decent fraction of the students to understand the nuances you are laboring over. They really don't care about different interpretations of 17th century statements, unless you plan to test them on that. And I don't know of any good way to test for an understanding of that. But I do know how to test for an understanding of newtonian mechanics, and that is anyway what I want them to learn. I taught introductory physics. If I were to teach a course on the philosophy of physics, or the history of physics, then that would be a different set of circumstances and expectations.

Here's an example of the weeds you can get into by trying to teach an understanding of statements made in the Principia. Before the above statement of the 1st Law, we find the following Definition IV:

"An impressed force is an action exerted upon a body in order to change its state, either of rest, or of uniform motion in a right line."

You see the circularity? Do you really want to expend precious resources in an effort to get students to understand why Newton did this?

What I did was start out by asking if any student can recite the 1st Law. At least one will be able to parrot the blurb they were taught in their previous science courses. I then explain to them that what I'm really trying to do is to get you to is understand the meaning of that blurb. I then go on to talk about the things I stated in the latter part of Post #6. I follow that up with examples of situations familiar to students where they've encountered an inability to determine the difference between being at rest or moving in a straight line at a steady speed.

Then, when teaching the 2nd Law, which is usually not in that same session, I spiral back to this idea by showing a series of examples where we calculate the acceleration of a cart with forces acting on it, approaching and finally ending with an example where the net force on the cart is zero. Students are then exposed to the result that the acceleration is zero, meaning that the cart is either at rest or moving in a straight line at a steady speed. So the conclusion is that the 2nd Law makes no distinction between these two states, and is therefore consistent with the 1st Law.

No, that's not the point. The point is that I think of these things now when I got the time. So I don't need to use the time in the classroom. I have a right answer before the class starts.

 

[pjhirv]

Great, but physicists don't read historical texts like gospels. And they are not afraid to challenge textbooks, mainly because during studies we read TONS of books and we see pretty quickly that there are a lot of inaccuracies even in the best ones. You can't became a physicist of you are not critical of what you read.
Anyway, I learned in high-school the form of 1st law as "intertial frame of reference exist" and I teach that to my students.

No, you understood me wrong again. I'm not talking about the physicist. I am talking about the students, great majority of them won't be physicists.

 

[Herman Trivilino]

No, that's not the point. The point is that I think of these things now when I got the time. So I don't need to use the time in the classroom. I have a right answer before the class starts.

What's not the point? The entire post is a discussion involving thinking about these things now. And getting a right answer before class starts.