Clarification regarding Newon's first law.

[sankalpmittal]

Hello ,
I am in class XI now and I am 15.

Ok , so I have a doubt :

Majority (no all !) of our textbooks state Newton's first law as follows :

"Every object remains in its original state of rest or uniform motion unless it is acted upon by a net external force."

But our teacher told us that the definition in each and every book (nearly) is incomplete.

He told us that correct definition will be to say :

"If observation is being made from an inertial or non accelerating frame of reference , then every object remains in its original state of rest or uniform motion unless it is acted upon by a net external force."

He then gave us the following examples :

Suppose a block is kept at rest on a floor. An observer observes this standing beside the block and thus concludes that the block is at rest. So |a| =0
Or |F| =0. So he says that Newton's first law holds.

Another observer in a car is moving with a velocity towards the block and observes the acceleration in the block as a.
So F = ma
So he says that the Newton's first law is wrong , as there is an external force being applied and object was at rest!

____________________________________________________________________

Now here are my views regarding this :

I think that the teacher is confusing relative acceleration or relative force with frame of references.
For example another observer in a car is moving towards the block with velocity v.
The velocity of block with respect to car is given by :

v_b/c = -v

If we differentiate this , we get :
dv_b/c/dt = -dv/dt

"a_b/c = -a"

Also relative momentum of block w.r.t car is :

p_b/c = -p

If we again differentiate this , we get :

F_b/c = -F

Thus the above expression gives force on block w.r.t car.

Also , I think that inertial frame means that Newton's first law isn't applicable inside the frame.

I think that every textbook just can't be incorrect.

Can someone clarify please ?

 

[Doc Al]

Another observer in a car is moving with a velocity towards the block and observes the acceleration in the block as a.
So F = ma
So he says that the Newton's first law is wrong , as there is an external force being applied and object was at rest!

Apparently that car was accelerating. Viewed from an accelerating frame, Newton's first two laws require modification.

 

[TobyC]

Newton's 1st law is actually more of a definition of which frames are not accelerating. It's true that Newton's laws have to be modified when you move into an accelerating reference frame, but what is an accelerating reference frame? Accelerating with respect to what? The best way that I've heard Newton's 1st law stated is "There exist frames of reference in which particles move at constant velocity when not acted on by external forces". Once you have established the existence of these frames, it is clear that all these frames must be moving at constant speed with respect to each other, so you can define this set of frames to be the non-accelerating frames and measure acceleration with respect to them.

I think that definition is superior to what you've quoted as the textbook definition, which is oversimplified because it doesn't acknowledge the fact that Newton's laws only apply to specific frames of reference. I think it is also superior to your teacher's quoted definition, because actually you can't really define absolute acceleration without using something like Newton's law as a reference point. Your teacher's definition appears to have it sort of backwards.

 

[krd]

No. Your teacher is wrong.

The way the law is written in every textbook assumes that the frame of reference the object is in is the entire universe.

Your teacher has it half baked.

The law states simply, objects do not change their course or velocity unless an external force acts on them.

He's wrong for other reasons. When you drop an apple, does it experience the force of its' mass by acceleration, or the mass of the Earth by the apple's acceleration. Do you feel the force of the moon's velocity.

With Newton's first law you're better off just to forget about acceleration - acceleration requires external force. Take an accelerating car, the extra fuel burned to accelerate the car is considered an external force for the purpose of the law.

 

[krd]

It's true that Newton's laws have to be modified when you move into an accelerating reference frame,

But then it's not Newton's first law.

You can't give an object energy just by observing it.


Damn I just thought of another problem for high speed space travel.

If you're in a space ship. traveling towards a galaxy, at near the speed of light - the normally pale photons from the galaxy, will be highly energised and melt you and your spaceship. Your velocity will Doppler shift them to a much higher energy. Not all that relevant but just thought of it.

 

[TobyC]

No. Your teacher is wrong.

Wrong isn't the word I would use, strictly speaking it's actually right, I just think it's badly phrased because it's better to use Newton's 1st law to define which frames are accelerating, not using the concept of acceleration to define Newton's 1st law.

The way the law is written in every textbook assumes that the frame of reference the object is in is the entire universe.

The textbooks assume an inertial frame of reference when they state their definition (I don't know what you mean by 'the entire universe') and that's why the textbooks aren't wrong either, they're just simplified so as not to confuse students starting out in physics.

But then it's not Newton's first law.

Exactly, Newton's 1st law only applies in special reference frames. If you start from scratch looking at the mess of particles moving through space and time that is the universe, there's no way you can initially decide which coordinates you should be using. There's nothing to use as a reference point to define absolute acceleration for instance. It turns out that there are special systems of coordinates in which the laws of physics take nice simple forms. You call these inertial systems, and in these systems only, Newton's laws including his first apply. In other systems the laws of physics describing how particles move would look very different. This gives you a way to define absolute acceleration, because you say inertial frames are not accelerating and you measure all other acceleration with respect to them. If you're in an accelerating car you can tell, and measure your absolute acceleration, by the fact that objects left on their own will accelerate backwards, not what you'd expect if Newton's first law applied.

 

[Dead Boss]

The law states simply, objects do not change their course or velocity unless an external force acts on them.

Of course this is only true in an inertial reference frame.

 

[D H]

No. Your teacher is wrong.

The way the law is written in every textbook assumes that the frame of reference the object is in is the entire universe.

The teacher was correct. The modern interpretation of Newton's first law is that it defines the inertial frames of reference. There is nothing in Newton's writing about a universal frame of reference; there's nothing in Newton's writing about frames of reference, period.

 

[krd]

Of course this is only true in an inertial reference frame.

The whole idea of the law is it's a simplified starting principle. There isn't anything in the universe that doesn't have an external force acting on it.

When you start dragging reference frames into it, you're muddying the water.

 

[sankalpmittal]

Apparently that car was accelerating. Viewed from an accelerating frame, Newton's first two laws require modification.

Ok , but that's not what I asked for. However thanks for clarifying this
. Please reread my post #1.

The teacher was correct. The modern interpretation of Newton's first law is that it defines the inertial frames of reference. There is nothing in Newton's writing about a universal frame of reference; there's nothing in Newton's writing about frames of reference, period.

Ok I get the idea regarding this. My guess is that every textbook states the definition of Newton's first law on "Earth". As we "practically" regard "Earth" as an inertial frame of reference , Newton's first law will hold true there. So the textbook's definition is simplified though. Am I correct ?

So teacher's definition is correct then.. However I am not at all convinced by the examples he gave :

He then gave us the following examples :

Suppose a block is kept at rest on a floor. An observer observes this standing beside the block and thus concludes that the block is at rest. So |a| =0
Or |F| =0. So he says that Newton's first law holds.

Another observer in a car is moving with a velocity towards the block and observes the acceleration in the block as a.
So F = ma
So he says that the Newton's first law is wrong , as there is an external force being applied and object was at rest!

So suppose we take the block kept on ground as reference frame ,S. Since Earth is practically an inertial frame , so frame S will also be an inertial frame. Also let the observer standing beside block be taken as frame S'. Since frame S' is at rest with respect to frame S , so frame S' will also be an inertial frame. Now another observer in car moving towards the frame S be taken as frame S''. Since frame S'' is accelerated with respect to frame S , so frame S'' is a non inertial frame. Now inside frame S'' Newton's first law just can't be applied.
How does he dragged concept of frames here.

Again he is confusing frame of references with relative force. Like I stated :

For example another observer in a car is moving towards the block with velocity v.
The velocity of block with respect to car is given by :

v_b/c = -v

If we differentiate this , we get :
dv_b/c/dt = -dv/dt

"a_b/c = -a"

Also relative momentum of block w.r.t car is :

p_b/c = -p

If we again differentiate this , we get :

F_b/c = -F

Thus the above expression gives force on block w.r.t car.

Also , I think that inertial frame means that Newton's first law isn't applicable inside the frame.

I think that every textbook just can't be incorrect.

Am I correct ?

So the mentors posts are stating that teacher's definition is correct , complying with TobyC and Dead Boss , however contrary to krd.

 

[D H]

When you start dragging reference frames into it, you're muddying the water.

Dragging reference frames into it is exactly what Newton's first law does. It is essentially a statement about frames of reference. Toby C put it very nicely in post #3 with his statement "There exist frames of reference in which particles move at constant velocity when not acted on by external forces."

 

[arildno]

The whole idea of the law is it's a simplified starting principle. There isn't anything in the universe that doesn't have an external force acting on it.

When you start dragging reference frames into it, you're muddying the water.

ACTUALLY, it is deep thinking about reference frames that clarifies the waters..

 

[Dale]

You can't give an object energy just by observing it.

Sure you can. Or more specifically, energy depends on the reference frame. It is frame variant, not an intrinsic property of the object itself.

 

[krd]

Dragging reference frames into it is exactly what Newton's first law does. It is essentially a statement about frames of reference. Toby C put it very nicely in post #3 with his statement "There exist frames of reference in which particles move at constant velocity when not acted on by external forces."

I don't know what Newton's original wording was, the law implies a frame of reference. And I don't think you need to explicitly state that the velocity can appear different in different frames of reference. I'm certain Newton was well aware of that too.

Other reference frames, in the instance of the law, are not relevant. They only become relevant when they need to be explicitly considered.

 

[jtbell]

I don't know what Newton's original wording was

http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_first_law

 

[krd]

Sure you can. Or more specifically, energy depends on the reference frame. It is frame variant, not an intrinsic property of the object itself.

I'm too tired for this kind of argument.


By referring to external force in the original wording of the law, the frame of reference is implied.

 

[krd]

http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_first_law

I knew it was in Latin.

Have you read Principia in the original?

 

[Dale]

By referring to external force in the original wording of the law, the frame of reference is implied.

That is not correct. Any real force exists in all reference frames, and it even has the same magnitude in all reference frames. There is nothing about a real force which identifies a specific reference frame.

 

[D H]

I knew it was in Latin.

Have you read Principia in the original?

You can download English translations from multiple sites. Just google Newton's Principia.

However, this thread isn't about how Newton expressed his laws, or what he thought of them. It's about the modern interpretation of Newton's laws. Newton's original formulation is a bit irrelevant. The sciences don't place near the emphasis on the wisdom of the great minds as do the humanities. The original formulations of most scientific works are oftentimes clumsy, verbose, and not quite scientific. Newton's physics is a prime example. Newton didn't use calculus much at all in his Principia; Newton preferred not to do so if at all possible. Newton rarely used the concept of energy; Newton didn't think energy was conserved. He didn't use vectors; the vectors we use so freely nowadays weren't invented until 200 years later. He didn't use the concept of a frame of reference; that too was something invented long after Newton's death.

Most importantly, Newton's concept of absolute space and absolute time are not scientific. This absolute space and time are not observable by means of any scientific experiment. The same applies to your concept of a reference frame based on the universe. The concept of an inertial system as one in which Newton's first law of motion is valid was introduced a couple of hundred years after Newton.

This concept is falsifiable, and it turns out, is false. It took a couple of hundred years to fully formalized Newtonian mechanics through Hamilton's principle, make it nice and compact through the use of vectors, and made scientific in Lange's reinterpretation of Newton's first law. Just when this work of two centuries was finished, it was shown to be false by relativity and quantum mechanics.

 

[arildno]

You can download English translations from multiple sites. Just google Newton's Principia.

However, this thread isn't about how Newton expressed his laws, or what he thought of them. It's about the modern interpretation of Newton's laws. Newton's original formulation is a bit irrelevant. The sciences don't place near the emphasis on the wisdom of the great minds as do the humanities. The original formulations of most scientific works are oftentimes clumsy, verbose, and not quite scientific. Newton's physics is a prime example. Newton didn't use calculus much at all in his Principia; Newton preferred not to do so if at all possible. Newton rarely used the concept of energy; Newton didn't think energy was conserved. He didn't use vectors; the vectors we use so freely nowadays weren't invented until 200 years later. He didn't use the concept of a frame of reference; that too was something invented long after Newton's death.

Most importantly, Newton's concept of absolute space and absolute time are not scientific. This absolute space and time are not observable by means of any scientific experiment. The same applies to your concept of a reference frame based on the universe. The concept of an inertial system as one in which Newton's first law of motion is valid was introduced a couple of hundred years after Newton.

This concept is falsifiable, and it turns out, is false. It took a couple of hundred years to fully formalized Newtonian mechanics through Hamilton's principle, make it nice and compact through the use of vectors, and made scientific in Lange's reinterpretation of Newton's first law. Just when this work of two centuries was finished, it was shown to be false by relativity and quantum mechanics.

I'd like to add to DH's excellent post:

The wisdom of the ancients is honoured in science as well, namely in recognizing who made giant efforts of clearing away intellectual rubble of THEIR OWN TIME, finding a constructive path for future generations to walk along.
-------------------------------------------------
Therefore, many of the great thinkers within science are easily misunderstood, because we have forgotten what they struggled AGAINST.

To take Newton's first law:
Essentially, it is a radical break with theories like the "impetus theory", and similar conception, namely that in order for objects to move at all, one thought there had to be some kind of self-propelling force WITHIN the object.

Not only do these theories conflate mechanisms for maintenance of velocity (the self-propelling force) with acceleration generators, but the theory also blocks the mentality for the true gem contained in Newton's 3.law:
IF one accepts a self-propelling force, then the whole idea of forces always acting IN COUPLES is very hard to come by, even sounding utterly false (the self-propellator is a counter-example, violating Newton's 3. law).
-------------------------------------------------------------
Of course, none of this is really necessary to know about in order to learn and master physics, but nonetheless of some interest.

 

[sankalpmittal]

Ok , enough of confusion already !

This thread is getting sophisticated now. Here , I found an English translation of Newton's original Principia. It asks for password , just click "cancel".

http://gravitee.tripod.com/axioms.htm

What I observed was that in his Principia he hasn't even mentioned about frame of reference nor about inertia. He has not proved his law , the way the textbook gives. He proved them experimentally and geometrically and declared them as axioms.

He never even mentioned about vectors laws and neither used them.

Now my question is just left unanswered in post #10. I am restating it here :

Originally Posted by Doc Al

Apparently that car was accelerating. Viewed from an accelerating frame, Newton's first two laws require modification.

Ok , but that's not what I asked for. However thanks for clarifying this
. Please reread my post #1.

Originally Posted by D H

The teacher was correct. The modern interpretation of Newton's first law is that it defines the inertial frames of reference. There is nothing in Newton's writing about a universal frame of reference; there's nothing in Newton's writing about frames of reference, period.

Ok I get the idea regarding this. My guess is that every textbook states the definition of Newton's first law on "Earth". As we "practically" regard "Earth" as an inertial frame of reference , Newton's first law will hold true there. So the textbook's definition is simplified though. Am I correct ?

So teacher's definition is correct then.. However I am not at all convinced by the examples he gave :

He then gave us the following examples :

Suppose a block is kept at rest on a floor. An observer observes this standing beside the block and thus concludes that the block is at rest. So |a| =0
Or |F| =0. So he says that Newton's first law holds.

Another observer in a car is moving with a velocity towards the block and observes the acceleration in the block as a.
So F = ma
So he says that the Newton's first law is wrong , as there is an external force being applied and object was at rest!

So suppose we take the block kept on ground as reference frame ,S. Since Earth is practically an inertial frame , so frame S will also be an inertial frame. Also let the observer standing beside block be taken as frame S'. Since frame S' is at rest with respect to frame S , so frame S' will also be an inertial frame. Now another observer in car moving towards the frame S be taken as frame S''. Since frame S'' is accelerated with respect to frame S , so frame S'' is a non inertial frame. Now inside frame S'' Newton's first law just can't be applied.
How does he dragged concept of frames here.

Again he is confusing frame of references with relative force. Like I stated :

For example another observer in a car is moving towards the block with velocity v.
The velocity of block with respect to car is given by :

v_b/c = -v

If we differentiate this , we get :
dv_b/c/dt = -dv/dt

"a_b/c = -a"

Also relative momentum of block w.r.t car is :

p_b/c = -p

If we again differentiate this , we get :

F_b/c = -F

Thus the above expression gives force on block w.r.t car.

Also , I think that inertial frame means that Newton's first law isn't applicable inside the frame.

I think that every textbook just can't be incorrect.

Am I correct ?

 

[krd]

Am I correct ?

I think you're correct. I also believe drawing accelerating frames into the law is absurd.

 

[sophiecentaur]

Aren't we putting a lot onto poor old Newton here? Why do we need something that was written hundreds of years ago to be 'correct' in modern terms? (same thing happens with Darwinism, too)
Newton's Laws are absolutely appropriate for a first stab at describing Motion in School. They are 'concrete' enough (Piagetianly speaking) for most kids to pick up. If the first thing you do is to use the term 'frame of reference' with a class full of twelve year olds then you will lose all but the nerds in the class. What's wrong with presenting things in historical order?
In the UK, there are so many Science Teachers who struggle with the Physics (teaching out of speciality) that it needs to be taught in very straightforward terms.
There is plenty of time to move on to frames of reference later and to discuss how Newton needs to be re-visited, on the way to Relativity. (Is there anything to be gained with the 'Newton was wrong' argument?) The OP has clearly shown, by asking the question, that he/she is intellectually well able to make the jump but many students would be struggling to Compare and Contrast Newton with modern ideas.

 

[arildno]

I agree with Sophiecentaur:
You can get a lot more students to actually learn and appreciate physics by only focusing on problems where you have an implicit Earth-fixed frame of reference.
This is actually to trade upon the "natural physics" natural selection has built into our brains; more subtle, and logical precise ideas as reference framescan be developed later on.
Even a genius like Newton had a rather implicit thinking concerning "reference frames", but he did manage to create a lot of important physical insights.

 

[krd]

You can download English translations from multiple sites. Just google Newton's Principia.

Thanks, I went looking for copies before and only found the original Latin.

However, this thread isn't about how Newton expressed his laws, or what he thought of them. It's about the modern interpretation of Newton's laws. Newton's original formulation is a bit irrelevant.

Yes, but you can't start dragging in every exception into the definition of the rule. Because, literally, where do you stop. What if the object in the first law has an internal engine - a spinning disc - it will change the velocity of the object without any external force acting on it? You have to make an assumption from the bare wording of the law, that that will not be the case.

The sciences don't place near the emphasis on the wisdom of the great minds as do the humanities.

This is true. To better understand an idea, it's good to know how and why the idea was arrived at.

The original formulations of most scientific works are oftentimes clumsy, verbose, and not quite scientific.

The other extreme is when a formula is presented to a student as complete. They can memorise it, and get full marks in an exam - but the only basis they have for the law being true is it was presented to them as true. And they memorised it. And their knowledge is as scientific as religious indoctrination.

I will put my hands up. I have done exams where I gave the correct answers to questions with no idea why the answers were correct.

 

[D H]

Ok , enough of confusion already !

This thread is getting sophisticated now. Here , I found an English translation of Newton's original Principia. It asks for password , just click "cancel".

http://gravitee.tripod.com/axioms.htm

You can find it elsewhere. Here's a digitized version at Google Books: http://books.google.com/books?id=KaAIAAAAIAAJ&pg=PA83#v=onepage&q&f=false. Scroll back a few pages to page 77 and you'll find Newton's scholium to his definitions. This scholium, or commentary, gives insight into Newton's thinking. It also gives insight into why Newton's first law has been recast along the lines of TobyC's statement in the third post in this thread, There exist frames of reference in which particles move at constant velocity when not acted on by external forces. Or, as my dusty old freshman physics text says,

Newton's first law is really a statement about reference frames. The first law tells us that it is possible to find a family of reference frames in which a particle [not subject to external forces] has no acceleration.​

What I observed was that in his Principia he hasn't even mentioned about frame of reference nor about inertia.

Well of course he didn't mention inertial frames of reference. How could be? The concept of an inertial frame of reference postdates Newton by 200 years or so. He did however mention the vis inertiae in his Definition III (page 74 of the version at Google Books), and he spent several pages on absolute space and time in his scholium to his definitions (page 77).

This concept of absolute space and time is not scientific. It is not observable. Newton said exactly that in that first scholium. It was this non-scientific basis of Newtonian mechanics that led 19th century physicists to recast Newton's laws into a scientific framework. Newton's first law is really a statement about about reference frames.

He has not proved his law , the way the textbook gives. He proved them experimentally and geometrically and declared them as axioms.

You do have a bad textbook if it claims that Newton proved his laws. Unlike mathematical theorems, scientific laws and scientific theories cannot be proven to be true.

This is a bit of an aside, but it is an important one. Scientific laws and theories can be shown to be false; all it takes is one lousy experiment to send theoretical physicists back to the drawing board. However, evidence in favor of a scientific law or scientific theory does not prove the law or theory to be true. This is a basic problem of science. As an example, suppose I have a theory that all swans are white. Just because I observe thousands upon thousands of swans, all of them white, does not prove this theory to be true. All it takes is one black swan, and oops, there goes my theory. For more, read up on the concept of falsifiability. A starter: http://en.wikipedia.org/wiki/Falsifiability.

He never even mentioned about vectors laws and neither used them.

Since the concept of vectors postdates Newton by about 200 years, it would have been rather tough for him to use vectors his Principia. Yet most every freshman level, calculus-based physics text uses vectors heavily. If Newton didn't use vectors, why do those texts do so? The answer is that science progresses. The giants in science such as Newton, and Newton certainly was a giant (maybe the biggest one of all) point the way. However, their writings, their thoughts, are not sacrosanct.

I also believe drawing accelerating frames into the law is absurd.

That's exactly backwards. Your concept of the frame of reference of the universe is not scientific (not testable, not observable). Dragging the concept of frames of reference into the picture is essential to making Newtonian mechanics scientific. There are obviously frames of reference in which Newton's laws do not apply. So what to do about these? The answer in the minds of most physicists is to recast Newton's first law as a statement about frames of reference.

The sciences don't place near the emphasis on the wisdom of the great minds as do the humanities.

This is true. To better understand an idea, it's good to know how and why the idea was arrived at.

That, too is exactly backwards. Pick up a freshman physics text. It won't spend a lot of effort on explaining Newton's thinking. It will instead explain Newton's laws in the form of calculus (Newton eschewed calculus in his Principia). It will spend a good amount of effort discussing frames of reference and vectors (concepts that postdate Newton by 200 years) before even beginning to dive into Newton's laws.

While I don't quite agree with Feynman (Philosophy of science is as useful to scientists as ornithology is to birds), he most certainly does have a point.

 

[sankalpmittal]

Ok , but still the following questions in this thread have been unanswered :

1. My guess is that every textbook states the definition of Newton's first law on "Earth". As we "practically" regard "Earth" as an inertial frame of reference , Newton's first law will hold true there. So the textbook's definition is simplified though. Am I correct ?

2. So teacher's definition is correct then.. However I am not at all convinced by the examples he gave :

He then gave us the following examples :

Suppose a block is kept at rest on a floor. An observer observes this standing beside the block and thus concludes that the block is at rest. So |a| =0
Or |F| =0. So he says that Newton's first law holds.

Another observer in a car is moving with a velocity towards the block and observes the acceleration in the block as a.
So F = ma
So he says that the Newton's first law is wrong , as there is an external force being applied and object was at rest!

So suppose we take the block kept on ground as reference frame ,S. Since Earth is practically an inertial frame , so frame S will also be an inertial frame. Also let the observer standing beside block be taken as frame S'. Since frame S' is at rest with respect to frame S , so frame S' will also be an inertial frame. Now another observer in car moving towards the frame S be taken as frame S''. Since frame S'' is accelerated with respect to frame S , so frame S'' is a non inertial frame. Now inside frame S'' Newton's first law just can't be applied.
How does he dragged concept of frames here.

Again he is confusing frame of references with relative force. Like I stated :

For example another observer in a car is moving towards the block with velocity v.
The velocity of block with respect to car is given by :

v_b/c = -v

If we differentiate this , we get :
dv_b/c/dt = -dv/dt

"a_b/c = -a"

Also relative momentum of block w.r.t car is :

p_b/c = -p

If we again differentiate this , we get :

F_b/c = -F

Thus the above expression gives force on block w.r.t car.

Also , I think that inertial frame means that Newton's first law isn't applicable inside the frame.

I think that every textbook just can't be incorrect.

Am I correct ?

I have been posting these questions thrice in this thread including this !

Note : Although krd has agreed with me , but I want to listen to views of other members regarding this.

 

[Dale]

Ok , but still the following questions in this thread have been unanswered :

1. My guess is that every textbook states the definition of Newton's first law on "Earth". As we "practically" regard "Earth" as an inertial frame of reference , Newton's first law will hold true there. So the textbook's definition is simplified though. Am I correct ?

If the textbook's definition is:

"Every object remains in its original state of rest or uniform motion unless it is acted upon by a net external force."

Then the teacher is correct that this is not a complete definition. I would not say that it is "wrong" just "incomplete". It is, IMO, a reasonable simplification for an introductory course, but as stated it is not always true. The teacher's version:

"If observation is being made from an inertial or non accelerating frame of reference , then every object remains in its original state of rest or uniform motion unless it is acted upon by a net external force."

is more accurate, but probably unnecessarily complicated for an introductory course.

2. So teacher's definition is correct then.. However I am not at all convinced by the examples he gave

If the example is:

Suppose a block is kept at rest on a floor. An observer observes this standing beside the block and thus concludes that the block is at rest. So |a| =0
Or |F| =0. So he says that Newton's first law holds.

Another observer in a car is moving with a velocity towards the block and observes the acceleration in the block as a.
So F = ma
So he says that the Newton's first law is wrong , as there is an external force being applied and object was at rest!

Then the example is a bad example, not necessarily because it is wrong, but because it is not clear enough to even know if it is wrong or right. Specifically the part highlighted in red. Is the velocity constant or changing? All we know from this description is that it is non-zero, but that is not what we need to know.

If the car is moving with a CONSTANT velocity then, the block is NOT accelerating in the car's reference frame, the frame is INERTIAL, and there is NO net force.

On the other hand, If the car is moving with a CHANGING velocity then, the block IS accelerating in the car's reference frame, the frame is NON-INERTIAL, and there IS a net force with one of the forces being a fictitious force which shows up only in non-inertial reference frames.

 

[krd]

That's exactly backwards. Your concept of the frame of reference of the universe is not scientific (not testable, not observable). Dragging the concept of frames of reference into the picture is essential to making Newtonian mechanics scientific.

No I meant it's absurd to expand the definition. Because you're getting into things like "light has a wavelength...but the length of that wavelength depends on your frame of reference"

While I don't quite agree with Feynman (Philosophy of science is as useful to scientists as ornithology is to birds), he most certainly does have a point.

No, because things go out the window quicker when you abandon philosophy than when you don't. It's not that long since God was taken out of science. Roll Feynman back a few centuries and any holes in his theories would have been filled by god.

Science is loaded with philosophy.

 

[Dale]

It's not that long since God was taken out of science. Roll Feynman back a few centuries and any holes in his theories would have been filled by god.

Science is loaded with philosophy.

I suspect that it won't be too much longer until philosophy is taken out of science too and the holes are filled with Bayesian inference.

 

[ZapperZ]

This thread needs to get back on topic or it will be locked. Discussion about philosophy and its function in physics does not belong here.

Zz.

 

[sankalpmittal]

ZapperZ seems to be correct. This thread is really going off topic , rather than being limiting itself to the scope of my question (OP's).

Ok , so my first question has been well answered however second question has not , for I've mistyped it.

Let me repeat it again :2. So teacher's definition is correct then.. However I am not at all convinced by the examples he gave :

He then gave us the following examples :

Suppose a block is kept at rest on a floor. An observer observes this standing beside the block and thus concludes that the block is at rest. So |a| =0
Or |F| =0. So he says that Newton's first law holds.

Another observer in a car is moving with an acceleration towards the block w.r.t ground and observes the acceleration in the block as -a.
So F = -ma
So he says that the Newton's first law is wrong , as there is an external force being applied and object was at rest!

Is he correct ?


Also , suppose a frame is situated in the ship which is non inertial. So Newton's first law will not apply inside the ship (frame) or also outside it ?

Please do not leave any question unanswered now, otherwise I'll have to re-post it.

 

[Doc Al]

Let me repeat it again :


2. So teacher's definition is correct then.. However I am not at all convinced by the examples he gave :

Suppose a block is kept at rest on a floor. An observer observes this standing beside the block and thus concludes that the block is at rest. So |a| =0
Or |F| =0. So he says that Newton's first law holds.

Another observer in a car is moving with an acceleration towards the block w.r.t ground and observes the acceleration in the block as -a.
So F = -ma
So he says that the Newton's first law is wrong , as there is an external force being applied and object was at rest!

Is he correct ?

Here's how I would describe those two cases:
(1) A box is at rest with no external forces* acting. It just sits there, so Newton's first law holds.
(2) That same box has no external forces acting yet is seen to accelerate. So Newton's first law does not hold.

Also , suppose a frame is situated in the ship which is non inertial. So Newton's first law will not apply inside the ship (frame) or also outside it ?

Not clear what you mean. If you are viewing things from a non-inertial frame, Newton's first law will not hold.

Edit: I meant, of course, no net external force acting.

 

[sophiecentaur]

ZapperZ seems to be correct. This thread is really going off topic , rather than being limiting itself to the scope of my question (OP's).

Ok , so my first question has been well answered however second question has not , for I've mistyped it.

Let me repeat it again :


2. So teacher's definition is correct then.. However I am not at all convinced by the examples he gave :



Is he correct ?


Also , suppose a frame is situated in the ship which is non inertial. So Newton's first law will not apply inside the ship (frame) or also outside it ?

Please do not leave any question unanswered now, otherwise I'll have to re-post it.

Threats now, eh?

How can Newton's First Law be 'wrong' about a situation involving acceleration when it specifically refers to a non-accelerating situation?

 

[sankalpmittal]

Here's how I would describe those two cases:
(1) A box is at rest with no external forces acting. It just sits there, so Newton's first law holds.
(2) That same box has no external forces acting yet is seen to accelerate. So Newton's first law does not hold.

But is that example correct ? I think that the teacher while giving that example , is confusing frame of references and relative force.

Not clear what you mean. If you are viewing things from a non-inertial frame, Newton's first law will not hold.

But what I am asking is this :

Earth is practically regarded as inertial frame of reference , right ? Suppose the ship is in acceleration with respect to earth. So ship is a non inertial frame. Now an observer in the ship cannot apply Newton's first law by observing object inside ship , or also cannot apply Newton's first law outside the ship ?