# Homework Help: Why do some people say that Newton's Second law is the real Law of Motion?

1. Jan 23, 2012

### Rakinniya

People say that Newton's Second law is the real Law of Motion because both the First and the third law can be proved from the second. If this is true why did Newton state them as separate laws if they are just special cases of the 2nd law? My teacher taught us this.
These are the proofs she taught....

1)1st Law frm 2nd

In the eq
F=ma
when we put F=0 we get Acceleration = 0 ----> the first Law of motion

okay this is fine.....

2)Now..3rd frm 2nd..
let two bodies interact
let F1 and F2 be the action n reaction
then according to the second law
F1=dP1/dt F2=dP2/dt

F1+F2 = d/dt (P1+P2)

when no external forces are acting.... according to the law of conservation of momentum Momentum must be conserved

P1+P2=const

F1+F2 = d/dt (P1+P2)

F1+F2=0

F1= -F2 ----->third Law

But the problem I find here is ..... We prove the Law of conservation of momentum by applying the third law ..... Then isn't it silly and incorrect to use the momentum law to prove the third law....?????? ....

.. Please tell me if there is some other valid proof...or any changes that can be made in this one:uhh:

2. Jan 23, 2012

### tsw99

Linear momentum is conserved because of Newton's 3rd law. I think it is on every physics textbook

3 laws are essential, they are not proved from each other.

3. Jan 23, 2012

### pabloenigma

Linear Momentum Of a System Of particles is conserved because Of 3rd Law.For a Single particle,2nd Law Suffices.
Conservation Of Momentum and Angular Momentum for a system of particles rest On third Law.I dont think Its derivable from the 2nd.Once upon a time I saw such a derivation in a very bad school text book.But it doesnt make any sense.

4. Jan 25, 2012

### Rakinniya

Thank you.......It's not there in my text book....our teacher taught it ...but I found it in a guide as well...and its de on some sites also...ofcourse it doesn't make any sense ...but what about the First Law???...Is it or not just a special case of the second....????????

5. Jan 25, 2012

### Ambidext

I see it as more of a "check" rather than a proof. physics can get very ambiguous sometimes. When you come across Daniel schroeder's thermal physics you'll see what i mean. :p

6. Jan 25, 2012

### Dickfore

This is true, but, perform the following experiment:

Inside a standing vehicle, put a ball on a horizontal surface. It should remain still. It means all the forces acting on it are in equilibrium. Then, the vehicle starts accelerating. You notice the ball starts to roll. If it starts to move, its velocity surely changes, therefore it acquires some acceleration. However, nothing changed with the forces acting on the ball before and after the acceleration. If before the acceleration Ʃ F = o, so should it be after the acceleration. But, now we have a contradiction: The sum of all the forces is zero, and the ball is accelerating. This means First Newton's Law is incorrect. Because you derived it from the Second Law, it means that one is incorrect as well???

You are right, you performed a circular argument. Third Newton's Law is related to Conservation of Linear momentum.

7. Jan 25, 2012

### Rakinniya

Dickfore .... I don't really understand the mistake you are pointing out..... and about what you said
"Then, the vehicle starts accelerating. You notice the ball starts to roll. If it starts to move, its velocity surely changes, therefore it acquires some acceleration."

Here you are talking about the velocity of the ball w.r.t the ground right??..so at first the ball is not at rest it is moving with respect to the ground... and though the train accelerates...the ball rolls because it has to stay its earlier state of uniform motion(cuz no force is acting on the ball)....so at that instant it is not accelerating with the train...............................

8. Jan 25, 2012

### spaghetti3451

Newton's first law is, in fact, a special case of the second. I understand that you are confused as to why we need a 'Newton's First Law' if it can be derived from 'Newton's second law'. In other words, you want to boil down the rules of mechanics down to the most basic, fundamental postulates/laws/assumptions and since you see that Newton's second law is the most fundamental, you want to rip the first law of its status as a fundamental law. That is all fine, and you are right to say that Newton's second law is the most fundamental postulate of Newtonian mechanics. I am going to refer you to this link https://www.physicsforums.com/showthread.php?t=165100. It has further discussions about the problem you mentioned. The discussion there should help you develop your own point of view regarding the issue.

Actually, Newton's Third Law is correct if and only if the law of conservation of motion is valid. One cannot exist without the other. Once you will have studied quantum mechanics and relativity, you will understand that the law of conservation of momentum is the more fundamental principle. The law of conservation of momentum has been taken to be one of the fundamental postulates of all of physics and Newton's laws have been modified to take into account some of the bizzare experimental results that have been noticed in the late 19th century. These experiments could not be explained using Newtonian mechanics so physicists had to resort to modifying Newton's laws themselves (to form what is called quantum mechanics and relativity) to explain the observations. However, even in the newer modern theories, the law of conservation of momentum is still valid.

Last edited: Jan 25, 2012
9. Jan 25, 2012

### spaghetti3451

Pardon me if I'm wrong, but the vehicle is a non-inertial reference frame when it is accelerating, isn't it? And we all know Newton's laws of motion are not valid in a non-inertial frames? :-)

10. Jan 25, 2012

### Dickfore

No, relative to you, while you are sitting still in the vehicle.

11. Jan 25, 2012

### D H

Staff Emeritus
No, it's not, for several reasons.

The most important reason is that Newton's second and third law are valid only in an inertial frame of reference. The modern interpretation of Newton's first law is that serves as a test of whether a frame of reference is inertial. Newton's first law tells us whether his other laws are even applicable.

It's also important to consider the historical background. Newton's first law went directly against the grain of Aristotelian physics. Newton was establishing a background for his other laws. (Aside: Only Newton's third law is Newton's. Newton explicitly attributed the first two laws to his predecessors in his Principia.)

A third reason is that there are some pathological cases such as Norton's dome where Newtonian mechanics from the perspective of Newton's second and third law only are non-deterministic. Add in Newton's first law and voila! those non-determinstic solutions disappear. Newton's first law in fact does add something that is not in Newton's second law.

12. Jan 25, 2012

### Rakinniya

Then its worng isn't it??....It is an accelerating frame of reference....provided we don't fall down

13. Jan 25, 2012

### Rakinniya

so which is more fundamental??? Newton's third law of Motion or The law of conservation of linear momentum???? Both can't be...

14. Jan 26, 2012

### Dickfore

They're equivalent within Classical Mechanics. However, when you consider particles interacting with a field, the field carries momentum as well. Total momentum is conserved, due to homogeneity of space, but Third Newton's Law may not hold anymore.

15. Jan 26, 2012

The above comment surprised me and seemed to be paradoxical.After a quick search I read something similar in a Wiki article.But I am still surprised.The problem as I see it is that the second law expresses the first law in that when F equals zero dv/dt=zero(in other words the object must be at rest or moving with constant velocity).If my interpretation here is correct we have the second law serving as a test to tell us whether the second law is applicable.

16. Jan 26, 2012

### Dickfore

I think you made a typo. If it was supposed to read the way I corrected it, then you're right.

17. Jan 26, 2012

Thanks for pointing out what you thought was a mistake but it's not a typo.The main point I'm making is that the second law expresses the first law.

18. Jan 26, 2012

### D H

Staff Emeritus
Don't look at it that way then!

I, along with others (you have found some of the others), prefer to look at Newton's 1st as distinct from the other two laws. Newton made his first law distinct from his second for some reason. Why? This is perhaps a bit of appeal to authority, but Newton was dang smart. He certainly was smarter than am I, smarter than almost everyone at this forum, and quite possibly the smartest person on the planet, ever.

Newton's 1st does a nice job of resolving some pathological cases in Newton's 2nd law. I've raised the specter of Norton's dome in various other threads. Here's a brief synopsis of a recent discussion:

19. Jan 26, 2012

### Fredrik

Staff Emeritus
I had never heard of Norton's dome, so thanks for mentioning that. I found a nice explanation of it on youtube.

The non-trivial solution that's presented at 8:40-9:00 has $dr^2/dt^2=1/12(t-T)^2$, so at t=T, i.e. the moment when the ball starts rolling, the acceleration is =0. So it doesn't look like Newton's first can rule out the non-trivial solutions.

I have personally never been a fan of the idea that Newton's second law should be reinterpreted as saying something very different from what Newton actually said. I would say that the first is a special case of the second. I think that a better way to teach mechanics (to an audience with some mathematical maturity) is to start by defining Galilean spacetime (instead of the first law) and then present Newton's second law in the form x''(t)=f(x'(t),x(t),t). Here it would be appropriate to also talk about the existence and uniqueness theorem for differential equations of this type. Mass can be introduced after explaining why we need it, as in example 3 here. I would of course mention the third as well, but I think of it as less significant than the others. It's just a statement about what sort of forces we should expect to encounter. Note that Newton's third is automatically present when we state Coulomb's law or Newton's law of gravity. (If the force that an arbitrary particle exerts on another arbitrary particle is given by the standard formula, then the statement isn't just about what A does to B, but also about what B does to A).

Last edited by a moderator: Sep 25, 2014
20. Jan 26, 2012

### D H

Staff Emeritus
Be careful there. There's a perennial argument over whether Newton said F=ma or F=dp/dt. In fact, he said neither. His Principia is for the most part calculus-free. Saying that Newton's 2nd is F=ma or F=dp/dt is a bit of historical revisionism.

That said, you can take Newton's 1st at face value as eliminating all but the trivial solution to Norton's dome. The ball is initially at rest perched atop the dome and the net force on the ball is zero so long as it remains perched atop the dome. Ergo the ball remains at rest perched atop the dome forever.

That is putting the cart before the horse, miles and miles before the horse!

Most students of science and engineering are taught Newton's laws in high school, well before they have had calculus at all, let alone the mathematical maturity you are talking about. They are re-taught Newton's laws in freshman physics at the same time they are just starting to learn calculus.