# The Naked Scientists Forum

### Author Topic: Newton’s third law of motion violates the law of Cause and Effect  (Read 2922 times)

#### CitronBleu

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##### Newton’s third law of motion violates the law of Cause and Effect
« on: 26/06/2014 12:13:00 »
Newton’s third law of motion violates the law of Cause and Effect, dominant principle of Classical Physics.

If Newton’s third law is correct, then how did momentum begin ?

Newton’s third law states:

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For every action, there is an equal and opposite reaction.

To make the paradox clear, we replace the words action and reaction with the word force, as we should. This gives us :

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For every force, there is an equal and opposite force.

#### alancalverd

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##### Re: Newton’s third law of motion violates the law of Cause and Effect
« Reply #1 on: 26/06/2014 16:56:44 »
No paradox. If you push something, you can feel it pushing back at you for as long as it is accelerating.

Momentum doesn't need to "begin" because it is a conserved vector, so the net momentum inside a closed system is zero. You push the car, you feel it resisting as it accelerates, and the earth moves ever so slightly backwards.

There are no paradoxes in classical physics, only misunderstandings and flawed observations. There are possibly still a few bugs in quantum physics but every discovery resolves another apparent paradox.

#### PmbPhy

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##### Re: Newton’s third law of motion violates the law of Cause and Effect
« Reply #2 on: 26/06/2014 18:00:16 »
Quote from: CitronBleu
Newton’s third law of motion violates the law of Cause and Effect, dominant principle of Classical Physics.
No, it doesn’t. Why would you make such a claim? What are you basing it on? I should point out that it doesn’t always hold. For instance, in some instances where two charged particles are moving relative to each other the force exerted on one charged particle is not equal and opposite to the force it exerts on the other charge. If the field itself didn’t have momentum then this would violate the principle of conservation of momentum.

Quote from: CitronBleu
If Newton’s third law is correct, then how did momentum begin ?
What do you mean “how did momentum begin”? Are you asking how it was originally defined? If so then the first hint of it appeared in Newton’s Principia where it was related to something called the quantity of motion which was mass times speed. In modern Newtonian mechanics we defined momentum as the product of mass times velocity. See http://en.wikipedia.org/wiki/Momentum

An acquaintance of mine wrote an article on this. I have it and will upload it onto my website if you’d like to read it. It’s called
Did Newton forget his own laws of motion? by A.P. French, Am. J. Phys., 52(1), Jan. (1984). The abstract is online at http://link.aip.org/link/ajpias/v52/i1/p13/s1
Quote from: CitronBleu
Newton’s third law states:
Quote
For every action, there is an equal and opposite reaction.
That’s how Newton phrased it. It’s not how physicists phrase it in modern classical mechanics textbooks. For example; in Classical Mechanics – Third Edition by Goldstein, Safko and Poole (2001) they define it on page 5 as follows
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…the forces two particles exert on each other are equal and opposite.
Quote from: CitronBleu
There’s no paradox here. Even from what you wrote, there’s no paradox. All you did was state Newton’s Third Law of Motion which when stated correctly is always correct. The correct statement which is always correct in classical mechanics, even in special relativity, is that when two particles interact by contact forces the force on particle one by particle two is equal and opposite to the force on particle two by particle one.

#### CitronBleu

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##### Re: Newton’s third law of motion violates the law of Cause and Effect
« Reply #3 on: 26/06/2014 18:25:13 »
Momentum doesn't need to "begin" because it is a conserved vector, so the net momentum inside a closed system is zero. You push the car, you feel it resisting as it accelerates, and the earth moves ever so slightly backwards.

Hi alancalverd,

What is a conserved vector if you don't mind me asking ?

I did a search for the terms online and could not find a clear definition:

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No paradox. If you push something, you can feel it pushing back at you for as long as it is accelerating.

Even in space ?

#### Bill S

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##### Re: Newton’s third law of motion violates the law of Cause and Effect
« Reply #4 on: 26/06/2014 20:36:54 »
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Even in space ?

I believe so.  In fact, in space you could be the one doing the moving.

#### Bracewell

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##### Re: Newton’s third law of motion violates the law of Cause and Effect
« Reply #5 on: 23/07/2014 11:47:32 »
It is the first law that may be hard to prove. Firstly a body at rest is needed otherwise everything is momentum.

#### alancalverd

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##### Re: Newton’s third law of motion violates the law of Cause and Effect
« Reply #6 on: 23/07/2014 13:52:35 »

Hi alancalverd,

What is a conserved vector if you don't mind me asking ?

(a) it's a vector - i.e. a quantity with magnitude and direction and (b) the total momentum of a system doesn't change when components of the sytem interact - it's conserved.

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No paradox. If you push something, you can feel it pushing back at you for as long as it is accelerating.

Even in space ?

Yes, that's how rockets work

I guess you are wondering how all the bits of the universe came to be moving in the first place. Consider a hand grenade. Total momentum equals zero. Then it explodes and bits of shrapnel fly in all directions. If you add the initial momentum of all the bits, the total is still zero but each bit now possesses momentum that it can pass on to a target.

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##### Re: Newton’s third law of motion violates the law of Cause and Effect
« Reply #6 on: 23/07/2014 13:52:35 »