How Newton's Laws work.

I s'pose ya could say, someone doesn't give a fig.

Let's start with Newton's Third Law. This is generally stated "for every action there is an equal and opposite reaction.". People sometimes misunderstand what the "equal and opposite reaction" means.

What this law actually means is that if A puts a force on B, then B puts a force on A (in the opposite directly). Examples of this...

- If your foot kicks a ball (putting force on said ball), the ball exerts an equal force on your foot.

- If an airplane uses jet engines to push back on the air (which is how jet engines work), the air puts an equal force on the airplane (pushing it forward).

Equal and opposite does not mean balanced... The confusion for some people is that if you had two equal forces acting on a soccer ball, by Newton's second law the ball wouldn't move (because they would cancel each other out). That is not what Newton's law says.

In the foot kicks ball example... one force is acting on the ball (and so the ball moves), the other force is acting on the foot (and the foot slows down). Neither of these forces is balanced.

There is an interesting consequence of Newton's Third Law when it comes to space travel. Since any action has an equal and opposite reaction, to propel a spacecraft (or anything else) forward, something needs to be propelled backwards with an equal force. For airplanes it is easy. As long as there is air around, you just push the air backwards. That is exactly what jet engines (or propellers etc) do.

Spacecraft operate in a vacuum... there is no air to push backwards. For this reason, a rocket needs to have what is called "reaction mass", this is matter that is expelled out of a spacecraft in order to accelerate it.

The problem is that once a spacecraft runs out of reaction mass, it can no longer accelerated (speed up, slow down, or change direction). At that point it is stuck in whatever course it happens to be on at that time.

In space, energy is not really a problem especially if you are able to collect light from a star. Reaction mass is expensive... and is a much greater challenge to longer space flights.

If a current passes through a magnetic field, the wire will move at a right angle to the field. But if a current passes through a disc from shaft to rim and the magnet is glued to the disc, it all turns with no equal and opposite force on anything. Pictures here:
https://www.youtube.com/results?search_query=faraday+motor

Ever heard of the perihelion precession of Mercury?

What's your point?

Of course there is an equal and opposite force for every force in this apparatus. This would be a basic question I would ask my high school Physics students., they would draw a force diagram (explaining all of the forces). It is pretty easy to show Newton's Third Law with magnets. If you hold two objects with magnetic attraction near each other and release them at the same time they both will move towards each other.

I could give you the answer if you want. In this case (as in every case) each force has an equal and opposite force.

Sure, relativity comes into it with Mercury, don't be a wise ass.

Every Physics student learns that Newton's laws can be easily derived from Einstein.

And yet Mercury ain't moving anywhere close to the speed of light, respective to the sun. Thus Newton's Laws don't apply in some circumstances other than near light speeds.

You are right Olivier, you caught me. Good boy!

Do you disagree that Newton's laws apply in almost any real world cirxumstances except near light speeds or in very deep gravity wells, or are you just being a smartass.

Congrats Max! It's the first you admit it when I proved you wrong. You're growing up. :-)

Congratulations Olivier, thank you for showing me up so that I can be a better person. If you ever make a mistake I will try to return the favor.

The point of this thread is to counter claims that Newtons Laws dont work in everyday circumstances like faraway motors or friction. And I wany to clarify how Newton's Laws actually work.

Do you at least support this aim?

You lose some, you win some.


They usually provide a good-enough approximation in everyday life (a formulation more precise than "they work" but it means the same thing), but not always. Eg GPS systems need to correct the clocks of satellites using general relativity, or the system would not work.

I don't think you know what you are talking about, Olivier. I don't know if you are trolling me... or if this is some sort of weird one-upmanship.

Early in any Physics class you run into the issue of of precision (as opposed to accuracy). You need to understand the difference in order to get the issue you are raising; that is, classical physics versus relativity. Newton's Laws highly accurate in all but extreme circumstances. You trust Newton's Laws every time you get onto an airplane or drive in a car. They got us to the Moon and to Mars.

Physics students, when they start to cover relativity, generally go through the exercise of calculating the difference between calculations using classical Physics (i.e. Newton) and Relativity. The point of this exercise is to give students an understanding of how accurate Newton is in almost every situation (outside of the extreme examples).

I don't know if you have every studied relativity and gone through this exercise (or if you are just googling exceptions to score points). If you did, I don't think you would use the phrase "good enough approximation".

There are correct answers in Science. And when you are talking about the motion of cars, airplanes, any existing spaceship, robots, birds, engines, motors, bullets, missiles, baseballs or flying fish.... Newton's laws will get you the correct answer.

I do think your are incorrect about this. That's all. I don't mind if you don't believe me.

And yes I have studied both the general and special versions of relativity, although the math of the latter is more familiar to me (I haven't studied non-euclidian geometry so I can't deal with the math of general relativity).

There is no such thing as a "correct answer" in science the way you appear to mean it, ie a theory that would predict observations with perfect accuracy and for ever. Only fools who've never practiced science believe in that.

Even general relativity does not perfectly fit astronomic measurements of Mercury's precession, when you take into account everything we know of, including the effect of other planets on Mercury's orbit. It comes much closer than Newton but not quite.

Newton laws offer a good-enough approximation of observed movements most of the times. But not always. The case of GPS satelites is well known, I'm surprised you never heard of it.

I am completely happy using the term "correct answer".

- When I was in college, I did problem sets that were marked with a check mark when they were correct, and an 'x' when they were wrong. I had no problem with this. I assume you did the same. As a teacher I marked student's work "correct" or "incorrect".

- When Jewels Verne says that a Faraday motor violates Newton's Third law, he is incorrect. Would you agree?

- When I get onto an airplane, the calculations say that the lift of the plane will be enough to keep it safely aloft. I never doubt that this is the correct answer... I trust it with my life.

Your problem is with the term "perfectly accurate". This is a term that I have never used. You don't need to be "perfectly accurate" in order to be correct.

You do need to be "correct" in order to fly robots to Mars. If you do your math incorrectly (and yes, they use Newton's laws) then your spaceship will crash.

This is devolving from a discussion of science to a discussion of the meanings of words. This is one of the reason that Physicists use math.

You don't know what you're talking about, rather.

I repeat: Newton offers a good enough approximation. You can call that "correct" because "planes don't fall to the ground", but in science there are theories and observations. And Newton's theory offers a good enough approximation of observations most of the time in our normal life, except for GPS.

And that, sir, is the correct answer.

And by the way, GPS systems could well use Newton only (and thus ignore time dilatation) and they would still "work". It's only that their accuracy in predicting your position on earth would slightly worsen year after year.

I'm bemused to observe this discussion which seems to be in the spirit of the pragmatists' one-liner.. 'truth is what works' .
Of course, 'what works' and ' why it works' are both negotiable !