Foundations: Newton's Third Law and time reversal invariance

Doc Al
Mentor

Do you understand the Third Law?

Your explanation should be like, "when you push the wall the wall pushes you back with the same force", shouldn't it be?
That's a reasonable description of the Third law. Where's the problem?

Reciprocal action means 'increase in your weight in the opposite direction'. So when you apply a force on a body, the body's weight increases in the opposite direction as explained in the above three examples.
Huh? What are you talking about? Sounds like you are confusing "true" weight (due to gravity) with "apparent" weight (a measure of the support or normal force). What does this have to do with the third law?

Your examples have more to do with the Second law than the Third. (And they are all "business as usual", covered in every freshman physics class.)

"When you push the wall the wall pushes you back with the same force"! What an explanation!
That's a statement, not an explanation.

Good night!
You'd better get some sleep!

vanesch
Staff Emeritus
Gold Member

If you followed the thread correctly, you would know its the primary schools kids we are talking about. In primary schools gravity is introduced as a force with direction of acceleration towards the centre of earth.
In primary school it is impossible to introduce "the direction of acceleration", as it is a vector derivative. Even the concept of "acceleration" in one dimension is not taught in primary school. The concept of uniform motion is only introduced in secondary school.

But in any case I think you are trolling, because according to you, one should talk about *spacetime curvature* in primary school now ???

And, btw, the third law has nothing to do with gravity.

I do accept that it would be better to use the term weight rather than gravitational mass for school kids easier comprehension.
That's the case, btw.

d'Alembert's force
Fictitious force
pseudo force
centripetal force
centrifugal force

They are just colorful jargons, they don't exist and would only make physics seem more complicated for kids. The only forces that exist in nature are:

Gravitational force
Electromagnetic force
Weak nuclear force
Strong nuclear force

Do you get it love?
One more such comment and you get infraction points.

Dale
Mentor

As far as the below terms are concerned

d'Alembert's force
Fictitious force
pseudo force
centripetal force
centrifugal force

They are just colorful jargons, they don't exist
This is a sticky subject and is not as simple as you make out here. The existence or reality of inertial forces is (like everything else) very strongly dependent on the definition of "exist" or "real".

Inertial forces (like time dilation and length contraction) are measurable, but they are also frame-dependent. Some people think that anything that can be measured must exist, so they would claim that inertial forces exist. Other people think that anything that is coordinate dependent cannot be real, so they would claim that inertial forces are not real.

Personally, I don't feel that there is a very good definition of "exist" or "real" so I am not prepared to make a strong claim either way; I just recognize the complicated nature of the issue.

Personally, I don't feel that there is a very good definition of "exist" or "real" so I am not prepared to make a strong claim either way; I just recognize the complicated nature of the issue.
When you start saying things like; 'good definition of exist or real', 'fictitious forces', 'pseudo forces', 'complicated nature of the issue', 'true vs apparent', 'ifs and buts'. Sorry to say, but they only mean one thing, you have failed to grasp it!

Imagine you are a judge, and there is a witness who uses phrases like the above, what impression would you get?

When you say 'it is complicated' what you really mean is 'it is complicated for me'. When it is complicated for you, tell me what would it be like for the kids, and this is the point here?

On the surface of earth or inside an accelerated spaceship, you feel the same thing; your weight or gravitational mass' where is the complicacy? You start using terms like d'Alembert's force and it is bound to look complicated.

one should talk about *spacetime curvature* in primary school now ???
Correct! spacetime curvature has a future, those large number of fictitious forces have no future. Spacetime is much easier to grasp and relate to (when you give the freely falling elevator anology) than a term like fictitious or pseudo.

btw, have you conducted any scientific research to find out what is good and what is not good for the kids? As for me, what has a future is good, what is fictitious is not good.

And, btw, the third law has nothing to do with gravity..
After which scientific experiment and analysis this conclusion was drawn? In physics we study Motion, here we are discussing the third law of Motion, and gravity causes Motion. Where is the inconsistency?

I think you are trolling. One more such comment and you get infraction points.
The same person has made the two statements is hard to believe.

"when you push the wall the wall pushes you back with the same force".
That's a reasonable description of the Third law. Where's the problem?
Except that it is useless (pushing the wall has no future) and we are talking about laws of Motion! Instead of just commenting back on me, why don't you try and explain it Doc?

Summary:

When you use the axiom "weight (gravitational mass) is proportional to acceleration in the opposite direction" you can explain all the three scenarios without using unwanted inventiones, and explain it in terms of motion. All the three scenarios I mentioned have a future, kids would study them in detail in the future.

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Doc Al
Mentor

Summary:

When you use the axiom "weight (gravitational mass) is proportional to acceleration in the opposite direction" you can explain all the three scenarios without using unwanted inventiones, and explain it in terms of motion. All the three scenarios I mentioned have a future, kids would study them in detail in the future.
Summary: Complete and utter gibberish.

Apparently, you've never understood the use of pseudo-forces and accelerating reference frames. Sorry, but personal theories are not permitted here.

Dale
Mentor

When you start saying things like; 'good definition of exist or real', 'fictitious forces', 'pseudo forces', 'complicated nature of the issue', 'true vs apparent', 'ifs and buts'. Sorry to say, but they only mean one thing, you have failed to grasp it!

Imagine you are a judge, and there is a witness who uses phrases like the above, what impression would you get?

When you say 'it is complicated' what you really mean is 'it is complicated for me'. When it is complicated for you, tell me what would it be like for the kids, and this is the point here?

On the surface of earth or inside an accelerated spaceship, you feel the same thing; your weight or gravitational mass' where is the complicacy? You start using terms like d'Alembert's force and it is bound to look complicated.
OK, since you have grasped the issue and it is so simple and you are so smart and I must be an idiot to have ever thought it was complicated please post your clear and easy definition of "exist" which can be applied to forces to show that the forces you listed do exist and inertial forces do not.

ZapperZ
Staff Emeritus