You know how newton's 1st law is redundant? it's just a special case of the 2nd, what are some other redundancies in physics that you know of?

@RonaldVilliers It's not redundant, see physics.stackexchange.com/q/122231/50583 and also physics.stackexchange.com/a/70188/50583 (linked already earlier today)

@ACuriousMind have you ever read principia? Do you have a preferred translation if so?

my interest is now piqued when i read that newton didn't actually write the laws as they are known today

nope

I'm gonna try andrew motte

500 pages ah crap that's a long read :P

And reading pdfs on my monitor is terrible for my eyes.. will put it on the list.

@Dabed there's no need to worry about more formal stuff if you're fine with the magic

@RonaldVilliers apart from the fact that it's not the case in your example, a lot of stuff is "just a special case" of something else

thermodynamics can be derived from statmech

general relativity from string theory

Also not trying to pretend I know anything about the newton example but Leonard Susskind said it was a special case of the 2nd law which is why I used that example :P

but it was in a lecture series on classical mechanics if maybe the truth lies in QM/QFT or whatever

Hi @JohnRennie when do we take -g and when g ?

@cOnnectOrTR12 Hi :-)

There are two ways people use the symbol g and it's important to be clear how you are using it.

We normally measure distance upwards from the ground e.g. an object on the ground is at height zero, and an object 1 m above the ground is at height +1m. Yes?

Yeah

So we are taking the upward direction to be positive i.e. vector pointing upwards is positive and one pointing downwards is negative.

That means an object moving upwards at 1 m/s would have a velocity +1 m/s while if it was moving down its velocity would be -1 m/s. OK so far?

Ok

And the gravitational acceleration is downwards so the value of the gravitational acceleration is -9.81 m/s².

But, sometimes people use g to mean this gravitational acceleration i.e. g = -9.81 m/s².

And sometimes people use g to mean the magnitude of the gravitational acceleration i.e. g = 9.81 m/s².

In that case the gravitational acceleration is -g.

Can we take -g in upward direction?

The problem arises becasue the symbol g is frequently used to mean two different things and people often don't make it clear what they mean.

@cOnnectOrTR12 If you are using g = -9.81 m/s² then an object accelerating upwards at 9.81 m/s² would have an acceleration of -g.

@JohnRennie meaning if we take the downward acceleration with sign it will be g=-9.8 and so upward acceleration will be - (-9.8)

Yes, exactly :-)

But when we throw something does it accelerate with 9.8 m/s^2

The acceleration is downwards, yes?

I mean when body goes up does it accelerate with 9.8

No. If an object is moving upwards then its velocity is positive, but the acceleration is still negative i.e. -9.81 m/s².

How? Acceleration should be + in upward direction

The gravitational acceleration is always downwards i.e. it is always negative.

The acceleration is a = dv/dt. Yes?

When the body is moving upward then is it accelerating or deaccelerating

I guess de accelerating

@cOnnectOrTR12 Correct. If you throw an object upwards then its velocity decreases with time i.e. dv/dt is negative, and since a = dv/dt that means the acceleration is negative.

And the acceleration of an object in Earth's gravity is just g i.e. -9.81 m/s².

So the acceleration is negative because it is equal to g and g = -9.81 m/s².

@JohnRennie if an object is moving upward is it subject to one downward gravitational acceleration or two accelerations one upward and one downward

There is only ever one acceleration .

And for an object thrown in Earth's gravitational field that accleration is g i.e. -9.81 m/s².

But if it’s a rocket then tere are two. Right?

A rocket has a motor as well, so there are two forces acting on it.

But when we throw something aren’t there two forces here also , so two acceleration . One force is from us and other is the one applied by the earth?

We are only applying any force to the object while we are holding it. As soon as it leaves our hand we can no longer apply any force to it and the only force is the gravitational force.

Okay

So there is one +velocity and -acceleration. Net result is its speed decreases?

Yes :-)

And the value of this acceleration in both cases upward and downward motion is -9.8

Correct!

And there is no acceleration in upward direction

Yes

Only downward acceleration which first causes the body to deaccelerate and then accelerate

Yes

Thanks man :)

as always

:-)

if I travel in same direction in this universe will the universe end?

@BannedUser see physics.stackexchange.com/q/24017/50583

@Dabed well you asked about this at the right time apparently

Nobel Prize in Physics 2021: Half of the prize is shared jointly between Syukuro Manabe and Klaus Hasselmann "for the physical modelling of Earth's climate, quantifying variability and reliably predicting global warming", the other half is awarded to Giorgio Parisi "for the discovery of the interplay of disorder and fluctuations in physical systems from atomic to planetary scales"

I really hoped for Parisi

so even with current tech we don't know if universe is finite or infinite ;_;

In the "O(3) nonlinear sigma model", what does O(3) refer to?

And I don't understand what ron is saying in that answer.....

@Ratman indeed, it was very well deserved. I was anticipating Aspect and Zeilinger for the other half, but perhaps that's for another year...

(I also realised I misspelt "Klaus" in my original message, could a mod please correct that :p)

holy molly how can i guy get suspended for like 3 century lol

@BannedUser Ah, the orthogonal group of course. I was mixing it up with something else

Hmm, that was supposed to be a reply to my own question, not BannedUser

you are ron maimon?

i forgot group theory years ago... I was at elementary school when I was learning it

@NiharKarve and here I was thinking "Kalus is a pretty cool name"...

@Ratman where can I keep a watch on the works of contenders of Nobel prize?

@cOnnectOrTR12 well I don't think there is a place for this, if you want to see what the winners worked on you can look at their articles in google scholar maybe

@NiharKarve maybe It Is even a matter of age of the candidates, this year winners are quite old

@fqq @fqq Cool thanks for mentioning it I was just reading about it but as I don't understand much I didn't make the connection between Parisi and the replica trick, putting those terms on google gives some nice looking lectures on arxiv "Replica Theory and Spin Glasses" by Parisi and others authors (arxiv.org/abs/1409.2722) hope someday will put time to try to understand this

@cOnnectOrTR12 I don't know about a technical coverage of their work but in case it is useful for a layman article I was reading about it in quantamagazine.org/…

@Dabed very nice, i'll ready it

My thesis included some work on the Kardar-Parisi-Zhang equation, so that part is the Nobel prize is neat

And while I don’t know replica stuff, my PhD advisor definitely did