The h Bar

8:00 AM

I'm of 10th grade but forced to study of JEE Advanced level, think.

user228700

@MAFIA36790 Yes, I didn't mean cramming; meant understanding and then keeping it in my head.

Carbonyls have a nice governing principle: Most of the reactions are nucleophilic substitution reactions. To first order, a nucleophile is usally a electron rich group such as N,O. This principle will help nail down most of the carbonyl reactions in one principle

As for the other ones, I need to know the exact syllabus of the reactions you covered before I can comment

Swapnil Das

@Secret Could you teach me? :P

user116211

@Kaumudi I must say though, I too had problem in memorising the reactions.

user228700

@Secret Oh, well, it's a very long list and I don't even everything that's in it.

user228700

8:02 AM

@MAFIA36790 Yeah :/ I think I'ma maintain a separate notebook and revise bit by bit everyday or something. That should help; that's how I was able to remember everything for my 12th finals :-P

Secret

just refer me to it, cause while my own list is quite extensive, I don't know if it covers more than you need

user116211

@Kaumudi Good.

user116211

Happy that I've come out of it.

user228700

@MAFIA36790 :-P Good for u.

Swapnil Das

Could anyone give links for crash course of OC? Please.

user228700

8:03 AM

@Secret Right. Okay, once I get a hold of the correct and complete list, I will ping you. Dunno how long it will take tho. Max. by the end of this week. I'm gonna learn Chemical Bonding now.

Swapnil Das

I'm having 2 weeks for studying the same.

Secret

Central guiding principle for most organic reactions: Electrons always want to move towards more positive groups

So if you have a group that is electron rich and want to donate electrons, it will attack groups that have a lack of electrons. Bonds will then shift accordingly as the reactions proceed

The leaving group is the one that basically sucks in all the electron pair, and breaks off from the group

As an illustrative example, the nucleophile can be an amine nitrogen atom, and the leaving group can be a Cl, the Cl will leave to become $Cl^-$ and the N will attack the C centre of the CO group

The +ve charge on the N then want to suck electrons, therefore (to first order) the N-H bond is going to be weak

Thus the H becomes relatively acidic and something more electronegative will want to abstract it (It can be the - charge but the solvent (in this case can be some alcohol) is more likely because the molecule is surrounded by it

now Cl, being very electronegative, want to suck away electrons, This makes the central carbon quite positive in the process, and thus the -charge will migrate into the C-O to form a double bond

This result in the amide as expected

user228700

8:19 AM

@Secret: This is my syllabus:

user228700

A smidge overwhelming, yes, but I'm fairly OK with the basics of most every on there. Still, I need to dig much deeper and be more comfortable with everything, which includes memorizing all the reactions.

user228700

@Secret: I imagine that u have a lot more in ur syllabus..?

user228700

And I have less that two months. Sigh, I hope I'm able to get it done.

user228700

8:27 AM

Also, wonder where JR is today :/

Secret

The syllabus is more or less simailr to me back then in high school, except possibly the detection of N,S and few named reactions liek the Kobe reacton and such

Let's see how to summarise this...

user228700

@Secret Oh, I see...

Secret

You guys are not taught anything about mechanisms and arrows (though at high school level they are not required to be wrote down)?

So for most things in the section called concepts:

user228700

@Secret Yes, we are! Don't think about this syllabus as pertaining to high school; a lot of advanced stuff is taught to us. Mechanisms are super important for the exam that I'm preparing for actually.

Secret

they are not that advanced. Yours is simialr to my high school level stuff. Meahcnism arrows can help you to group many reactions together

*continueing analsyis*

user228700

8:34 AM

@Secret Yes, the syllabus is relatively not that advanced, I guess, but then again, you're from Hong Kong, right? So I imagine that our syllabus will be at the same level...

Secret

Ok for anything electronic (bonds, stability etc.) in the concept section, there's a guiding principle:
1. You don't want charges to be localised (think electrostatic repulsion). Thus the stability pattern of sp3>sp2>sp of carbocation is explained by you have more groups around the central carbon atom to delocalise the charge

2. For bond clevages, think abotu where electrons are moving when you break the bond. Hetero (means different) lytic clevage means you end up with cationion and anion intermediates, whereas homo (means same)lytic you will end up with radicals (unpaired electrons)

sigma and pi bond are related to the orbitals involved. pi is caused by p type orbitals overlap sideways whiel sigma is head on overlapping. multiple bond are p bonds

hybridisation means mixing s and a number of other orbitals to give some hybrid orbitals that acts like something in between s and p. There geometry at the carbon centre follows the principle of minimising electron repulsion, using this you get the tetrahedral, triangle and linear shaps of sp3 sp2 and sp respectively

Inductive effects can be phrased in terms of what groups are electronegative, and how they will distort the electorn density in the molecule. If a group is near a highly electronegative atom, the bonds in that group is goin to be weaker in msot cases

user228700

Omg. Omg. Omg. Are u attempting to give me an intro to the basic ideas behind OC?

Secret

Kinda, I am hellping you to join the dots (I think)

user228700

Omg. U are too kind :-O

Secret

Typical example is the acidity. COOH is going to be more acidic OH because COOH has more oxygen, thus the OH bond is going to be weaker than that of OH in OH compounds

user228700

8:45 AM

(Screenshoting all this)

user116211

@Kaumudi You can bookmark the conversation too and then save the page.

user228700

@MAFIA36790 But to refer to it later, I'd need to use the internet, no? I prefer screenshots.

user116211

@Kaumudi save the page.

Secret

Resonance is a extremely fancy term for delocalisation of electrons in the molecule. You can represent it using resonance structures. A molecule with resonance is more stable because the electron are delocalised. Resonance is important in explaining why amides are not very basic, as most electrons are delocalised between the OCN bonds

user116211

@Secret Resonance is only for valence bond theory.

user116211

8:50 AM

Delocalisation is inherent in MO Theory.

Secret

It also explains why benzene con't act like a "cyclohexatriene", thus it does not do alkene reactions

MAFIA: I know, but her syllabus mentioend about that, and her sylabus does nto have MO stuff in it

user228700

@Secret Actually, it does :-P

user116211

@Kaumudi No, you don't.

user116211

You have MO, that's right.

user228700

@MAFIA36790 I think I know what I have :-P

user116211

8:51 AM

But the syllabus doesn't have the rigorous approach of MO in analysing molecules.

Secret

The combustion stuff relies on how CO2 is linked to the number of C atoms, H2O relate to the number of H and O atoms. It usually wont get mroe complicated than that. The ratios of the moles of these are important in using to figure out a CnHmOp molecule formula

user116211

You just need to know the resonance approach, that's it.

Secret

You need newman projection to assign R,S configurations, so if you can handle Newman you can handle R,S

user228700

@Secret Hm, OK...

Secret

For Eand Z, I use the mnemonic: Z looks like S, thus "same" hence same side

The various types of isomers you kinda need to commit to memory. But they should not deviate from conformers, enantiomers, diastereo(iso)mers

user116211

8:56 AM

@Kaumudi, Secret is giving you an awesome lecture ;D

Secret

These are all stereoisomers, which means they only differ in the way the atoms are arranged. This is contrasted with geometic isomers like E and Z, where the atom connections and bonds are different

IUPAC naming, you just have to get used to it. Luckily, after but, things follow the usual numebr for numbers. Always name the longest carbon chian first, then name the side groups

Wow that's "Concepts" in a nutshell

Now for the next sections:

user228700

@MAFIA36790 IKR?! So awesome :-D

Secret

alkene and alkynes behave similarly because they can open their multiple bond to accept two groups (4 for alkynes)

Terminal alkynes have acidic H because of the huge electron density in the triple bond, thus the C-H bond is rather weak

user228700

Where did that guy @SwapnilDas go? Missing this lecture of sorts :P

Secret

For acid catalysed hydration of alkynes, you just kinda hav to remember that the metal is mercury. This is the commony thing about msot reagents and conditions. They jsut work and the explanation on whay they work is way too much to worth the trouble

though one common thing about metal species in reactions is that they have a lot of protons, thus they really help at opening otherwise very storng CC bonds

similarly for KMnO4 reaction. A rather simple mechanism can be drawn, but I simply remeber it as the Mn cleave the bond and then replace the ends with OH groups

Ozone does simialr things, they like to chop open bonds. You can imagine that two of its oxygens get inserted in between the bonds to give two carbonyls

electrophiles like electons, thus they add onto alkyenes and alkene in the expected way. Hydrogenation is catalysed usually by Pt or C and because the molecule is attached on the Pt or C surface flat sided, the H can only add on in a cis fashion

The topic of metal acetylides I am not famialr with (not covered in any of my uni courses), but since terminal alkynes are quite acidic and electron rich, the H can be easily substituted by a metal

Details will depend on what type of metal acytylide you covered in the syllabus

Benzene reactions:

All substitution reactions, the m,p directing groups can be summarised into one single package by the reaction mechanism and the inductive effects caused by the groups. Drawing multple resonance structures to work out where the partial positive charges in the intermiedtae is important

If a electron donating group is next to a partial positive charge, the reaction is going to be favoured

user228700

9:17 AM

Omg, I cannot believe this is happening. @Secret: Are u revising ur notes alongside? :-P :-)

Secret

The Reimer Tieman reaction is new to me, thus I cannot said much about it. However there's a way to understand why KOH and Cl3CH. The chloroform is quite acidic due to all those Cl around it, thus KOH can easily abstract the H away to form a anion intermediate. Also note that phenol is quite acidic thus you should expect the H to be lost. Antying else I don't fully understood but should be able to reaosn with pushing arrows

(For anyting I said is new, have use the mighty google pplus my background knwoledge to quickly assemble something rough), for anything else it is mostly in memory

The Kolbe reaction is new to me. In fact, back then we are nto taught phenol reactions, but pyridine reactions, which behave oppositely to benzene

Charactristic ractions of the following:

user228700

@Secret Huh. OK...

Secret

Alkyl halide reactions are mainly based on the idea of carbocation and leaving groups. There are mostly substitution reactions

Grignard reagents are basically sources of carboanion (otherwise to first order, why will they like to bond with metal ions like Mg?), thus they are nucleophiles

Alcohols:

If you want to dehydrate something, you need to make the OH group leave. OH is a sigma bond and it quite storng bonded in the C, thus you are going to heat the mixture. The Al2O3 I don't recall any intuitive way to remember thus you just have to go with it

Oxidation is relatively straightforward. Basically, H is removed and you keep inserting oxygens. This explains why OH becomes =O and finally COOH (if it is an aldehyde, thus there is an H to leave with O to form water)

In high school level, KMnO4 is a very strong oxidiser thus when in doubt, use it (unless the compiund is so sensitive that you need a weaker reagent)

Reduction is the oposite, removing O and adding H. This is usually carried out by reducing agents like LiAlH4

Reaction of alcohols with alkali metals are rather unique, thus there is no way to derive that this reaction exists. However to rationalise the products, it means that alkali metals are basic enough to rip off the H form the alcohol formign an alkoxide

In fact, if you think about it, the first member of the series formed by lienar alcohols is water, thus alcohol react similarly as water in most cases

user228700

9:35 AM

@Secret Yes, right...

Secret

If you have a compound with a lot of chlorine atoms bonded to some non C atoms, chance are this thing is going to put chlorines into you molecule. The reason why pyridine is needed is foun the the mechanism, but that might be a bit too mcuh to derive in exam conditions

similarly if other reagents full of Br etc. are used

Also remember metal groups in general like to suck electorns away, thus it is going to make some bonds weaker for things to be plugged in

Williamson ether synthesis: you need to make alkoxide, thus you need to pull H away. Try something basic, You also need something for the alkoxide to attack. By pushign arraows, you need something good at leaving such as halide

oximes and hydrazones: general mnemonic: if you see -NH2 paired with O=C- then O+2H give water, buss these off and then click the gorups together

(The reaction is of course more involved but this is one of the class of reactions that has a plug and play like way to remebr it)

user228700

I'll brb...

Secret

keto enol forms: Extremely important, the enol acts as a nucleophile and ready to attack electrophile such as C=O

It can be done by first protonating the =O and then arrow push

Anilines: Use inductive effects to explain basicity. Going from NO2 to NH2 means reduction

Amine reaction with nitrous acid is kinda new, however part of the product can be guessed by the fact that nitrous acid is oxidising

Azo coupling: The azo is +ve charge, it is going to be attacked by a nucleophile, push arrow to get result

Sandmeyer reaction is too complicated to summarise

Carbylamine reaction: chloroform is quite electrophilic, get an amine to attack it and then arrow push

haloarene have electron withdrawing X, thus nucleophilic thus do nucleophilic substitution reactions. The intermediate is a sign reversed version of the benzene one

Carbohydrates:

glyclosidic bonds invovle OH and OH snapping together, and give water

There is no easy way to memorise the names of the mono and disacchraides

hydrolysis is the opposite of glycosidic bond formation

Practical organic chemistry: I belive they have ran labs to have you remeber these?

user228700

10:08 AM

@Secret Yes :-)

Secret

Wurtz reaction is new and is a radical reaction, thus arrow pushing need to be modified. One way to reason this is that Na want to lose one electorn only, thus R becomes R .

(All alkali metals tend to lose one electron)

R. and .R are perfect in clicking together

UV is high energy, can excite the alkane bonds for the halogen substitution to happen. Your lab should have a contrast between alkene halogen and alkane halogen reaction to help you remeber that

user228700

@Secret Actually, we haven't done those many labs in OC...

Secret

Decaboxylation to form alkane: This is new casue I only have learnt decarboxylation of dicarbonyls (which has a nice 6 membered ring mechanism), thus I cannto say much about it

and finally:

Carbonyls: For Perkin, Cannizarro, Haloform, Aldol, amide, acid choride, haloform reaction:

4 basic rules dhouls be enough to group them all together via arrow pushing:
1. enol is a good nucleophile
2. The C in C=O is always being attacked by nucleophile and then reform, which kick away the leaving group
3. conjugate double bonds are more stable, thus if you end product can be made into conjgate double bonds, lose water to do so
4. Base is often used to deprotonate the reagent to give nucleophile

user116211

10:24 AM

WTH!!

user116211

@Secret, What have you done with h bar?

Secret

Basically given a lecture on org chem to Kaumudi

user116211

I could have send my mafias to teach you a lesson for doing that ;P

user116211

We discuss only maths here ;))

user116211

Anyways. continue; won't intervene....

user228700

10:28 AM

@MAFIA36790 IKR?! :-O @Secret is too good.

Secret

Core guiding principle of organic chemistry:
1. Electron like to minimise repulsion and (to first order) will redistribute to do so
2. Factors that affect stability and rate of something
3. Metals like to coordindate to bonds and electorn rich stuff
4. When in doubt, *push arrows*, use MOs
5. Most reaction conditions need to be memorised, some can be reasoned with chains of logic

user228700

@Secret: How the heck am I supposed to thank u for this?! :-O >.<

user228700

?!?!

user228700

?!

user228700

Thanks so effing much! I mean, what the hell?! You are very kind and generous, omg. Thanks so much again! :-D _/_

Secret

10:39 AM

np

Secret

11:53 AM

NOTHING: The Science of Emptiness

►

heather

12:05 PM

@Secret, fits how the hbar is right now

Secret

I migth ask PhSE a nothing question soon

There is something called nothing exists (Logic bomb)

user116211

12:26 PM

@Secret You mean vacuous truth?

Swapnil Das

Hi.

0celo7

Wtf is that above

user116211

Don't know.

user116211

Some nonsense philosophy?

Secret

nope, it's something more than that, basically going as far to say there is a "thing" called "nothing"

user116211

12:27 PM

oh Jesus ;/

Swapnil Das

Philosophy is dead: Agree?

user116211

@SwapnilDas Not at all.

Secret

[Hence the logic bomb tag]

Swapnil Das

@MAFIA36790 Hmm OK.

Secret

Swpnil, in respond to your question: see previous conversation of me with Kaumudi for org chem in a nutshell

user116211

12:29 PM

You will be pleased to know I'm working on linear algebra @0celo7.

Swapnil Das

Thanks @Secret

user116211

A matrix is always defined over a field.

Swapnil Das

@MAFIA36790 why is linear algebra called so?

user116211

Because it is linear in nature.

Swapnil Das

I see.

Secret

12:39 PM

After reading 7 out of 12 pages of a chemistry paper that is for my phd proposal, I realised i really have not read this paper before

looks like i need to reread it

user228700

@0celo7 Hang on, do u mean what Secret posted or the Chem. lecture? :-P

0celo7

Yes

user116211

@0celo7 He spoilt h bar.

Secret

$\lvert\hbar\rangle = \bigotimes_{i\in S} \frac{1}{\sqrt{N}}\sum_{i=1}^{N}\lvert \textrm{subject}_i\rangle$

user116211

@Secret \textrm

user116211

12:45 PM

Apart from maths, all the other states are not even the eigenstates of the discussion operator.

Secret

typo: $j \in S$

Infinity: The Science of Endless

►

Secret

1:03 PM

quora.com/…

Nothing as a state of absence or negation vs nothing as the actual thing

How about: Boundless, Endless, Timeless, Spaceless Meta Nothingness

user116211

2:02 PM

1

Q: Affine connection in general relativity

In The GR lecture my teacher deduced the relation between affine connection and the metric tensor according to the following way: He firstly wrote the relationship of two tensors like this (I understand): $$A^{\mu }{}_{[P\to Q]}=A^{\mu }{}_{[P]}-\Gamma _{\nu \sigma }^{\mu }{}_{[P]}A^{\nu }{}_{[P]...

0celo7

2

Q: Affine connection in general relativity

In The GR lecture my teacher deduced the relation between affine connection and the metric tensor according to the following way: He firstly wrote the relationship of two tensors like this (I understand): $$A^{\mu }{}_{[P\to Q]}=A^{\mu }{}_{[P]}-\Gamma _{\nu \sigma }^{\mu }{}_{[P]}A^{\nu }{}_{[P]...

general-relativity differential-geometry metric-tensor coordinate-systems differentiation

what is that notation

user116211

Which one?

0celo7

see my comment

user116211

ohh.

0celo7

Why do GR posts always seemingly devolve in a mess of indices

Slereah

2:16 PM

Because nobody wants to use math notation for GR

Except for topology posts I guess

0celo7

can you understand the notation

Slereah

From point $P$ to $Q$ I guess?

0celo7

What does that mean

Slereah

Maybe it's a geodesic tangent vector

0celo7

o.o

user228700

2:23 PM

Damn, where is @JohnRennie today?!

user228700

Is anybody here interested in talking about the centre of mass of a system for a bit?

0celo7

Dead :(

I've been saying it all along

user116211

This man asked Donald Trump a question and became a meme

0celo7

wow what a loser

user228700

@0celo7 Dude, he's not even 60 yet! I hope he's well and alive :/

user228700

2:30 PM

An athlete uses physics to shatter world records - Asaf Bar-Yosef

►

user116211

Maybe he found some major alien bugs in his servers.

user228700

I was wondering if anybody would like to discuss this..?

user228700

@MAFIA36790 Yeah, that's what I've been thinking too. But he comes here a bit before he starts his work so :/

user116211

Greys might have abducted him.

0celo7

JD finally got to him

user116211

2:33 PM

@0celo7 Oh my...!

0celo7

I miss JD

I should send him an email

user116211

Send him some Digital Papers of Einstein.

QuantumBrick

@0celo7 because GR IS a mess of indices

0celo7

@QuantumBrick Differential geometry is not a mess of indices

It's the insane way that physicists think of it that turns it into one

QuantumBrick

GR is not about differential geometry =/
It's about riemannian geometry. I've studied Riemannian Geometry in the math department in my Msc and it WAS bloody horrendous.

The calculations were really, REALLY huge.

Not as big as in GR, but they were ugly, too.

0celo7

2:44 PM

Then you had a terrible course, I'm sorry

there's a few bad computations (Bianchi identities, I'm looking at you)

but nothing with indices

and if you did geometry analysis, then yeah, some stuff there gets horrible

but not really index horrible

@QuantumBrick Can you name a bad computation in Riem geo?

QuantumBrick

@0celo7 I remember some particular demonstrations that were horrendous: Bianchi Identities, Rauch's theorem, and an example about hyperbolic spaces gave me a headache for about a year.

0celo7

Rauch is long, yes, but there are no indices there

Ok, if you explicitly computed stuff for hyperbolic spaces that was messy.

QuantumBrick

Ah, ok... Indeed. If your problem are the indices, then you're right: GR is the worst. But, really... Hyperbolic spaces were the same as GR: they made me cry and call for my mother.

0celo7

@QuantumBrick The worst computations I've done were in Brendle's Ricci Flow and the Sphere Theorem

I'm not sure how the proof works because I just remember terrible inequalities

stuff like this

QuantumBrick

@0celo7 D=

0celo7

2:52 PM

I checked a few of these...

user116211

OMG!

QuantumBrick

I remeber I had to prove the Ricci tensor's invariance under conformal transformations, once. I believe this was the word thing I've ever done, but nothing compared to what your sending.

0celo7

@QuantumBrick Ok, right.

I've managed to mostly avoid...wait

I think I did that proof once.

QuantumBrick

I hate numbers and calculations. Riemannian geo and GR were the tip of the iceberg for me: I'm NEVER ever going to touch any spacetime that isn't flat.

0celo7

:(

I'm a Riemannian geometer

QuantumBrick

2:54 PM

HAHAHAHAHA

0celo7

I have some computations for conformal stuff somewhere

I wrote it while I was bored I guess

trying to find it

QuantumBrick

@0celo7 I should have talked to you when I was studying that suicidal subject.

brb

0celo7

@QuantumBrick Wald actually has a complete derivation of the Riemann tensor for a conformal metric

awesome, I do have it typed out

QuantumBrick

OH no

it was for the WEYL tensor

now I remember.

0celo7

Yeah, the Weyl tensor is invariant.

Well, compute the transformation rules for the other tensors, then plug everything into the definition of the Weyl tensor

Trivial ;)

QuantumBrick

2:58 PM

That's what I wrote in my exam.

0celo7

@MAFIA36790 Yes. Geometric analysis is horrible at times

QuantumBrick

Teacher didn't like it, though

0celo7

How did he want it done??

Did you memorize the transformation rules for all the other curvature tensors?

QuantumBrick

This was the last question in a set, and they asked you to show the invariance of each and other tensor required to prove weyl's tensor invariance... I can only remember that ONE of them was not invariant (I think it were the christoffel symbols), and you needed to look for an actual cancellation of terms in the Weyl calculation.

Man, don't make me go back to that. I can still feel the pain.

My brain still swells a little.

Let's talk of cats and trees.

0celo7

quantum mechanics and weed?

QuantumBrick

3:01 PM

hahahaha that's a good starter.

Why did the superposed cat cross the street? To buy weed.

0celo7

Snoop catt

@QuantumBrick I can't think of an area of physics that doesn't involve horrible calculations from time to time.

user228700

@DanielSank: Hello! :-)

Slereah

What's the heuristic behind not allowing QFT to be done on operator valued smooth functions again?

But only distributions

QuantumBrick

@0celo7 MIne! hahahah Just kidding. But indeed, I almost never deal with horrible calculations.

John Rennie

3:18 PM

@0celo7 I'm not dead, I just smell that way

user228700

@JohnRennie You're alive! :-D I was just going through the transcript and realized in horror that u last spoke on Saturday afternoon and it's Monday night now.

Slereah

Where is Yuggib when you need him

John Rennie

@Kaumudi I've been visiting relatives and drove home yesterday. By the time I got home I was too tired to have much interest in anything other than my armchair and a quiet beer :-}

user228700

@JohnRennie Ah, I see, OK :-)

user228700

Are u well enough to maybe have a tiny discussion now..? Sheepishly smiles :-P

DanielSank

3:29 PM

@JohnRennie That's called a beerchair.

John Rennie

@DanielSank :-)

DanielSank

@Kaumudi Hey! Red card! Sheepish smiles are not allowed to be used to get help with physics.

John Rennie

@Kaumudi Yes, fire away. Though it must be getting late in Tamil Nadu ...

Slereah

@yuggib @ACuriousMind halp

user228700

@DanielSank What?! :-O Why?!

user228700

3:30 PM

@JohnRennie Not really. It's only 9PM :-)

user228700

I'm learning about the center of mass of a system now and I happened to watch a video about the Fosbury Flop.

user228700

And in the video:

user228700

1 hour ago, by Kaumudi

An athlete uses physics to shatter world records - Asaf Bar-Yosef

►

John Rennie

OK

user228700

I learned that Mr. Fosbury was able to break world records because of the way his body is oriented during the jump.

user228700

3:34 PM

That while jumping, his COM doesn't go all the way up with him; that it actually stays below the bar, while the rest of his body curves away from the bar, above it.

John Rennie

OK

user228700

And in the video, they say that because of this, he(his muscles) needn't exert the force required to raise his COM up above the bar as well.

user228700

Or another way of saying this is that with the same amount of force, he's able to go much higher than when he did it facing the bar.

John Rennie

Yes, all OK so far. And your question is?

user228700

What does that mean? I mean, force is a very real, tangible concept; but the COM of a system is just something we defined(made up) and it doesn't make sense to me when they say that with the same amount of force, he's able to lift his body much higher, simply because his COM isn't in his body. I mean, it's not like his mass is going somewhere...or is it?

John Rennie

3:43 PM

Suppose I throw a long bendy pole into the air. The trajectory of the pole looks like (diagram to follow)

Shing

center of mass is a useful tool (that defined by human) tells us a lot about real world.

user228700

Awaiting one of ur terrific diagrams :-D (:-P)

John Rennie

I throw the pole up to it bends into a U at the apex and straightens back out again as it falls back. Does that make sense?

user228700

Yes.

John Rennie

The height of the apex is $h$ as labelled on the diagram. However the centre of mass of the pole never gets as high as $h$. Is it clear why that is?

user228700

3:50 PM

@JohnRennie No. Hang on...

John Rennie

Hanging ...

user228700

Is it because it bends the way it does..?

John Rennie

@Kaumudi Yes

user228700

OK...

John Rennie

The height of the centre of mass is sort of an average height of the whole pole.

Some bits of the pole are at height $h$, but most of the pole is at a height less than $h$ because of the way it bends down.

user228700

3:52 PM

@JohnRennie Right...

John Rennie

So when we average out the height of all the bits of the pole we get a centre of mass height that is less than $h$.

user228700

@JohnRennie OK...

John Rennie

And this is how the Fosbury flop works. The jumper starts out leaping straight up, but at the apex they bend their body downwards just like the pole in my drawing.

user228700

Hm...

John Rennie

So the uppermost part of their body is above their centre of mass.

user228700

3:55 PM

OK..?

John Rennie

You remain unconvinced?

user228700

Yes. My original question wasn't about how it is that the COM is below the body:

user228700

16 mins ago, by Kaumudi

What does that mean? I mean, force is a very real, tangible concept; but the COM of a system is just something we defined(made up) and it doesn't make sense to me when they say that with the same amount of force, he's able to lift his body much higher, simply because his COM isn't in his body. I mean, it's not like his mass is going somewhere...or is it?

0celo7

@JohnRennie Fix that please

@JohnRennie what

user228700

@0celo7 What?

John Rennie

3:59 PM

@0celo7 You want me to die or use deodorant?

0celo7

What?

user228700

@0celo7 You can watch that video. It's fascinating.

0celo7

@JohnRennie if deodorant works, use that

If it's bad enough that it won't work...you'd best consider the other option

user228700

@0celo7 But why?! Is this an inside joke that I dunno about? Seems like it.

John Rennie

@Kaumudi male humour - best ignored

user228700

4:01 PM

@JohnRennie Alrighty :-P

John Rennie

Back to the jumper - let me draw another diagram ...

user228700

Oh, OK...

John Rennie

Suppose we turn off gravity so our pole/jumper is floating in space.

user228700

OK...

John Rennie

On the left we have the jumper with their body straight.

And the dotted horizontal line shows the height of the centre of mass.

user228700

4:06 PM

@JohnRennie OK...

John Rennie

Now the jumper/pole bends the top half of their body downwards as shown on the right.

The height of the centre of mass can't change because there is no external force acting.

user228700

No gravity. OK...

John Rennie

So the centre of mass for the body on the right remains at the level of the dotted line.

But by bending the top half of their body down the jumper has pulled the bottom half of their body up.

Slereah

Is the distribution necessity just because of the CCR

user228700

@JohnRennie OK...

John Rennie

4:09 PM

Now turn gravity back on, and this is basically what the jumper does in the Fosbury flop.

user228700

@JohnRennie Damn. Okay, but what about what I said about the mass not being there and all?

John Rennie

They jump upwards so their arms go higher than the bar, then they pull their arms down over the bar and this pulls the rest of the body upwards.

As each bit of their body passes over the bar they bend it downwards on the other side and that pulls the bit of their body that hasn't reached the bar yet upwards.

user228700

@JohnRennie Uh huh...but what about my query regarding mass..? What point am I completely missing?

John Rennie

You asked:

> What does that mean? I mean, force is a very real, tangible concept; but the COM of a system is just something we defined(made up) and it doesn't make sense to me when they say that with the same amount of force, he's able to lift his body much higher, simply because his COM isn't in his body. I mean, it's not like his mass is going somewhere...or is it?

But I don't understand what you are asking.

The COM is most certainly not just something we have defined. For example the change in gravitational potential energy is determined by the height of the COM.

user228700

@JohnRennie Yes, this. Can you please elaborate..? It's not something we defined?

John Rennie

4:16 PM

No. Once the jumper leaves the ground (and ignoring air resistance) their centre of mass moves in the same parabola as if all their mass was concentrated as a point at the centre of mass.

user228700

@JohnRennie Huh. I'm finding it a little difficult to get my head around this. See, the first time I read this, I thought I understood most of what it is, but having thought about it some more, I most certainly do not understand :/ Wait, who came up with this idea? And why is it so important..?

John Rennie

Who came up with the idea of the centre of mass?

Apparently it was the ancient Greeks

user228700

Right. Why is this idea so important..? And how, sir, do u make sense of it in ur head..?

John Rennie

The way I think of it is that the centre of mass of an object is the average position of all the mass in it. If we were to replace the object by a small dense sphere then that small dense sphere would be located at the object's centre of mass.

0celo7

Tfw there's a blind old man just picking his nose

@JohnRennie are you like that :/

user228700

4:24 PM

@JohnRennie And that's it..?

John Rennie

@Kaumudi Yes.

user228700

@0celo7 If that also is supposed to be humorous, then there's definitely something wrong with ur taste, man.

user228700

@JohnRennie Hm...

John Rennie

An object in free fall cannot change its centre of mass, so when studying the trajectory of the object you can ignore how the object is flailing its arms and legs around and just consider the motion of the centre of mass.

user228700

@JohnRennie Hm, right...

user228700

4:28 PM

Okay, I guess as I work with this more, I'll understand better. Thanks sir! :-) Will u be back t'row, as usual?

John Rennie

I should be here tomorrow around 05:30 BST as usual :-)

user228700

OK :-) Good night!

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