JEE ADVANCED 2020 SOURCE QUESTION | CORRECT PREDICTION | Surface tension pathfinder

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Q : An amount of water is poured on top circular face of a horizontal glass cylinder . Excess amount of water sills over the edges and a little amount of water remains there in a thin uniform layer, a potion of which is shown in the figure . If contact angle is and surface tension of water is , the vertical thickness of the water layer far away the edge is t. the point in the plane of the diagram on the liquid protrudes the most is D. The vertical depth of this point from the horizontal free surface of the water layer is d , is density of water and g is acceleration due to gravity . Mark the correct relation(s)
(a) (b)
(c) (d)
/ @physicssirjee
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[@therock8408]

This question came today in advance 2020..nailed it sir !!!..I watched this video a day before the exam and fetched me 4 marks 🤩🤩

[@PHYSICSSIRJEE]

Great to hear👍 that I could be of some help , it definitely motivates me to better on my channel 😀. Thanks for letting me know 🙏

[@bharghavaoruganti8325]

PHYSICS SIR JEE - JANARDHAN - IIT JEE and Olympiad Yes sir this came did it

[@neilhanda8903]

what is your rank by the way ?

[@therock8408]

@@neilhanda8903 4527 crl

[@Sam-zi2mx]

nice prediction sir u are awesome

[@lakshyasharma8519]

OMG similar question in jee advance 2020. GR8 SIR JEE

[@simisimi9362]

Beginning with the first concept of the channel. Am grateful enough for such nice problems sir.

[@percyyay980]

Thank you very much sir.. 💙 I'm not sleeping without watching atleast 3 videos of ur channel daily sir..😁❤

[@vedantkokate971]

we could even find the quation of the curve since we have a relation between theta and y coordinate and we can write the slope in terms of theta

[@shivx3295]

Best teacher ❤❤❤❤ I can't thank you enough how nuch you have helped me and also created a love in my life for physics 💗 I will definitely in future repay the debt i have with you ❤

[@user-ew2tk8mn2w]

Sir for example if we take the the entire thickness t to a long distance like in second fbd you had taken only till d but, would there be a surface tension force acting on the bottom left part of it? 4:45. Sir in this case you had taken d fbd of only the meniscus but if we take an entire body of long length wouldnt there also be a surface tension force on its bottom left corner connected to the surface, because sir isnt the part connected to the surface also not in bulk of the fluid?

[@boominadharshankarunakaran7979]

It will be completely vertical, wouldn't affect the horizontal eqm

[@swathik8422]

SIR @ 10:43 IN THE SECOND FBD WONT THERE BE A SURFACE TENSION ON THE RIGHT ACTING DOWNWARDS?PLEASE CLARIFY...OR IS IT LIKE THAT THE FORCE IS THERE BUT YOU CHOSE NOT TO SHOW IT BECAUSE WE ARE NOT INTERESTED IN Y AXIS FORCES

[@govinddugaye5042]

yes last part of ur statement true

[@anmoldeepsingh9281]

the profile of the protrusion will be asymmetrical right? thats why we didn't just halve the t length... what is the reason of this asymmetric nature, is it because the internal pressure along the depth of protrusion is varying hence the excess pressure will be different and simultaneously the radius of curvature will be different across the entire monoclinic surface?

[@PHYSICSSIRJEE]

Yes👍

[@seediscute]

but again, why is the excess pressure varying and not like the usual spherical shapes?

[@hetalparekh9472]

Sir A doubt in the second part of the question ( @10:07 )where the depth d was found in the FBD for the surface Tension alpha* dl and P0* dl*d has been taken if instead we were to take the entire circular part(annular donut shaped part) of it it would have been alpha* 2pi*r and P0* 2Pi*R*d my point being that the radius would be different at both the places won't this be an inconsistency since now that dl won't come out common, when I had thought of solving the problem I had thought we should take an angle d theta and then multiply by radius at the points we want to write the force. Hovewer with that logic I couldn't proceed further. Please could you explain what is wrong with what I have been trying. I understand that you have taken a piece that is not strictly circular but because it is in differentials it can be approximated like that ,Also if the radii were taken very close to each other then also the problem gets fixed however shouldn't the FBD be true even if the radii were at different parts that is the issue I am not able resolve . Could you please help resolve it sir. (I had come from the AITS video of the sphere that gets submerged in the water and there I had got this doubt there was similar fbd, in that you had mentioned this video but this doubt still persists) Thank you sir

[@arvindpathak918]

Sir please also explain it using Laplace formula

[@sunritroykarmakar4406]

Sir can you give some theory on this section..would be really grateful

[@JEE-zm3sh]

9:38 1.Sir in second FBD why is surface tension at right bottom not being considered? because due to asymmetry they would not cancel out right

[@amritaghosh7541]

YOU ARE A GENIUS!

[@samarthtankasali8298]

Wow sir you predicted so accurately.

[@PHYSICSSIRJEE]

😁🙌

[@vedant6598]

Now I am addicted to this channel more than anything ❤️❤️

[@Prashant-fh6px]

i solved similar type of problem in 500 challenging problems by dc pandey and anurag mishra illustrations which helps me in solving this ques in first go :) your explanation is awesome

[@ayushsarangi4095]

Anurag mishra illustration me fully explain kiya h sb?

[@girdhansingh1814]

Awesome physics explanation sirjee ❤️

[@simisimi9362]

Revised this one nd so happie i was able to do this time on my own. Thank you sir 🙌

[@LuciferFlux]

Sir which resources should I refer to in order to get a proper understanding of this topic ? I feel I'm lacking conceptual clarity to solve problems like these which are up to the level of JEE advanced and olympiads.Thank you!

[@schrodingerscat4971]

I am addicted to your videos sir

[@PHYSICSSIRJEE]

It's not a bad addiction 😛

[@schrodingerscat4971]

@@PHYSICSSIRJEE yes sir I have improved a lot in physics after following your channel you are one of the few on youtube who actually gives advance level content..thank you so much sir❤

[@sanchitmukherjee2410]

Superb explanation. And a good prediction before JEE Advanced 2020.🤎🤎🤎

[@vedbrahmbhattiit-j4497]

OK

[@sat20y]

Bro u omnipresent op

[@18bhavin80]

Cant we write a force eqn in vertical direcn? @PHYSICS SIR JEE

[@PHYSICSSIRJEE]

Discussed it in a future video of JEE advanced question solution... You can find it in the playlist of 'surface tension' in the description of above video

[@chanakyavasantha8134]

verey level🥰🥰😍

[@prmomaths757]

Sir please also make solutions of physics fiitjee aits and GMP.

[@muqeemkhanghauri]

Horizontal forces are balanced in first case but vertical forces are not balanced i think at top contact there must be some contact angle so that vertical forces balance the weight of elemental liquid

[@PHYSICSSIRJEE]

Please refer to the comments section below by a subscriber named varidhisahaya ... Raised a similar valid doubt. I need to have a relook at the solution in a new video as I mentioned in reply there🙂... Till then you may refer to the alternative FBD provided there as a reply. I am providing the same here too👇

[@PHYSICSSIRJEE]

I know, my drawing here is a bit crude but this should help 👇, the slant section is taken instead of vertical section and horizontal Force equilibrium is considered... As mentioned in the previous comment it includes the entire height/ thickness t. drive.google.com/file/d/1IdW9d58J4giqgIKawuYOoNwE3nb657eG/view?usp=drivesdk

[@akanshkh26_08]

Sir I have a little doubt here At 10:22 when you started doing the same thing for a depth d instead we can do it for any x (less than t) and end up getting the same answer. That is d=x then how can we say that this is the maximum protruding distances depth. I mean that for any general depthx less than t I end up getting the same value you calculated for depth d Why is this so? Am I missing somewhere 🤔

[@PHYSICSSIRJEE]

If you are taking any distance , then horizontal component of surface tension force will exist at right most part of fbd. For the max protrusion point the tangent at right most point is vertical , so in balancing horizontal forces , the surface tension force at right most point won't contribute

[@akanshkh26_08]

@@PHYSICSSIRJEE sir surface tension acts for some contact between liquid and solid but at the rightmost part at depth x there's no contact so why any kind of surface tension acts other than the upper horizontal one?

[@rahulpatne7793]

@@akanshkh26_08 Not only solid liquid contact but at liquid-liquid contact also surface tension acts... That's how we initially defined it.. S= force due to surface of liquid on one side of an imaginary line on surface on the other side per unit length of imaginary line.

[@akiiiphysics3345]

Can you solve this particular question by Laplace sir... Thank you

[@PHYSICSSIRJEE]

Yes , but we need to know the radius of curvature at D . It's not a spherical shape

[@akiiiphysics3345]

@@PHYSICSSIRJEE yes sir, someone sent me wrong solution through Laplace that's why asked .. I do agree with you... Though from force method had done. Thank you sir

[@shravanram5464]

How is the liquid buldge in vertical equilibrium because I took a vertical component of The upper surface tension

[@mvkgcs]

sir,I think there should be one more 'alplha.dl' in vertically downward direction in the second free body diagram .But it's not of any use as we are considering the horizontal forces only . am I right sir?

[@simplephysics104]

Simply brilliant and awesome

[@prateekbheda9802]

thank you sir very nice explaination 😁

[@dhirendrakumar3767]

How contact angle is drawn , one side in solid and other toward bulz part , then theta will not be same

[@jeeranker1167]

AMAZING!!!

[@anishtakshak6515]

Thank you sirji 😊❤

[@juliancarax4797]

sir you are an amazing teacher

[@shwetasinha2506]

Hello sir, Sir can we say that the angle made by liquid in the lower half is theta, whereas there is no angle made by the liquid in the upper half because of the weight of protuding part? If iam wrong please correct me sir. Regards

[@MayankGoel447]

Sir is there any way find out the shape of the buldge

[@Xjdhdhxhdhdd]

Thank u sir🙏🙏👌👌

[@radewoosh3356]

Sir in first fbd, why aren't vertical forces balanced?

[@PHYSICSSIRJEE]

They are balanced but will contain weight of FBD and is not of any use for solving the problem

[@dontopenthisaccount1637]

Even we don't know abt normal force no sir

[@shaileshgupta2343]

Superb sir ...

[@nikhilchauhan2097]

SIR I GET CONFUSED ON WHICH SURFACE TO TAKE CONTACT ANGLE AND ON WHICH DIRECT TANGENTIAL LIKE IN THE UPPER N BOTTOM SURFACE PLS WAITING FOR UR REPLYಠ_ಠಠ_ಠಠ_ಠಠ_ಠಠ_ಠ

[@PHYSICSSIRJEE]

Watch the surface tension videos in order in playlist properly 👍in order

[@Fighter_Believer_Achiever]

Very well explained

[@rajjoshi3431]

Thanks, Sir.

[@rudrakshsingh6614]

Sir me too also watch yesterday

[@_TheInquistive]

Sir there wouldn't be some adhesive forces at the contact surface

[@PHYSICSSIRJEE]

Adhesive force and cohesive force concepts are microscopic concepts and what we draw here are “macroscopic force” fbds … difference is explained in the next videos uploaded in the surface tension playlist in description below. In macroscopic form the force between the solid and liquid is related to “ Laplace pressure “ and that’s also discussed in a later video on contact angle. Please visit the playlist

[@_TheInquistive]

@@PHYSICSSIRJEE Thankyou so much sir I am new to this channel and will explore the channel

[@afzal1104]

sir, at 7:17, how can we be sure that the upper alpha*dl will be horizontal only, and not at some angle, like the lower alpha*dl ?

[@PHYSICSSIRJEE]

It was assumed. It should be mentioned in the question

[@devanshdwivedi623]

Sir in first fbd why are we not taking surfact tension force of sides of bulged element

[@PHYSICSSIRJEE]

Time stamp of your doubt?

[@devanshdwivedi623]

5:11 Where green arrows are present If we visualise in 3 d Surface tension force should be acting there also

[@PHYSICSSIRJEE]

@@devanshdwivedi623 they will into the plane (behind) and out of the plane ( in front of the figure ) , get cancelled. Nothing in the rightward or leftward direction to effect the equation we wrote at 5:11

[@vgeetha7840]

Sir may I know any course for jee 2023 going on in your channel

[@vgeetha7840]

Sir i am unable to get it lesson wise questions

[@shiv3992]

really amazing sir

[@Fighter_Believer_Achiever]

Thank you very much sir 🥳🥳🥳🥳

[@varidhishayana6890]

Could you kindly comment on the vertical force-balance of the 1st FBD (entire protruding portion of the liquid)? Especially if the vertical inner boundary is moved slightly outwards (radially), so that the free-body is not touching the sold floor anymore...

[@PHYSICSSIRJEE]

Vertical force balance concept ( Laplace pressure ) upon insistence of students is covered in later videos of surface tension... It's in the playlist

[@PHYSICSSIRJEE]

kzbin.info/aero/PLtzNDt-x8_02O5K_5QwtMLJhCVPTuwiwM Please go through the last two videos of the playlist (starting with JEE advanced 2020 Sol) ... Also in the comments of "contact angle" video, I remember sharing a document regarding the same. Please go through those videos there and comment there if any further help is needed 🙂

[@varidhishayana6890]

@@PHYSICSSIRJEE I did go through the other videos, but my concern remains. Consider the picture 5:00 minutes into the video. In the 1st FBD, let's focus on the surface-tension forces at the top and bottom, both of magnitude (alpha * dl). In another video, you have correctly argued that the lower surface-tension force cannot possibly be horizontal (even though a problematic JEE question supplies that false assumption). You have nevertheless assumed that the upper surface-tension force points horizontally to the left. I would argue that this force cannot be horizontal either; in fact, its vertical (upward) component must exceed the vertical-downward component of the lower surface-tension force, by an amount proportional to the weight of the protruding portion of the liquid. Hence, the upper surface of the liquid, at height 't', cannot be entirely flat. Let's argue this by contradiction: Suppose the upper surface directly above the circular base is indeed completely flat (as shown in the picture on the left, in the video at 5:00 minutes). Then, the Laplace pressure at that top portion must be zero; hence there can be no pressure difference at the top surface. So, the pressure at the bottom surface (circular base) of the liquid is (Po + rho * g * t). The resulting normal reaction (N) is just enough support the weight of the (cylindrical column of) liquid just above the circular base. There is nothing to support the weight of the protruding portion (beyond the circular base). In other words, theta cannot be obtuse. Even if you could assume a vertical (upward) line-force acting along the rim of the circular base, in order to compensate for the weight of the protruding portion of the liquid, this would create a torque (couple), that would topple the protruding portion. Please let me know where my reasoning fails. Thanks. ... P.S. In your other video on weight of a liquid drop, you have assumed nonzero curvature at the top of the liquid drop. This curvature is directly related to the weight of the protruding portion of the drop.

[@varidhishayana6890]

I have verified my assertions with numerical computation of the shape of the water drop. The point on the drop surface directly above the point where it separates from the floor cannot be flat. It should slope even more sharply, towards the vertical. If you flip your first FBD vertically, you get a more realistic shape. Let me know if you disagree. Best ...

[@PHYSICSSIRJEE]

@@varidhishayana6890 your analysis seems to be spot on upon initial reading especially the argument using "weight of drop video"... There is nothing I can disagree with 👍🏼. There is a need to revisit this problem situation in a separate video. I will try to do it in upcoming months once I am free from my commitments 🙂.Thank you for taking so much time and effort in expressing your views 🙏

[@srisaisubramanyamdavanam9912]

Sir force due to surface tension is t*alpha na sir. I think so there is no need of consideration of dl

[@PHYSICSSIRJEE]

It's alpha.dl . Please check FBD in the video again 👍

[@PHYSICSSIRJEE]

@@techtronics6121 time stamp of your doubt please ?

[@Sonu-hv4ef]

Sir; in 1st fbd which force is balancing the weight part?

[@PHYSICSSIRJEE]

Already discussed it in a future video, please check out the surface tension playlist in description... It's the concept of Laplace pressure

[@Sonu-hv4ef]

@@PHYSICSSIRJEE thanks alotttt sir, I’ll check it out.

[@jaishankar5450]

Great sir ji

[@indiann871]

Really helpful ☺️❤️

[@Nick-lg7rz]

♥️♥️👌👌 thank you sir

[@KapidhwajSonithB]

Sir, how can we say that, if we draw a vertical line from the point where liquid touches the surface ... It will meet the upper surface of liquid perpendicularly?

[@PHYSICSSIRJEE]

Good question, it is assumed that when the liquid is protruding like that the curve at the top remains horizontal beyond table edge and then curves down... It's from diagram but it would have been better if mentioned in the question. We assumed because no information of angle at top is given ,like they did at bottom

[@KapidhwajSonithB]

@@PHYSICSSIRJEE ok sir!!

[@ankitsoni9275]

Thanks ❤️

[@ankitsoni9275]

Why earthmetic mean is (p° + pgh/2) Why not (P°+pgh)/2.....

[@shraddhapathak187]

Sir, how to decide the direction of surface tension force , which act at an angle pi- theta. Why it is not opposite in direction to that what is. Shown. And one more doubt is that, is surface tension used in this question is T(lg).

[@PHYSICSSIRJEE]

Surface tension always tries to act as a pull . Yes Tlg

[@shraddhapathak187]

@@PHYSICSSIRJEE then sir in cappillary rise it acts opposite to the direction as in this question

[@dheeraj839]

Sir pls don't put that images like no one can do and tough pls put as akash goyal sir as kept only the question that is best .

[@PHYSICSSIRJEE]

Where?

[@sunritroykarmakar4406]

woah this came in advanced right?

[@ayush1643]

yup

[@endlesseducation2845]

#23 For my revision

[@gauravbanik8240]

Omg this question came in jee adv 2020

[@vamsichappidi7960]

Electric field last question in pathfinder

[@simplephysics104]

Really brilliant sir...you are top notch simply.... thank you very much sir

[@omjoglekar3677]

what a question !! so amazing.

[@PHYSICSSIRJEE]

Thank you 🙂👍🏼

[@sakshampaliwal8170]

Easier type of this question asked in jee advanced 2020!!!

[@ayush1643]

yup

[@AnirudhVermaBCS]

Are you in aakash saksham paliwal?

[@sakshampaliwal8170]

@@AnirudhVermaBCS yy

[@sakshampaliwal8170]

@@AnirudhVermaBCS do I know u?

[@AnirudhVermaBCS]

Are you in spotlight batch?

[@charchitjainiitm]

Maja aa gaya

[@harshitmishra7138]

i think i saw you in iitian explain's advanced booster checklist video also, altho i am not sure...anyways, seeing people like you who watched the same videos i am watching rn and then ending up in IIT's, sure does boost my confidence..your hard work paid off man

[@charchitjainiitm]

@@harshitmishra7138 yes, i followed iitian explains throughout my jee journey, this channel and mka sir channel are like gems, their content is relevant and unmatchable.

[@harshitmishra7138]

@@charchitjainiitm i agree man. this channel, iitian explains, pwag, insp, all of em are gems.

[@deepabisht5449]

Wow 🤩🤩🤩🤩

[@00ANURAG15]

Sir I need an important suggestion. Sir I am an early starter,although I think quite far for future preparation. So one thing which always hits my mind is that in order to get top 150 rank, 1)What should be correct sequence:Irodv -krotov then Pathfinder MCQs or Irodov then directly Pathfinder MCQs 2)Which should be purchased for advanced:Last 48 years PYQ or Last 15 yrs PYQ . Please reply sir 🙏

[@PHYSICSSIRJEE]

You may skip krotov if you don’t have time. 15 year pyq is enough in general. But for information heavy topics like modernist physics, thermodynamics, solving all pyqsis recommended

[@PHYSICSSIRJEE]

Already answered at other place

[@user-ew2tk8mn2w]

Sir for example if we take the the entire thickness t to a long distance like in second fbd you had taken only till d but, would there be a surface tension force acting on the bottom left part of it? 4:45. Sir in this case you had taken d fbd of only the meniscus but if we take an entire body of long length wouldnt there also be a surface tension force on its bottom left corner connected to the surface, because sir isnt the part connected to the surface also not in bulk of the fluid?

[@trunalwandile4656]

sir where did you use the information that bulge at D is maximum ??

[@PHYSICSSIRJEE]

Please read my 3 weeks old reply to same doubt of akansh khandelwal

[@trunalwandile4656]

@@PHYSICSSIRJEE yes sir,got it