Please Help me in this Problem...

A water drop of surface tension T=0.07N/m and have mass m=0.2gram is placed two glass plates. The distance Between them is 0.01 cm .Find the Force of attraction between the plates ?

A)-2.8N

B)-3.5N

C)-0.7N

D)-1.25N

Answer is option-A (2.8N)

Please Post solution........

Thanks

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## Comments

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TopNewestLook at the figure. The force is due to the excess pressure outside as compared to inside the water droplet. We can calculate the pressure difference. We can also calculate the area of contact by approximating the shape as a cylinder. Then just multiply to get force.

@Karan Shekhawat

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Looks similar to Einstein rosen bridge :p

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yep answer is 2.8 N, The only tricky thing here is to find pressure difference inside the approximately cylindrical bubble , let me show you with a diagram

The concept is just as Raghav told

but i dont know hot to upload images (forgot )

so till you tell that, consider the equations

Total force due to surface tension

= 2*2.S.pi.R (the extra two since there are two contacts at two plates)

where R is NOT the radius of curvature but the radius of the approximately cylindrical dropand effective area =2.pi.R.H (the curved surface of cylinder

(where H is the distance between the plates

so pressure = force/area = 2S/H

i suppose now you can proceed

(Note- the likely mistake is to assume that this works just as a bubble and use 2S/R and then claim logically 2r=h and get 4S/H )

(here R is the radius of curvature )

the reason it doesnt work is because the curved surface is not spherical (yes , it matters)

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Can we the maximum distance between plates so that water is in contact with both plates?

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ideally, it is infinite, no matter how far you take, the water column will become thinner and thinner but still wet both sides till it becomes negligibly thin far far away, but ofcourse not really

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Thanks... you both @Mvs Saketh @Raghav Vaidyanathan ... I got it !

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And Just a little query .. what % marks are you getting in JEE advance Test's in ur coaching or in any Test series or etc... And what is your Target (I know there is time to strenghten our Practice and concepts... but still what are Probabble guess according to Tution Test's or any Test Seriees Based test's .... ?)

@Mvs Saketh @Raghav Vaidyanathan @Krishna Sharma ?

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I want to get \(300+\) out of \(360\). Or it's equivalent in whatever the total marks we have this year. Yeah, it is a bit high, but I want it. Also, my current percentage in coaching institute is nowhere near goal.

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Also is Shashwat and you both are in same Institute ? Since I saw that you are from same city .. am i right ?

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@Raghav Vaidyanathan

And It is great , I mean 300+ means you are Planing To get Below Air 25 .... am I rightLog in to reply

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Around 250 is good score for me, in coaching tests I m facing some problems with chemistry otherwise I m comfortable with P and M.

ALL THE BEST Raghav, Karan, Mvs for advanced :D

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@Shashwat Shukla @Mvs Saketh @Raghav Vaidyanathan @Ronak Agarwal @Sheshansh Agrawal

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If you are referring to an AIITS problem by FIITJEE, most of my friends are of the opinion that it is wrong.

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No It is not from AITS ... And I 100% Sure that this is correct one ... But I did not posted exact language ... So wait I'am Posting Question as It , again with data and options also ... So that you can match it ...

Btw what was AITS Question ...? Please tell

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Yes, please post original problem. A similar question appeared in AI2TS. Let's see.

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btw , What answer do you getting ?

what is concept in this question...

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This thing is correct. I will post solution. @Karan Shekhawat

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