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

## Comments

Sort by:

TopNewestyep 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) – Mvs Saketh · 2 years, 5 months ago

Log in to reply

@Mvs Saketh @Raghav Vaidyanathan @Krishna Sharma ? – Karan Shekhawat · 2 years, 5 months ago

Log in to reply

ALL THE BEST Raghav, Karan, Mvs for advanced :D – Krishna Sharma · 2 years, 5 months ago

Log in to reply

– Raghav Vaidyanathan · 2 years, 5 months ago

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.Log in to reply

– Mvs Saketh · 2 years, 5 months ago

i want the same, i am doing whatever i can for it, let us see what happens ,Log in to reply

Also is Shashwat and you both are in same Institute ? Since I saw that you are from same city .. am i right ? – Karan Shekhawat · 2 years, 5 months ago

Log in to reply

– Raghav Vaidyanathan · 2 years, 5 months ago

We are classmates man, for the last two years.Log in to reply

– Karan Shekhawat · 2 years, 5 months ago

Just curious ... How much he genrally get in tution ...?Log in to reply

– Raghav Vaidyanathan · 2 years, 5 months ago

It's high enough for other people to be bothering themselves with it. If you know what I mean.Log in to reply

@Raghav Vaidyanathan – Karan Shekhawat · 2 years, 5 months ago

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

@Mvs Saketh @Raghav Vaidyanathan ... I got it ! – Karan Shekhawat · 2 years, 5 months ago

Thanks... you bothLog in to reply

– Krishna Sharma · 2 years, 5 months ago

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

– Mvs Saketh · 2 years, 5 months ago

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 reallyLog in to reply

Look 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 – Raghav Vaidyanathan · 2 years, 5 months ago

Log in to reply

– Krishna Sharma · 2 years, 5 months ago

Looks similar to Einstein rosen bridge :pLog in to reply

This thing is correct. I will post solution. @Karan Shekhawat – Raghav Vaidyanathan · 2 years, 5 months ago

Log in to reply

If you are referring to an AIITS problem by FIITJEE, most of my friends are of the opinion that it is wrong. – Raghav Vaidyanathan · 2 years, 5 months ago

Log in to reply

Btw what was AITS Question ...? Please tell – Karan Shekhawat · 2 years, 5 months ago

Log in to reply

– Raghav Vaidyanathan · 2 years, 5 months ago

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

– Karan Shekhawat · 2 years, 5 months ago

I have posted it... Please do check it ..Log in to reply

– Karan Shekhawat · 2 years, 5 months ago

Please Reply .. wheather it is correct or not ....?Log in to reply

– Raghav Vaidyanathan · 2 years, 5 months ago

can you double check data?Log in to reply

btw , What answer do you getting ?

what is concept in this question... – Karan Shekhawat · 2 years, 5 months ago

Log in to reply

@Shashwat Shukla @Mvs Saketh @Raghav Vaidyanathan @Ronak Agarwal @Sheshansh Agrawal – Karan Shekhawat · 2 years, 5 months ago

Log in to reply