Calculating the surface tension using the method of Lecomt de Nouy

[arhzz]

Homework Statement

The surface tension can be determined using the Lecomte De Noüy ring method. The mass of the ring is m = 4.9 g and the diameter d = 64.1 mm. The tensile force just before tearing is in water at 25 ° C is F = 0,089 N. Calculate the surface tention

Relevant Equations

y = F/4*pi*r

Hello! I am having unexpected difficulties with this problem.A friend showed me an article regarding the Lecomde ring method,and at the bottom was this little problem that to solve.I've done it like this.

$$ y = \frac{F}{4\pi r} $$ where F is the given force and r is = 0,0325 in meters. The value I get is y = 0,22 N/m. But according to the solutions this is wrong,the right solution should be y = 0,1016 N/m.I highly doubt they are wrong,which means I made a mistake somewhere but I don't see where.The only formula that appered in the article was this and the usual surface tension.What am I i missing here.Thank you!

 

[haruspex]

You are given the mass of the ring. Does the force you are given include the weight of the ring?

 

[kuruman]

Homework Statement:: The surface tension can be determined using the Lecomte De Noüy ring method. The mass of the ring is m = 4.9 g and the diameter d = 64.1 mm. The tensile force just before tearing is in water at 25 ° C is F = 0,089 N. Calculate the surface tention
Relevant Equations:: y = F/4*pi*r

What am I i missing here.

You are misinterpreting the formula. The one here gives the expected answer and explains how the terms are put together.

 

[arhzz]

Okay so I've tried using the formula where the mass of the ring is take into account.

$$ F = m + 4
\pi * r * y $$

If I try to get y out of this it should look like this; $$ y =
\frac{F-m}{4
\pi r} $$

But I still do not get the wanted result.

 

[kuruman]

The weight is $mg$ not $m$.

 

[arhzz]

The weight is $mg$ not $m$.

Ohh,it said wait not mass,my bad I didnt read carefully.Now I get the right result,sorry for the silly mistake and thank you for your help!