Question

Calculate the force required to separate the glass plate of area 10−2m2 with a film of water 0.05 mm thick surface tension of water is 70×10−3Nm−1:

a) 25N

b) 20N

c) 14N

d) 28N

Answer 1

As we know that surface tension is given as: σ=Fl, where σ is surface tension of a fluid, F is the force applied and l is the length.

So, using the above equation we can find the force as: F=σl

Firstly, we need to find the length of the glass plate, by using the formula A=l×b

and then substitute the value in F=σl to get the force required.

Complete solution:

As we have the area of plate = 10−2m2 and thickness = 0.05 mm = 0.05×10−3m.

Surface tension = force / length

It is a glass plate, hence we have to consider both the areas of the slab ,

2 ×area = length × thickness

So, by using the formula:

A=l×b,

the length of given plate is, length =2 ×10−2 ×1000/0.05×10−3=400 m

Now, substitute the value of length in the surface tension formula to get the force required.

Since we have: σ=70×10−3Nm−1 and l = 400m,

So, we can write:

F=σl=70×10−3×400

F=28 N

Hence, the force required is 28N.

So, the correct answer is “Option D”.

Note: Surface tension is the attractive force exerted upon the surface molecules of a liquid by the molecules below which tends to draw the surface molecules below. This helps the liquid to acquire the least possible area. This phenomenon can be seen widely in nature. Such as in water droplets, air bubbles etc. Also, surface tension is helpful to various organisms such as insects that can float or slide over a denser water body due to this property of the fluids.