Question

A stick 2m long is floating on the surface of the water. The surface tension of water is $0.07{\text{ N/m}}$. By putting the soap solution on one side of the sticks, the surface tension is reduced to$0.06{\text{ N/m}}$. The net force on the stick is
A. 0.01 N
B. 0.02 N
C. 0.03 N
D. 0.04 N

Answer 1

Find the force on the surface due to surface tension in both the cases by using the formula $F = T.l$ , where l is the length of the object, and T is the surface tension, and the find the net force on the stick.

Complete step by step answer:
The length of the stick $l = 2m$
The surface tension of the water on the stick ${T_1} = 0.07{\text{ N/m}}$

The surface tension of the water on the stick when soap is applied on one side of the stick ${T_2} = 0.06{\text{ N/m}}$
We can see when the soap is applied to the stick, the surface tension on the stick decreases.
When the surface tension is applied on the surface, then surface fells a force on it, and this force is given as $F = T.l - - (i)$
Here when soap is applied on the surface, the surface tension changes and force on the surface also changes, hence the net force on the stick due to change in tension
$\left( {{F_1} - {F_2}} \right) = \left( {{T_1} - {T_2}} \right)l - - (ii)$
Now by substituting the value of tension in both cases, we get

$$\Delta F = \left( {{T_1} - {T_2}} \right)l \\\ = \left( {0.07 - 0.06} \right) \times 2 \\\ = 0.01 \times 2 \\\ = 0.02N \\\$$

Hence the net force on the stick is $= 0.02N$

Option (B) is correct

Note: Students must note that surface tension is caused due to the effects of intermolecular forces at the interface, and it depends on the nature of the liquid, temperature, and the surrounding environment.