Problem 29 - Master Resource Book in JEE Main Physics

Question

Two pieces of glass plate one upon the other with a little water in between them cannot be separated easily because of (a) inctia (b) pressure (c) surface tension (d) viscosity

Step-by-Step Solution

Answer

The glass plates cannot be easily separated because of surface tension.

Step 1: Understanding the Problem

Two glass plates are placed on top of each other with a small amount of water between them. We need to understand why they cannot be easily separated.

Step 2: Identifying Forces at Play

There are a few possible forces at play here: inertia, pressure, surface tension, and viscosity. Let's break these down: - **Inertia** is the resistance of an object to any change in its state of motion or rest, which does not apply to the static situation described. - **Pressure** is the force exerted per unit area. While pressure is present, it does not explain the difficulty in separation on its own in this context. - **Surface Tension** is the elastic tendency of a fluid surface which makes it acquire the least surface area possible. This is due to the cohesive forces between liquid molecules. - **Viscosity** is the measure of a fluid's resistance to deform under shear stress, but this affect primarily flow behavior rather than static adhesion.

Step 3: Explaining Surface Tension

The presence of water between the glass plates introduces surface tension. This force arises because the water molecules prefer to stick to each other rather than to the air, creating a 'film' that holds the glass plates together. It acts along the surface of the water, pulling the plates together and making them difficult to separate.

Step 4: Conclusion

Given the context and understanding of these forces, the main reason the glass plates are difficult to separate is due to **surface tension** of the water between them. The cohesive properties of water act to keep the plates together, effectively "gluing" them in place.

Key Concepts

Cohesive Forces

Cohesive forces are the attractive forces that occur between molecules of the same substance. In the case of liquids, these forces play a significant role in determining how the liquid behaves on surfaces or in contact with other materials. A classic example is water, where cohesive forces are mainly due to hydrogen bonding.
These forces make molecules "stick" to each other, creating a phenomenon observable as surface tension. Surface tension is essentially a manifestation of cohesive forces, which results in minimizing the surface area of a liquid.
For instance:

• Raindrops form because of surface tension, making them round as the liquid tries to minimize surface area.
• When glass plates are held together by water, it's these cohesive forces that effectively "glue" the plates by forming a film of water that resists being pulled apart.

Understanding cohesive forces helps in appreciating why liquids exhibit certain traits, like clinging to themselves and forming droplets.

Fluid Mechanics

Fluid mechanics is the branch of physics that studies how fluids (liquids and gases) behave and interact with forces. It's a massive field that encompasses various principles and laws aimed at predicting and explaining fluid behavior.
Key aspects include:

• Motion and flow of fluids.
• Forces exerted by and upon fluids.
• Energy transferred through fluids in motion.

The study of fluid mechanics can explain phenomena like water flow through pipes, air dynamics over wings, and how forces like pressure and surface tension work in contexts like our water-ensnared glass plates example.
Cohesive forces in fluid mechanics are critical when dealing with surface phenomena, such as the difficulty in "unsticking" two glass plates with water between them. Here, the fluid mechanics' principles express how the cohesive nature of water molecules leads to increased surface tension, creating a resistant film between the plates.

Properties of Liquids

Liquids, as a state of matter, exhibit distinct properties that differentiate them from solids and gases. These properties are largely influenced by the intermolecular forces present and the mobility of particles within the liquid.
Some fundamental properties are:

• **Viscosity** - A measure of a liquid's resistance to flow. High viscosity means a thicker fluid, like honey, while low viscosity means a thinner one, like water.
• **Surface tension** - As previously discussed, this is the elastic tendency of liquid surfaces that tries to minimize surface area, playing a vital role in creating droplets or glue-like films.
• **Density** - It quantifies how much mass the liquid contains in a unit volume. It affects buoyancy and pressure characteristics.

Each of these properties can be observed in everyday life:

• When cooking oil spreads over water due to its lower density.
• When syrup resists pouring quickly due to high viscosity.

Overall, the properties of liquids define much of their behavior in natural and engineered systems, demonstrating the latent power of intermolecular attractions that drive fascinating effects like surface tension.