# NCERT Class 11 Physics Solutions: Chapter 9 – Mechanical Properties of Solid-Part 5

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**Question 9.12:**

Compute the bulk modulus of water from the following data: Initial volume litre, Pressure increase Final volume litre. Compare the bulk modulus of water with that of air (at constant temperature). Explain in simple terms why the ratio is so large.

**Answer:**

Initial volume,

Final volume,

Increase in volume,

Increase in pressure,

Bulk modulus

Bulk modulus of air

This ratio is very high because air is more compressible than water.

**Question 9.13:**

What is the density of water at a depth where pressure is , given that its density at the surface is

**Answer:**

Let the given depth be

Pressure at the given depth,

Density of water at the surface,

Let ρ2 be the density of water at the depth

Let be the volume of water of mass m at the surface.

Let be the volume of water of mass m at the depth

Compressibility of water

For Equation and ,we get;

Therefore, the density of water at the given depth is .

**Question 9.14:**

Compute the fractional change in volume of a glass slab, when subjected to a hydraulic pressure of

**Answer:**

Hydraulic pressure exerted on the glass slab,

Bulk modulus of glass, Bulk modulus of copper

Where,

Fractional change in volume

Hence, the fractional change in the volume of the glass slab is