# NCERT Class 11 Physics Solutions: Chapter 9 – Mechanical Properties of Solid-Part 2

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Question 9.4:

Read the following two statements below carefully and state, with reasons, if it is true or false.

The Young’s modulus of rubber is greater than that of steel;

The stretching of a coil is determined by its shear modulus.

The Young’s modulus of rubber is greater than that of steel: False

Explanation:

For a given stress, the strain in rubber is more than it is in steel.

Young’s modulus,

For a constant stress:

Hence, Young’s modulus for rubber is less than it is for steel.

The stretching of a coil is determined by its shear modulus: True

Explanation:

Shear modulus is the ratio of the applied stress to the change in the shape of a body. The stretching of a coil changes its shape. Hence, shear modulus of elasticity is involved in this process.

Question 9.5:

Two wires of diameter , one made of steel and the other made of brass are loaded as shown in Figure. The unloaded length of steel wire is and that of brass wire is Compute the elongations of the steel and the brass wires.

Elongation of the steel wire

Elongation of the brass wire

Diameter of the wires,

Hence,

Length of the steel wire,

Length of the brass wire,

Total force exerted on the steel wire:

Young’s modulus for steel:

Where,

Change in the length of the steel wire

Area of cross-section of the steel wire

Young’s modulus of steel,

Total force on the brass wire:

Young’s modulus for brass:

Where,

Change in the length

Area of cross-section of the brass wire

Elongation of the steel wire

Elongation of the brass wire