# NCERT Physics Class 11 Exemplar Ch 9 Mechanical Properties of Solids Part 6

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Q.26 A steel rod of length , cross sectional area A and mass M is set rotating in a horizontal plane about an axis passing through the centre. If Y is the Young’s modulus for steel, find the extension in the length of the rod. (Assume the rod is uniform.)

Answer: Consider an element at r of width dr. Let be the tensions at the two edges.

Let the increase in length of the element

The total change in length is

Q.27 An equilateral triangle ABC is formed by two rods AB and BC and one Al rod. It is heated in such a way that temperature of each rod increases by . Find change in the angle ABC. [Coeff. Of linear expansion for is Coeff. of linear expansion for ]

Answer: Let

Differentiating

Now,

And

Putting