Question

For a perfect rigid body, Young’s modulus is

(a) zero
(b) infinity
(c) 1
(d) –1

Answer 1

A perfectly rigid body means the body which have negligible deformation or zero deformation . means change in Length for perfectly rigid body = 0
e.g., ∆L = 0

We know, Young's modulus = stress/strain
And strain is the ratio of change in length to original length
e.g., strain = ∆L/L

Now, for rigid body , Young's modulus = stress/strain
= Stress/{∆L/L}
= Stress/{0/L} [ ∵∆L = 0 ]
= stress/0 = infinity

Hence, Young's modulus of perfectly rigid body is infinity .
so, answer is option (b)

Answer 2

The correct option is - b. infinity

Young's modulus = stress / strain

The body is perfectly rigid so it will not experience any change in length

Therefore strain = 0

According to the formula

Young's modulus = stress / 0

                            =  infinity