Question

The young's modulus for a perfect rigid body is infinite.How?

Answer 1

For Young modulus and Bulk modulus of elasticity is defined as ,
Young modulus=StressStrainBulk modulus=StressStrain
Now as we know that for a perfectly rigid body it will experience no change in length so the strain = 0 .
So the young modulus and bulk modulus of a perfectly rigid body is equal to infinity .

Answer 2

$Y = \frac{ \sigma}{ \epsilon} \: \: \: \: or\\ \\ Young's\ modulus = \frac{stress}{strain}$

For perfectly rigid body, strain will be 0 as there is no deformation in its length or size.

You can go on increasing stress as much as you like, but strain remains 0.

So young's modulus = stress/0 = infinity