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Also, what can we say about the Young's modulus and Shear Modulus of a perfectly rigid body and why?

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    $\begingroup$ Can you please tell me what do you understand by a rigid body? $\endgroup$ – Knight Dec 4 '19 at 14:08
  • $\begingroup$ A rigid body cannot change its shape and distribution of mass. I do not know any formal definition sadly. $\endgroup$ – aditya_stack Dec 4 '19 at 14:10
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    $\begingroup$ Try a reading of Wikipedia article on rigid body (just first few lines). $\endgroup$ – Knight Dec 4 '19 at 14:12
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    $\begingroup$ Well the bulk modulus is just $$ E_v = \frac{stress}{strain}$$ and as you have said that distance between two points doesn’t change that means strain is zero and anything divided by zero is infinite. Therefore bulk modulus is infinite. $\endgroup$ – Knight Dec 4 '19 at 14:18
  • $\begingroup$ "The distance between any two given points on a rigid body remains constant in time regardless of external forces exerted on it." Okay this line answers the question, and there is also a semi-formal derivation given by @VK_fan. Thanks. $\endgroup$ – aditya_stack Dec 4 '19 at 14:18
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Bulk modulus is defined as $$B=-V\frac{P}{\Delta V}$$ It is obvious that we can't change the volume of an ideal rigid body so $\Delta V$ tends to zero hence magnitude of bulk modulus tends to $\infty$

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