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If force is applied to a rigid body and the body moves/remains still/vibrate or anything. But, we assume there is no strain.

But, even if we can't see any strain in the naked eye, isn't there some sort of strain in the molecular level?

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    $\begingroup$ A Rigid Body Does not Go any Change in Shape or Volume when external forces are applied on it $\endgroup$ – Ganesh Feb 14 '16 at 10:23
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    $\begingroup$ Do you mean specifically rigid or solid? $\endgroup$ – Jaywalker Feb 14 '16 at 10:34
  • $\begingroup$ He says "molecular", so I think that he means solid. But we won't know until he tells us. $\endgroup$ – garyp Feb 14 '16 at 12:53
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    $\begingroup$ In the classical ball-and-spring model of a solid, every stress, no matter how small, will cause an elastic strain. I'm not sure what quantum mechanics says. The stress would have to be large enough to excite a low frequency acoustic phonon. The energies of acoustic phonons goes to zero at zero wave vector, so one might think local deformation would always occur. But there's a case, the Mossbauer effect, in which gamma rays are emitted from a nucleus in a solid, but no local recoil occurs. There is no strain caused by recoil. $\endgroup$ – garyp Feb 14 '16 at 13:09
  • $\begingroup$ @garyp It would be better ig you expand your last comment to answer and explain a bit more clearly. $\endgroup$ – Mockingbird Jun 26 '17 at 9:43
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If you believe in Newton's third law then a body must deform in some way or other to apply a reaction force to the external action force.

Without the force acting the molecules will be at an average equilibrium separation with the (average) net force on each molecule being zero.
Applying an external force will change the separation so there will be forces produced by the block which will be opposite in direction to the external force.

So an perfectly rigid body only produces a reaction force because that instruction appears in its user manual "How to be a perfectly rigid body whilst making it appear that the laws of Physics are obeyed"

Of course it is not a perfectly rigid body rather it is a real body whose shape does not change very much on the scale of the other changes taking place. So a compression of 1 $\mu$m will usually not make much of a difference if all the other lengths are of order metres.

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Quoting wiki -

"In physics, a rigid body is an idealization of a solid body in which deformation is neglected. In other words, the distance between any two given points of a rigid body remains constant in time regardless of external forces exerted on it."

A rigid body is basically an assumption and hence in the boundaries of this assumption it does not undergo any strain even in the molecular level.

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All molecules in a solid are held together by intermolecular bonds. When the solid is pulled in one direction, these bonds are indeed put under some stress but provided the external acceleration does not exceed the magnitude of the internal forces, the object is able to maintain its shape.

But consider a bullet hitting a steel plate for example. The force of impact is often so large that the intermolecular forces will not be able to keep the bullet in shape and it mushrooms or even splatters across the plate.

So indeed under very extreme accelerations solids can lose some of their "solid behaviour".

This does not apply for rigid bodies because they are a theoretical approximation of a solid.

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    $\begingroup$ Small forces will elastically strain a solid. One doesn't need "extreme accelerations". $\endgroup$ – garyp Feb 14 '16 at 12:59
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    $\begingroup$ The deformation becomes apparent at extreme accelerations. Of course there is some deformation also at lower accelerations but it is very hard to observe in most solids. $\endgroup$ – Jaywalker Feb 14 '16 at 13:14
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A rigid body cannot be deformed.But the stress is same as of elastic body(follows hooks law) along the length.enter image description here

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