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?
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It only takes a minute to sign up.Sign up to join this community
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.
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.
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.