Is it necessary that when an external force is applied on a solid body, it always gets deformed however small the deformation may be?
If the material is highly elastic like steel, even then there will be a deformation of the body for negligibly small forces.
Please explain theoretically i.e in terms of the structure of the solid (in terms of chemical bonds, potential energy curve etc) why this happens.
I think the deformation should start when a certain particular force is applied, because to disturb the packed lattice a certain force is required. But I am not sure, it is also possible that lattice gets instantaneously disturbed (eg for polymers). Is is really confusing.