Final answer:
When a plastic ruler is bent, it undergoes plastic deformation and acquires a permanent deformation that it does not recover from.
Step-by-step explanation:
When a plastic ruler is bent, it undergoes plastic deformation. Plastic deformation means that the material deforms irreversibly and does not return to its original shape and size even when the bending force is removed. In the case of a plastic ruler, when it is bent, it acquires a permanent deformation and does not regain its original straight shape.