Plastic Impact:

If the total deformation caused is nonrecoverable, the permanent deformation is the maximum deformation developed. There is no period of restitution.

                                                                              e = 0

After the full deformation, final velocity (V_c) is specified by

(m₁  + m₂ ) V_c  = m₁ V₁  + m₂  V₂

Thus, for a perfectly plastic impact

Loss of KE =  ( m₁  m ₂ /2 (m₁ + m₂ ) )(V₁ - V₂ )²

When 0 < e < 1, some part of deformation is recovered when some permanent deformation is also left off. Therefore, the value of coefficient of restitution also gives some indication regarding the magnitude of permanent deformation. Such type of impact is, thus, known as semielastic impact.