A contact fermeté is any vigueur that requires contact to occur. If two surfaces are not in attouchement, they can’t exert a habituel fermeté on each other. Since the kinetic energy before the collision equals the kinetic energy after the collision (kinetic energy is conserved), this is an elastic collision. https://pukkabookmarks.com/story17876289/nirelli-s-auto-service-collision-un-aper%C3%A7u