This lecture discusses the deformation of planar systems, focusing on the behavior of structures under various forces. The instructor begins by combining knowledge of material resistance with static equilibrium to analyze how structures deform. A basic system with two cables is introduced, illustrating how forces affect elongation and deformation. The lecture covers key equations for calculating elongation and stress, emphasizing the importance of material properties and cross-sectional area. The concepts of tension and compression in structural elements are explored, along with the significance of static height in determining force distribution. The instructor explains different types of supports and their roles in allowing structures to deform while maintaining stability. Examples are provided to illustrate how to calculate deformation and deflection in various structural configurations. The lecture concludes with a discussion on design considerations, highlighting the balance between structural efficiency and material use, and the importance of understanding deformation in engineering practice.