Fig 14Forces inside a parthow materials carry loadσ = F / A
Forces don't stop at the surface. Inside a part every slice pulls, pushes or slides against its neighbours. Tension stretches it, compression squeezes it, and shear slides one layer past the next. A bent beam does both at once: compression along the top, tension along the bottom, nothing in the middle. That's why I-beams put their material at the top and bottom. Force per area is stress. Steel yields around 250–350 MPa (mild) to over 1,000 MPa (alloyed), and its stiffness E is about 200 GPa.