Statics: why the bridge stands

Statics = sum of forces zero: what pushes down must be held up. Beam bends: top compressed, bottom stretched — center neutral. Hence I-beam: material where it works, empty in the middle. Lever: F1×l1 = F2×l2 — the jack lifts 2 tonnes with your hand. Triangle = rigid — bridge trusses triangular. Free-body diagram: draw all forces on the part — problem solved.

• ●Moment = force × lever: 30 cm wrench doubles force vs 15 cm
• ●Center of gravity decides stability: lower = more stable
• ●Safety factor usually 1.5–3: “seems to hold” doesn’t count