Équilibre statique - II

Static equilibrium requires that both net force and net torque on an object or system equal zero. This means linear and angular accelerations are both zero, so the object remains at rest or its center of mass moves at constant velocity. Both conditions must be satisfied simultaneously for true static equilibrium.

When equal weights attach to both sides of a rod at equal distances from the pivot, gravitational forces act downward while the normal force at the pivot acts upward, balancing net force. The torques from each weight are equal in magnitude but opposite in direction, making net torque zero and establishing static equilibrium.

Moving one weight closer to the axis reduces its torque. Although net force remains balanced, the unequal torques create net torque that rotates the rod. The system violates the second equilibrium condition, so static equilibrium is lost despite balanced forces.

Static equilibrium ensures structures like bridges, towers, and skyscrapers remain stable under load. Engineers design these systems so net force and net torque balance despite tremendous forces from the structure's weight and external loads. Understanding equilibrium prevents deformation, fracture, and structural failure.

Bridge cables are elastic materials that can flex under load while maintaining equilibrium. Engineers design cables to withstand stress from the bridge's weight and moving loads. This elasticity allows the structure to absorb forces without permanent deformation, keeping the system in stable equilibrium.

Free body diagrams visually represent all forces acting on an object, making it easier to verify equilibrium conditions. By showing gravitational forces, normal forces, and their directions, diagrams help identify whether net force and net torque are zero, confirming static equilibrium.

Cranes use complex systems of cables and pulleys to balance forces and torques while lifting loads. The crane's structure, cables, and counterweights are arranged so net force and net torque remain zero, keeping the system in equilibrium despite the tremendous weight of construction materials being lifted.