Skyscrapers, bridges, and other structures are in static equilibrium—a state characterized by having no tendency to accelerate or rotate. Thus, objects in static equilibrium must be subject to zero net force and zero net torque. It’s the job of engineers to design structures so these conditions are met, thus ensuring that structures will be in static equilibrium. But even that’s not enough: equilibrium should be stable, meaning that small disturbances that disrupt the system’s state of equilibrium will result in the system returning to equilibrium.