“…Next, we prove the inherent quadratic nonlinearity of the arch regardless of its actuation method (i.e., whether there is a DC electrostatic force or not), which shows a softening nonlinear response. The nonlinear equation governing the transverse motion w ( x , t ) of the arch beam based on no axial inertia can be written as [ 41 , 42 , 43 , 44 , 45 ]: where x is the spatial position, t is time, and w 0 is the initial curvature of the arch. The arch has a Young’s modulus E , a material density ρ, and is assumed to have a rectangular cross-sectional area A and a moment of inertia I .…”