A rectangular fiberglass plate of size 1 m x 2 m is hinged to a rigid wall at corner O and supported on a vertical metal post of height 3 m at corner B where OB is horizontal (see Figure P4.11). Initially, the system is unstrained (neglect the weight).

When a horizontal force is applied to corner C of the plate, point C moves through a horizontal distance of δ _c   = 4 mm. Also, the post assumes a normal strain ε _q  = 500 με (tensile) as a result. Determine the "average" shear strain in the plate with respect to corner O.