四、說明寒害造成植物傷害的機制，另請舉出五種易受寒害之園產品及其臨界溫度。(25分)

2. A simple beam consists of a wood member that is reinforced by 0.25 in. thick steel plates on top and bottom, as shown in Fig-2. \( E_s=30 \times 10^6 \) psi and \( E_w=1.5 \times 10^6 \) psi, respectively. Calculate the maximum bending stresses \( \sigma_s \) in the steel plates and \( \sigma_w \) in the wood member due to the uniform load. (20%)