(2) 推導伯努利方程式時，沿著流線方向之受力只有壓力和重力需要被考慮，這是基於哪一個條件？(3%)

1. A punch for making holes in steel plates is shown in Fig.1a. Assume that a punch having diameter d=20mm is used to punch a hole in an 8mm plate, as shown in the cross-sectional view (Fig.1b). If a force P=110 kN is required to create the hole, what is the average shear stress in the plate and the average compressive stress in the punch? (20%)