4.6 A 4 kg vase is placed on a shelf at the position shown in Fig. ure 4.12. If the shelf itself has a negligible weight, what is the force Fnail with which the upper nail must hold to prevent being pulled out of the wall? Fa 0.2 m pivot mg 0.3m Figure 4.12 A vase sits on a shelf. The lower block acts like a pivot.