Your friend balances a package with mass m = 10 kg on top of his head while standing. The mass of his upper body is M = 55 kg (about 65% of his total mass). Because the spine is vertical rather than horizontal, the force exerted by the sacrum on the spine ( in Fig 8.33) is directed approximately straight up and the force exerted by the back muscles ( ) is negligibly small. Find the magnitude of.