There are two factors of two involved in the interpretation of this equation. First, the maximum force on the top piece of protection is roughly 2 F max, since the gear acts as a simple pulley. Second, it may seem strange that even when f=0, we have F max =2 mg (so that the maximum force on the top piece is approximately 4 mg). This is because a factor-zero fall is still a fall onto a slack rope. The average value of the tension over a full cycle of harmonic oscillation will be mg, so that the tension will cycle between 0 and 2 mg.