College Physics (4th Edition)

$F_s = 637~N$
The force exerted by the sacrum on the spine is equal in magnitude to the sum of the upper body weight and the weight of the package. We can find $F_s$: $F_s = (M+m)~g$ $F_s = (55~kg+10~kg)(9.80~m/s^2)$ $F_s = 637~N$