To calculate how far the person can walk, we compute for the tipping point where: tccw = tcw
tau - plank = tau - person
r1 F1 = r2 F2
r1 m1 = r2 m2
(0.5 m) (100 kg) = r2 (50)
r2 = 1 m is how far the venture past the edge of the building on the plank before the plank just starts to tip.
