Question

A 5-meter uniform plank of mass 100 kilograms rests on the top of a building with 2 meters extended over the edge as shown. How far can a 50-kilogram person venture past the edge of the building on the plank before the plank just begins to tip? 

(A) 0.5 m 

(B) 1 m 

(C) 1.5 m 

( D) 2 m 

(E) It is impossible to make the plank tip since the person would have to be more than 2 meters from the edge of the building.

Answer 1

Correct option (B) 1m

Apply rotational equilibrium using the corner of the building as the pivot point. Weight of plank (acting at midpoint) provides torque on left and weight of man provides torque on right. 

(m₁g) • r₁ = (m₂g) • r₂ 

(100 kg) (0.5m) = (50 kg) (r) → r = 1m