An 80-N uniform plank leans against a frictionless wall as shown. The torque (about point P) applied to the plank by the wall is:
A) 40 N.m
B) 60 N.m
C) 120 N.m
D) 160 N.m
E) 240 N.m

Question

An 80-N uniform plank leans against a frictionless wall as shown. The torque (about point P) applied to the plank by the wall is: 
A) 40 N.m 
B) 60 N.m 
C) 120 N.m 
D) 160 N.m 
E) 240 N.m

Quizplus · Accepted Answer

The torque applied by the wall is equal to the product of the force applied by the wall and the perpendicular distance between the point of application of the force and the pivot point (point P). In this case, the force applied by the wall is equal to the weight of the plank (80 N) and the perpendicular distance between the point of application of the force and the pivot point is equal to the length of the plank (2 m) divided by 2 (since the plank is leaning against the wall at a 45-degree angle). Therefore, the torque applied by the wall is:

Torque = Force x Perpendicular distance from pivot
Torque = 80 N x (2 m/2)
Torque = 80 N x 1 m
Torque = 80 Nm

Converting Nm to NF1F1F1S1F1F1F10m, we get:

Torque = 80 NF1F1F1S1F1F1F10m/1 Nm = 80 NF1F1F1S1F1F1F10m

Therefore, the correct answer is C) 120 NF1F1F1S1F1F1F10m.

An 80-N Uniform Plank Leans Against a Frictionless Wall as Shown