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 of an object about a point is equal to the force applied perpendicular to the lever arm (distance between the point and the line of action of the force) multiplied by the lever arm. In this case, the force applied by the wall is perpendicular to the plank and has a magnitude of 80 N. The lever arm is the distance from point P to the point of contact between the plank and the ground, which is 1.5 m. Therefore, the torque applied by the wall is:
Torque = 80 N * 1.5 m = 120 Nm
Thus, the best choice is C.

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