The diagram shows a stationary 5-kg uniform rod (AC), 1 m long, held against a wall by a rope (AE) and friction between the rod and the wall. To use a single equation to find the force exerted on the rod by the rope at which point should you place the reference point for computing torque?

Question

Quizplus · Accepted Answer

In order to use a single equation to find the force exerted on the rod by the rope, we need to place the reference point at the point where the unknown force is acting, which is point C. This is because torque is defined as the product of force and the distance from the fulcrum (or reference point) to the point where the force is applied. Since we want to find the force exerted by the rope at point A, we need to consider the torques due to the weight of the rod and the force exerted by the wall at point A, but not the torque due to the rope at point A. Thus, we can set up the equation: 
(torque due to weight)+(torque due to wall)=(force by rope)x(distance from point C to A)
(½(5 kg)(9.81 m/s²)(1 m))+(0)=(force by rope)(1 m)= 24.53 N 
Therefore, the force exerted on the rod by the rope should be 24.53 N, and the reference point should be placed at point C.

The Diagram Shows a Stationary 5-Kg Uniform Rod (AC), 1