11.4 Problem solving
The process for solving problems with torque is nearly identical to the process for solving problems with force (see section 9.4). However, instead of a free body diagram—which represents the system as a single point—we’ll use an extended free body diagram (E-FBD); with torque, the location where the force is applied is important. You’ll see an example in the next section.
11.5 Equilibrium revisited
An object is in static equilibrium when the following conditions are true:
- The net torque acting on the object is zero
τnet = 0
- The net force acting on the object is zero
Fnet = 0
Remember that force is a vector, so both both force components (Fx and Fy) are zero.
When an object is in static equilibrium, and not rotating at all, we are free to choose a convenient reference for calculating torques. This of course raises the question, “what makes a convenient choice?”
Often, you won’t immediately know the magnitude of a particular force exerted on an object. If the object is in static equilibrium, you can use the point where this force acts as the reference point for torque calculations, totally bypassing the problem of not knowing anything about that force! How convenient!
example
Example
Note: in the previous example, the denominator for the tension calculation uses (1 – f) where f is the fraction of the length of the boom corresponding to where the cable is attached; this is incorrect. The denominator should simply use the value given for f. For example, if the cable is attached to the boom 3/4 of the way from the wall, the denominator should be (3/4)Lsin(θ).