### Problem 6.2.1

Here's another hint that might help with Problem 6.2.1.
The component of the virtual work associated with force f is computed as \delta W = \delta c * f.
Ignore the hint in the textbook, and instead focus on the expression of c=wi^T*d_ij. Take a delta of c to get \delta c, and then use the information provided on Slide 4 of Nov 10 lecture to come up with the expression of the generalized force acting on body i and body j.
Note that when you compute \delta c, you should express this quantity as a sum of terms that look like \delta r_i^T*(something 1) + \delta \phi_i *(something 2) + \delta r_j^T*(something 3) + \delta \phi_j *(something 4). The generalized force acting on body i is going to be Q_i = [f*something 1; f*something 2], while for body j you'll have Q_i = [f*something 3; f*something 4].

I hope this helps, post follow up questions here.
Dan