1. The problem statement, all variables and given/known data The system shown in Fig. 2-6 is in static equilibrium. Use the principle of virtual work to find the weights A and B. Neglect the weight of the strings and the friction in the pulleys. Quick aside: This is straight out of the Exercises for the Feynman Lectures on Physics, in which he introduces virtual work as an imaginary displacement that let's us find the weight or height of objects in what he calls a "reversible machine". I know how to do this equilibrium problem using force vectors, but am not sure where to begin with virtual work. Can anyone help me to solve the question using this method? 2. Relevant equations Ug = mgh Total Ug = 0 3. The attempt at a solution Right off the bat I know the answer to this question, I'm desperate for a solution/explanation. I begin by imagining the 1kg weight falls 1 unit, which means that weight A must rise 1/2 a unit based on the angle of the string. Then I don't know whether or not I should include weight B in this displacement, or if it should fall, or if the entire right side of the system should rise according to the displacement of weight A in order to preserve the geometry of the system. After a bit of easy mathy stuff you can get the equation 0 = -1(1) + 1/2(A) + (whatever B does)B. I've tried many different imaginary displacements for B, but can't seem to get the right answer. This isn't very difficult using force vectors, but the question specifically asks for it to be solved using the method of virtual work, which I am admittedly unfamiliar with. What do I have to imagine the system does here? Maybe my original guess that weight A should rise a distance of Sin30 is wrong as well.