संरचनात्मक यांत्रिकी में आभासी विस्थापन के सिद्धांत (principle of virtual displacement) का प्रतिपादन किसने किया था?

What is the value of maximum shear force for a simply supported beam of length 6.0 m, subjected to a uniformly distributed load (w kN/m) throughout the span, for which the bending moment equation at a section from the left support situated at 'x' m is Mx = 30x - 0.5(wx2)?

Method of joints is applicable to calculate member forces of a truss, when the number of unknown forces at the joint under consideration is not more than: