Two people weighing W each are sitting on a plank of length $$L$$ floating on water at $$L/4$$ from either end. Neglecting the weight of the plank, the bending moment at the center of the plank is

The major and minor principal stresses at a point are $$3MPa$$ and $$-3MPa$$ respectively. The maximum shear stress at the point is

The number of independent elastic constants for a linear elastic isotropic and homogeneous material is

For the truss shown in the figure, the force in the member QR is