Consider the potential energy function for a 2-d mass-spring system given by; u-2(x2 + y2). a) what is the force constant for this spring? b) start with this potential and derive an expression for force vector. c) plot the equipotential curves for u-0, 10, 20, 30, 40, 50, 60 j and draw d) e) representative force vectors at these locations how do you see the magnitude of force in relation to density of lines? where is the equilibrium position of the system and is it stable?