If an undamped spring-mass system with a mass that weighs 6 lb lb and a spring constant 4 is suddenly set in motion at t = 0 by an in external force of 72 cos(8t) lb, determine the position of the mass at any time t. Assume that g = = 32 and t is in seconds. ft Solve for u, where u is in feet 82. u(t) =