Amass that weighs 8 lb stretches a spring 24 in. the system is acted on by an external force of 4sin(4t). if the mass is pulled down 6 in and then released, determine the position of the mass at any time t. assume that the u-axis is directed downwards and g = 32 ft/s^2. express your answer in as a linear combination of sin at and cos at. u(t) = where u is in feet and t is in seconds. determine the first four times at which the velocity of the mass is zero. exclude t = 0 as trivial. first zero: t =