Amass weighing 4 pounds is attached to a spring whose constant is 2 lb/ft. the medium offers a damping force that is numerically equal to the instantaneous velocity. the mass is initially released from a point 1 foot above the equilibrium position with a downward velocity of 6 ft/s. determine the time at which the mass passes through the equilibrium position. (use g = 32 ft/s2 for the acceleration due to gravity.)