21. A weighing 4 is attached to a spring whose constant is 2 lb In. Ihe medium offers a damping force that is numerically equal to the instantaneous velocity. Ihe mass is initially released from a point I foot above the equilibrium position with a dovvnward velocity of 8 ft.'s. Determine the time at which the mass passes through the equilibrium position. Find the time at which the mass attains its extreme displacement from the equi- libriurn What is the position Of the mass at this instant?