In the unstretched position the coils of the $3-1 b$ spring are just touching one another, as shown in part $a$ of the figure. In the stretched position the force $P,$ proportional to $x,$ equals 200 lb when $x=$ 20 in. If end $A$ of the spring is suddenly released, determine the velocity $v_{A}$ of the coil end $A,$ measured positive to the left, as it approaches its unstretched position at $x=0 .$ What happens to the kinetic energy of the spring?