Because of the sign in this line. (minus) (minus) = plus. It means that energy is continuously injected into the system, and destabilizes it.
(-k/m)*(x1 - y1)
function [] = call_spring()
t = 0:0.001:10;
y0 = [0, 1];
x1 = 2.5*cos(4*pi*t);
[t, y] = ode45(@spring, t, y0);
plot(t, y, t, x1)
end
function dzdt = spring(t, y)
k = 342;
m = 2.853;
x1 = 2.5*cos(4*pi*t);
y1 = y(1);
y2 = y(2);
dzdt = [y2;
(-k/m)*(x1 + y1)];
end
Result
Also, please double check your equation. Could be this one too:
(-k/m)*y1 + (1/m)*x1
