function y=rk4step(t,w,mass,h)
%one step of the Runge-Kutta order 4 method
s1=ydot(t,w,mass);
s2=ydot(t+h/2,w+h*s1/2,mass);
s3=ydot(t+h/2,w+h*s2/2,mass);
s4=ydot(t+h,w+h*s3,mass);
y=w+h*(s1+2*s2+2*s3+s4)/6;