model Pendulo
  Real x(start=0.3,stateSelect=StateSelect.always),y,vx,vy,F;
  parameter Real L=1,g=9.81,m=1;
equation
  der(x)=vx;
  der(y)=vy;
  m*der(vx)=-x/L*F;
  m*der(vy)=m*g-y/L*F;
  x^2+y^2=L^2;
end Pendulo;