function Fint = InternalForce(N,K2,K3,ui) FL=zeros(N,1); FNL=zeros(N,1); for i=1:N if i==1 FL(i)=-K2*(ui(i+1)-ui(i)); elseif i==N FL(i)=K2*(ui(i)-ui(i-1)); elseif i>1 && i1 && i