Let's say we have the stiffness matrix K $$ K = \left[ {\begin{array}{ccccc} k_{11} & k_{12} & k_{13} & k_{14} \\ k_{21} & k_{22} & k_{23} & k_{24}\\ k_{31} & k_{32} & k_{33} & k_{34}\\ k_{41} & k_{42} & k_{43} & k_{44}\\ \end{array} } \right] $$
and a displacement vector u $$ u = \left[ {\begin{array}{ccccc} 0 \\ u_{2} \\ u_{3} \\ u_{4} \\ \end{array} } \right] $$
The result for $F = Ku$ would be like this:
$$ F = \left[ {\begin{array}{ccccc} F_{1} \\ F_{2} \\ F_{3} \\ F_{4} \\ \end{array} } \right] $$
What I don't understand: How is it possible that although $u_{1} = 0$ that $F_{1} \neq 0$. That means there is a force but no displacement? Does not every force cause a displacement? Can you explain this for me?