Hello When I had relativity course and I read some books and sources like : Rindler W. Relativity.. special, general, and cosmological (2ed., OUP, 2006) I found that in these texts authors talk about Newton second law and explain why this law is not Lorentz invarient , but never use this new form of second law in the problems. In example I couldn't find resolving two body problem in those texts. Clearly I think there is a problem between special relativity and meaning of force wich we find in Newton laws. What are these problems ? Is Lagrangian form of mechanics more elegant when we solve relativistic problems? Sorry for bad English!