A method for calculus of Internal Forces

A new method to compute internal forces in a multi-bodies system is presented in the paper. Lagrange equations are used to study the motion of a system under the action of known external forces. If an internal force has to be found, a supplementary mobility is considered in the system and the corresponding internal force for the new mobility is found for null value of the mobility, as well as its first and second orders of derivatives. The method is a general one, but a particular case of mechanism used in the dynamics of the airplane elevator is analyzed to verify the validity of the proposed method.