# Mechanical sub-systems and mechanical coupling

## Introduction

In order to understand well the difference between an internal sub-system and an external mechanical sub-system, several reminders on the mechanical coupling in Flux are presented in this paragraph.

For more details, see chapter « Kinematic coupling: principles »

## Kinematic coupling: reminders

The diagram below emphasizes the manner in which motion is approached in Flux:

• the fixed part, the mobile part and the zone of compressible air belong to the finite element domain
• the mobile part can be connected to an external device (coupled load) that does not belong to the finite element domain. The essential mechanical and kinematic characteristics to solve the fundamental motion equation are as follows:

• the mass or the moment of inertia of the mobile part (internal) and of the coupled load (external)
• the forces or the torques exerted upon the mobile part (internal) and of the coupled load (external)
• the initial conditions: initial position and initial velocity of the mobile part

## Mechanical sub-system: definition

The external part of a mechanical assembly in motion is called the external mechanical sub-system.

There are as many external mechanical sub-systems as mechanical assemblies in motion. The internal part of a mechanical assembly in motion belongs to the internal sub-system.

## Regarding applied forces

The forces applied to the mechanical system can be of different types.

• The return force of the spring is a conservative force deriving from a potential energy (stored energy).
• The friction force is a dissipative force (dissipated energy).

Attention: if the user introduces a force by means of a formula, this force is seen as a dissipative force.