Frames and Machines

A frame or a machine is an engineering structure that that contains at least one member that is not a two force member.

Not a two force member
This beam is connected to other members (where normal forces would exist) at more than two locations. This beam is therefor not a two force member.
Not a two force member
This beam has two connection points, but a force is acting on a third point. Therefore the beam has forces acting on it at more than two locations and it is not a two force member.

A frame is a rigid structure, while a machine is not rigid. This means that no part can move relative to the other parts in a frame, while parts can move relative to one another in a machine. Though there is a difference in vocabulary in describing frames and machines, they are grouped together here because we use the same process to analyze both of these structures.

Stool Licensed under Creative Commons Attribution-Share Alike 2.5 via Wikimedia Commons
This stool contains non-two force members (the legs) and no part can move relative to the other parts (it is rigid). Therefore this is a frame.
Locking Pliers Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons
This pair of locking pliers contains non-two force members and has parts that can move relative to one another (it is not rigid). Therefore this is a machine

Analyzing Frames and Machines

When we talk about analyzing frames or machines, we are usually looking to identify both the external forces acting on the structure and the internal forces acting between members within the structure.

The method we use to analyze frames and machines (no special name here) centers around the process of breaking the structure down into individual components and analyzing each component as a rigid body. Where the components are connected, Newton's Third Law states that each body will exert an equal and opposite force on the other body. Each component will be analyzed as an independent rigid body leading to equilibrium equations for each component, but because of Newton's Third Law, some unknowns may show up acting on two bodies.