A structure is a system of interconnected members such as beams, struts, joints, etc., designed to support or transfer forces and safely withstand loads acting on them. Dismantling the structure and analyzing the free-body diagrams of individual members or a combination of members helps determine the internal forces of a structure. Structures are classified into three categories: trusses, frames, and machines. Trusses are stationary structures and remain rigid when subjected to force. A truss consists of straight members connected by joints at the ends of each member. So, the members of a truss have two equal and opposite forces acting along the member. A frame is a rigid, stationary structure designed to support loads. However, it always has at least one member on which three or more forces act. At least one of these forces is not directed along the axis of the member. Machines are non-rigid structures having moving parts designed to transmit and modify the effects of forces. At least one multi-force member is present in a machine.