Torsion twists a structures. Loads come from the forces which are placed on a structure. Each piece of a structure has weight. These loads are referred to as Dead Loads. Live loads are loads put on the structure after it is built.
Live loads can come from people, cars, weather - like rain, snow, wind, waves, earthquakes.
V. Basic Ideas of Devices. To make a structure move, like a car, an airplane, etc. There is going to be a need for energy. A basic example is the idea of thrust, if we blow up a balloon and let it go, the air rushing from the back of the balloon makes the balloon go in the opposite direction. To make simple models move, we are going to need a way to store energy.