STRUCTURES
Forces
Structures
Stresses
Forces in nature
Gravity 
Wind
Mechanical forces
Pressures of gases in closed containers
The force created by our own muscle.
Forces caused by the effects of heat
Definition: A force is anything that can deform a body (static effect) or change its state of movement or rest (dynamic effect).
All bodies must have a structure that can resist forces. There are two types of forces: natural structures that are not made by humans and artificial, human-made structures. A structure is the set of elements in a body that are intended to resist the effects of the forces that act on it. A structure prevents a body from breaking or becoming too deformed.
Loads
The forces that act on a structure are called loads.
There are two main types of loads.
Fixed or permanent loads don't
vary over time They always affect the structure in the same way.
Variable loads are occasional and changeable. For example, the weight of snow on a root, the pressure of water on a dam (the water level can vary).
Ways in which loads affect the structure direction, intensity, the point where the torces are applied, the shape of the structure. Loads act on the bodies, causing internal tensions, which vary depening on how they're applied, and the specific shape of the objects. These tensions are called stresses.
Definition
This internal tension is called stress. It happens to all bodies subjected to the applying of one or more forces on them.
Livings things feel this effect - it can be very slight or if it is strong it can even hurt us O biects do not have feehngs, but forces act on them in a similar way and produce deformations. In both cases this internal tension is called stress.
Main types of stressees
Traction
Bending
Compression
Torsion
Shear or cutting
Forces try to stretch the object they are acting upon. Opposing forces move outwards away from the body along the same trajectory but in opposite directions. The body tends to become longer.
Forces try to bend a body. This stress acts on elements that are supported at their extremities and have a weight applied to them. Normally, there are two forces that are separated by a certain distance and a third force acting in the opposite direction.
Forces try to flatten or compress a body. Opposing forces move inwards towards one another along the same trajectory and in opposite directions The body tends to lose its shape and becomes compressed.
Forces try to twist a body. They try to turn the ends of the body in opposite directions.
The forces act like two scissor blades: they are very close together, moving in opposite directions, as they try to cut an object. This stress is the same when we put an object over the edge of a table and hit n to try and break it.This is also called tensile stress.
Buckling or warping is a stress that combines both flexion (bending) and compression. It's very dangerous and occurs when compression stress is appalied to an object that is capable of bending. If enough compression is applied, the material bends to the point of breaking, often quite violently.