Force
We cannot see force, but we can see their effects around us, we can identify that a force is acting upon an object when we can see one of the following effects:
The object undergoes a change in speed (i.e the speed increases or decreases)
The object's trajectory changes (i.e the object turns left, right, up or down)
The object's shape changes (the object undergoes a deformation)
In nature there exist four or five fundamental forces, they are: gravitational, electromagnetic, weak nuclear force, strong nuclear force and dark force. In our daily life
these forces are identified as non-fundamental forces. There are many, but for us the most important ones are:
- Weight: Makes things fall down.
- Friction: makes a ball stop rolling on the floor
- Wind: Makes a windmill go around.
- Elastic: Makes a spring or rubber band stretch or shrink
- Pressure: Makes the cork of a bottle of champaign fly away
Structure
A structure is the skeleton that allows all the objects to withstand their own weight, to keep their shape and to resist external forces such as wind, impacts etc without breaking or falling down.
Examples of structures are: Our skeleton, a bridge, an cathedral, a tower crane, a dam a plane etc:
Structures are made up of different elements, the most important are:
- Beams: Are rigid elements placed horizontally.
- Columns: Are rigid elements placed vertically that support the weight of the structure above them.
- Wires: Are flexible elements, (i.e ropes, steel wires, strings etc).
Loads
The forces acting on a structure are called loads. There are two types of loads:
- Fixed Loads: When the load doesn't change over time: I.e the own structure's weight
- Variable loads: When the load varies over time: i.e the water pressure on a damp depends on the reservoir water level
Stress
As a result of the loads on a structure, all of its elements receive their share of force, the individual force acting on a structural element per square centimeter of cross section is named stress. As a result of the stress acted upon by the elements that make up the structure (i.e beams, wires and pillars) experience a small deformation. Depending on the type of deformation that appears in a structural element we can see the type of stress that is present on it. The three main types of stress on structural elements are:
- Traction: When the force applied to the element tries to stretch it. In structures wires or beams support traction stresses.
- Bending: When the force applied to the element tries to bend it. In structures beams support bending stresses.
- Compression: When the force applied to the element tries to compress or shrink the element. In structures columns support compression stresses.
To understand the type of stress imposed on a structural element we can think of it as being made out of rubber: If we replace the real element in the structure by the imaginary one made of rubber three things can happen:
1- The element will stretch, that means ìt's subjected to traction stress.
2- The element will bend, that means it's subjected to bending stress.
3- The element will shrink, that means it's subjected to compression stress.
Try to derive the type of stress placed on the structural elements of a swing:
Characteristics of a structure
The most important characteristics of a structure are:
1- Resistance: It has to resist its own weight and external forces such as wind, impacts, external weights etc. To withstand all these forces the structure has to have the right shape and size,
and has to be made out of the right materials
2- Stability: In order to prevent an object from falling down, a structure has to have the right distribution of weight, a big enough base, foundations and possibly struts or braces.
3- Rigidity: The structure has to be able to keep its shape, without bending or warping, to achieve this goal it has to be based on triangular patterns.
Links:
Cargo Brigde a game to build triangulated structures.
West Point Bridge designer, a program to design an simulate bridges
Mechanics of materials video: link
Stress and strain: link