4th Post: Machines and Mechanisms

    
          Machines & Mechanisms 

         1-.When a structure is stable?
             - It is stable when it remains in equilibrium even when a force is applied to it.

         2-.  What is the centre of gravity? 
             - Is the point where the weight of an object is considered to be concentrated.

                       
                          

        3- Machine: We use it to describe a set of elements or parts that work together and use energy                to perform a task amd obtain a result:

• Driver element: Feeds energy to the machine
• Driven element: Receives the force produced in the machine 
• Converting and/or transmitting element: Transmits and/or converts the motion from the driver element to the driven element.

             Old machine:

                                  

      4- Mechanism: Devices that transmit and/or convert motion.

      5- Differences between: 

        

         -Motion transmition: In this type, the mechanism is fed energy from the driver element 

and simply transffers it to the driven element

         -Motion conversion: In this type, the mechanism converts the movement from the driven                                                     element into a movement that the driven element can use

      6- Lever: Is a rigid bar that can pivot on a point supported called a fullcrum. It is used to lift load 

by applying force to a point on the bar. We call the applied force effort, whereas the load                      to be lifted or the resistence that must be overcome is known as the resistance.

   
      7- Pulley: Are used to lift heavy loads . They are wheels with a grooved rim.

       8- Belt and pulley: 

           - This system is made up of two pulley wheels joined by a belt that slots into the groove ring                of the wheels. 

             Example: When one of the wheels is rotated, the belt transmite the motion and makes the other wheel rotate (Like friction wheels)

       9- Gears: Is a mechanism consisting of two wheels with teeth.

                      Example: Just like friction wheels, the two wheels rotate in opposite directions.

                                       (They are used in vehicle gearboxes, watches and clocks, toys,..

                                             

3rd Post: Stresses and rigidity

 
     Stresses: Are the physical demands that a body or object must withstand when one or
                             or more external forces are applied to it.

      Basic Stresses:
              
          1- Compression:  This is caused by forces acting on a body that tend to flatten it or reduce its                            lenght or thickness. It happens in structures that need to support weights.
                                        

      
                       For Example: When we lean on a table, it undergoes compressive stress;                                             columns are also subject to compressive stress.

            2- Tension or traction: Its caused by forces acting on a body that tend to stretch it.
                                                     
                                         

                       For Example: When two people pull on an object from opposite
                                                      directions.
                 
          3- Bending:  This is caused by forces acting on an element that make it curve or bend. It is                                       done by supporting the two ends and applying force in the centre.
                                         
         
            4- Torsion: Its caused by forces acting on a body that make it twist. 
     
                                          
           
                     For Example: When two people hold a towel at either end and twist it to wring out; a                                  crank is also subjected to torsion.

            5- Shear: This is caused by two equal forces applied in opposite directions that act on lines of
                            of action close to each other. Shear stress will tend to cut the material if it cannot                                   twist or bend.
                        
                                                     

                   For Example: When we cut paper with a pair of scissors, or the point
                            where a beam rests on a column.

      Rigidity: If a material is rigid, it does not change shape when force is applied to it. By contrast,         if the material does change shape, we say that is deformable. 


            pag 95 exercise 12:
            
             - Think about the following objects and state whether they are rigid, plastic or elastic:

• Balloon: Elastic
• Empty drinks can: Plastic
• Window frame: Rigid
• Clay: Rigid
• Catapult: Rigid
• Door: Rigid

             
                    

2nd post

 Structure: Is a collection of elements arranged in an optimum way to support a body or an object.

   
                   In this pictures , we can see the structure of the Presa del Atazar.

                            

              

In the morning                                      In the night

           The Different types of Structures: (mass , frame , truss or suspended)

•   Mass structure: Are solid, heavy and very sturdy structures. Examples include, walls, dams,etc...

•  Frame stuctures: These stuctures are formed by columns and beams that act as supporting elemnets. One example might be the structure of a building

• Truss structures: These structures are compossed of bars that form triangles. They are lightweight and sturdy. Examples: Include cranes, electricity pylons, etc.

• Suspended structures: These are held in places by cables and suspenders attaching to sturdy supports. Examples: Include suspensions bridges, tents, marquees, etc.

               Force and stresses:

    

          Force: Is an influence that can deform a body or chande its movement, or produce motion in              a body at rest.

          Stresses: Are the physical demans that a body or object must withstand when one or more                  external force are applied to it.

Presentation

Hello, we are Nerea and Lorena and we are going to talk about structures.