4th Post: Machines and mechanisms

A structure is stable when it remains in equilibrium even when a force is applied to it.

The centre of gravity is the point where the weight of an object is considered to be concentrated.

Machine is a set of elements or parts that work together and use energy ton perform a task and obtain a result. The machines have three components:

- Driver element: feeds energy to the machine.

- Driven element: receives the force produced in the machine.

- Converting and/or transmitting: transmits and/or converts the motion from the driver element to the  driven element.

Mechanisms are devices that transmit and/or convert motion. We can divide mechanisms by their function into:

- Motion transmission mechanisms: In this type, the mechanism is fed energy from the driver element and simply transfers it to the driven element. Examples are pulleys, levers, etc. 

- Motion conversion mechanisms: In this type, the mechanism converts the movement from the driver element into a movement that the driven element can use. Examples are cams, eccentric, etc.

A lever is a rigid bar that can pivot on a point of support called a fulcrum.

 











Pulleys are used to lift heavy loads. They are wheels with a grooved rim.



 Belt and pulley system: This system is made up of two pulley wheels joined by a belt that slots into the grooved rim of the wheels. Examples are washing machines, cassete tapes, etc.

A gear is a mechanism consisting of two wheels with teeth. Examples are toys, watches, etc.

3rd Post: Stresses and Rigidity

Stress is the physical demand that a body or object must withstand when one or more external forces are applied to it. There are five types of stress:

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

- Tension: This is caused by forces acting on a body that tend to stretch it.

- 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 teh centre, as shown in the figure to the right.

 












 - Torsion: This is caused by forces acting on a body that make it twist.

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

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 it is deformable.
  
Exercise 12. Think about  the following objects and state whether they are rigid,plastic or elastic: balloon, empty drinks can, window frame, clay, catapult, door.
- Balloon: elastic
- Empty drinks can: plastic
- Window frame: rigid
- Clay: plastic
- Catapult: rigid
- Door: rigid 

2nd Post

-A structure is a collection of elements arranged in an optimum way to support a body or an object.

 Mass structures: These structures are solid, heavy and very sturdy. For example walls, dam, etc

Truss structure: The structures are composed of bars that form triangles, lightweight and sturdy. For example cranes, electricity pylons, etc.

Frame stucture: These structure are formed by columns and beams that act as supporting elements. Examples are buildings, etc

Suspended structure: These structure are held in place by cables and suspenders attached to sturdy supports. Examples are bridges, tents, etc.

A force is an influence that can deform a body or change its movement, or produce motion in a body at rest.

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

Presentation

Hello, we are Luis and Aitor and we are going to talk about structures.