4TH POST

Machines and mecanism 

• When a structure is stable?

Is stable when it remains in equilibrium even when a force applied to it.

• What is the centre of gravity?

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

Machine:

We use the word machine to describe a set of elements or parts that work together and use energy to perform a task and obtain a result.

The principal components of a machine: 

 -Driver element

- Driven element

- Converting and/ or transmitting element

Mechanism:

Is device that transmit and/or convert motion.

The diferences between motion transmission and motion conversion:

-The motion transmission is when the mechanism is fed energy from the driver element and simply transfers it to the driven element

-The motion conversion is when the mechanism converts the movement from the driver element into a movement that the driven element can use.

What is a lever?

Is a lever is a rigid bar can pivot on a point of support called a fulcrum. It is used to lift load by appying force to a point on the bar.

  

What is a pulley?
Pulleys are used to lift heavy loads. They are wheels with a grooved rim.

Explain belt and pulley system:
This system is madre up of two pulley wheels joined by a belt that slots into the grooved rim of the wheels.Examples: washing machines, cassette types, fans...

Explain what gears are:
Is a mechanism consisting of two wheels with teeth. Example: You have two red gears that you want to keep synchronized, but they are some distance apart. You can place a big gear between them if you want them to have the same direction of rotation

3RD POST

STRESS 

The basic stresses that a body may have to support are compression, tension, bending, torsion and shear .

Basic stresses:

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. For example, when we lean on a table, it undergoes compressive stress; columns are also subject to compressive stress.

Tension (or traction) : This is caused by forces acting on a body that tend to stretch it. For example , when two people pull on an object from opposite directions.

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, as shown in the figure to the rights. This is what happens to a shelf when we put lots of heavy books on it.

Torsion: This is  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 it out ; a crank is also subjected to torsion.

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 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 it is deformable.

Exercise

12. Think about the following objects and state whether they are rigid, plastic or elastic: ballon, empty drinks can, window frame, clay, catapult, door.

Rigid                                  Plastic                    Elastic

window frame             empty drinks can          ballon

catapult                                                             clay

door

2ND POST

                                 STRUCTURE

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

-Types of structures:

1. Mass structures. These are solid, heavy and very sturdy structures. Examples include walls, dams,etc.
   
2. Frame structures. These structures are formed by colums and beams that act as supporting elements. One example might be the structure os a building.
   
3. Truss structures. These structures are composed of bars that form triangles. They are lightweight and sturdy. Examples include cranes, electricity pylons, etc.
   
4. Suspended structures. These are held in place by cables and suspenders attached to sturdy supports. Examples include suspension bridges, tents, marquees, etc.

-Force is an influence that can deform a body or change its movement, or produce  motion in a body at rest

 -Stresses are phisical demands that a body or object must withstand when one or more external forces are applied to it-

PRESENTATION

Hello, we are Nicole and Victor, we talk about the Structures