Machines and mechanism

A stable structure : we say that a structure is stable when it remains in equilibrium even when a force is applied to it.

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

Machine: a machine a set of elements or parts that work together and use energy to perform a atask and obtain a result.

Main component of a machine: -Driver element
                                                    -Driven element
                                                    -Converting and/or transmitting element

Mechanism: they are devices that transmit and/or conver motion

Difference between motion transmission and motion conversion:

-Motion transmission is fed by energy from the driver element and simply transfers it to the driven element.

Example: sprocket systems

-Motion conversion converts the movement from the driver element into a movement that the driven element can use.

Example: slider system

Lever: it is a rigid that can pivot on a point of support called a fulcrum. It is used to lift load by applying force to a point on the bar. 

Pulley: it is an instrument used to lived heavy loads. They are wheels with a grooved rim.

Belt and pulley system: this system is made up of two pulley wheels by a belt that slots into the grooved rim of the wheels. 

Example: a bycicle

Gears: a gear is a mechanism consisting of two wheels with teeth. 

Example: a tape dispenser.

Stresses and rigidity

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

The basic stresses are:

Compression: this is caused by forces acting on a body that tend to flatten it or reduce its lenght or thickness. It happends in structures that need to support weights.

Tension: this is caused by forces acting on a body that tend to strecht 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 the center. This is what happends 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.

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.

Rigidity: it is an esential characteristic for any strong structure 

Exercise. 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: Elastic.

DOOR: Rigid.

Structures

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

-Mass structures: These a solid, heavy and very sturdy structures. 

-Frame structures: These structure are formed by columns and beams that act as supported elements

.

-Truss structures: These structures are composed of bars that form triangles. They are lightweight and 

sturdy.

-Suspended structures: These are held in place by cables and suspended attached to sturdy supports.

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

-Stresses: they 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 Raúl Gómez and María Ängeles Magro and we are going to talk about structures.