Understanding Structures, Forces, and Mechanisms

A structure is a collection of elements arranged in an optimal way to support a body or an object. When we design a structure, we need to combine maximum strength with the minimum amount of material. This makes the construction as economical and lightweight as possible.

Types of Structures

Mass Structures

These are solid, heavy, and very sturdy structures, such as walls and dams.

Frame Structures

These are formed by columns and beams that act as supporting elements, like in buildings.

Truss Structures

These are composed of bars that form triangles. They are lightweight and sturdy, like cranes and electricity pylons.

Suspended Structures

These are held in place by cables and suspenders attached to sturdy supports, such as suspension bridges, tents, and marquees.

Forces and Stresses

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. When we lean on a table, we apply force to it, and the table is subjected to stress.

The strength of a material is its ability to withstand external forces without breaking.

Basic Types of Stresses

• Compression: Caused by forces acting on a body that tend to flatten it or reduce its length or thickness.
• Tension: Caused by forces acting on a body that tend to stretch it (e.g., two people pulling on an object from different directions).
• Bending: Caused by forces acting on an element that make it curve or bend, such as supporting the two ends and applying force in the center.
• Torsion: Caused by forces acting on a body that make it twist.
• Shear: Caused by two equal forces 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.

Machines and Mechanisms

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.

All machines have three parts:

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

Types of Motion

• Linear motion: The object moves in a straight line.
• Rotary motion: The object moves in circles.
• Reciprocating motion: The object moves forward and backward in a straight line.
• Oscillating motion: The object moves backward and forward tracing a curve.

Mechanisms by Function

1. Motion Transmission Mechanisms

In this type, the mechanism is fed energy from the driver element and simply transfers it to the driven element.

• Linear transmission mechanisms: These are a type of simple machine (lever).
• Rotary transmission mechanisms: These include friction wheels, belt and pulley systems, gears, chain and sprocket systems, and worm gears.

2. Motion Conversion Mechanisms

In this type, the mechanism converts the movement from the driver element into a movement that the driven element can use.

Levers and Pulleys

A lever is a rigid bar that can pivot on a point of support called a fulcrum. It is used to lift a load by applying force to a point on the bar. We call the applied force effort, whereas the load to be lifted is resistance.

The law of the lever, which describes the forces of a lever in equilibrium (balance), states that: the greater the distance between the applied force and the fulcrum, the less effort is required.

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