TECHNOLOGY NOTES 2022 GRADE 9 TERM 1 STRUCTURES UNIT 1.1 – GRAPHIC COMMUNICATION Orthographic projection is a way of drawing an object from different positions so that all sides or views of the object can be seen. Usually front, side and plan views are drawn. Orthographic drawings are useful when drawing designs that are almost ready to be manufactured or built. There are two kinds of orthographic drawings: first angle and third angle. We will focus on the first angle drawings. DESCRIPTION Continuous thick Continuous thin (Straight or curved) DRAWN LINE GENERAL APPLICATION - Visible outlines. - Visible edges. - Imaginary lines of intersection. - Dimension lines. - Projection lines. - Leader lines. - Hatching. - Outlines of revolved sections in place. - Short centre lines. - Bending lines. - Limits of partial or interrupted views and sections, if the limit is not a chain line. - Break lines. - Hidden outlines - Hidden edges Continuous thin, freehand Continuous thin (Straight with zigzags) Dashed thin Dashed thick - Hidden outline - Hidden edges - Centre lines - Lines of symmetry - Trajectories Cutting planes Chain thin Chain thin, thick at ends and changes of direction Chain thick Indications of lines or surfaces to which a special requirement applies SCALE It is not possible to draw all objects to their real-life size. Some objects need to be drawn smaller, such as a house, whilst others need to be drawn larger in order to see more details clearly. This is why we make use of scale in our drawing. FORCES, STRENGTH AND PROPERTIES P.15 STATIC FORCE A static force is a load that doesn’t change. A static force can be the structure’s own weight or the load that is being carried by the structure. Example, a person standing hold a bag of groceries. DYNAMIC FORCE A dynamic force is force that changes. This force is a moving force. Example, traffic crossing a bridge. Dynamic forces can be destructive, for example, the load may exert too much force and cause buckling or fractures in the structure. A high wind could cause a bridge distort. INTERNAL AND EXTERNAL FORCES Internal – inside External – outside Internal forces There are a number of forces inside a structure, which are called internal forces. When a load is applied to a structure, these forces inside the structure resist the load. External forces External forces are forces outside the structure. When a car crosses a bridge, it exerts a force on the bridge from outside. This force is from the outside of the structure and is called an external force. TENSILE This is how good the material is at withstanding pulling force or tension force. A material that is bad at withstanding this force has a low tensile strength. A material that is good at withstanding this force at a high tensile strength. The tensile strength of a material is the force needed to pull it apart by breaking all the bonds between atoms. Metals such as iron and steel have extremely high tensile strengths and can withstand heavy loads. COMPRESSIVE This is a measurement of how good a material can be withstanding a compressive or squeezing force. When a material is compressed, the bonds squashed and atoms are pushed together. If the material is soft, it may be changed shape. If it is brittle, it may splinter or break. STRENGTH Is a measurement of how good a material is at resisting being alterated in the shape or form when a force is applied to it. There are two kinds of strength: 1. Tensile 2. Compressive DUCTILITY Ductility is the material’s ability to be drawn into wire or tube without breaking. MALLEABILITY Malleability the material’s ability to be hammered or rolled without breaking. CONDUCTIVITY Conductivity is the material’s ability to allow heat and/or electricity to pass through it. TOUGHNESS Toughness is the material’s ability to withstand knocks without being damaged or fractured. Material’s that are tough do not crack when exposed to shearing or bending forces. BRITTLENESS Brittleness is material’s ability to snap or fracture easily with little or no deformation. DURABILITY Durability is the material’s ability to withstand wear or tear, such as corrosion and weather conditions, and last long time. STABILITY Stability is the material’s ability to resist changes in shape and size. This is usually caused by heat and moisture, for example, wood warps in heat or when it gets wet. WEIGHT/MASS Weight/Mass describe how some materials are light while others are heavy. For example, aero plans are made of light materials while an axe is made of heavy materials. DENSITY Density is how densely (closely) the atoms are packed together in a material. The more dense a material is the more difficult it is to break. Density is the mass of 1 cubic centimeter of substance. To calculate a material’s density, we use the formula: Density = mass/volume FLEXIBILITY Flexibility is the materials ability to bend or give up to a certain point without breaking. STIFFNESS Stiffness is the materials ability to withstand a force without bending. HARDNESS Hardness is when a material is not soft or malleable and cannot easily be bent or change shape. It can withstand being cut, scratched or dented. CORROSION Corrosion is the ability of the material to resist the process of rusting. For example, iron rusts when it is exposed to oxygen in the air and makes a substance called iron oxide. If left long enough it will cause the iron to begin disintegrating. PROPERTIES OF WOOD AND METAL P.26 WOOD OAK PLYWOOD CHIPBOARD HARDBOARD Very strong, heavy and durable. Stays flat in large sheets strong when thin; flexible. Not very strong; difficult to join; cheap. Not very strong; cheap. METAL STEEL CAST IRON Tough, ductile and malleable; easy to join. Hard skin, but brittle inside; strong under compression; rusts and corrodes. END!!!!
