Science 7: Unit D – Structures and Forces Topic 1: Types of Structures

Structures  Structure – an object with a definite, unchanging size and shape which serves a purpose.  Function – The purpose of a structure. A structure can have one or more functions.  Load – The weight that is carried/ supported by the structure. Eg. The roof of Legal School is a load for the walls of Legal School.

Forces  Force – a push or pull which changes the speed or direction of an object.  The most famous example of a force is gravity.  Gravity is a pulling force which pulls all objects towards the centre of the Earth.  Force depends on two things: the mass of an object and how fast the object is being sped up or slowed down.

Weight  Weight – the amount of gravity-force acting on an object.  Weight, being a force, depends on your mass and how strongly you’re being pulled.  On Earth, the formula for force is:  Weight = mass*g, where g = 9.8 N/kg.  Weight is also known as the force of gravity, Fg.  The unit for all forces, including weight, is the newton, N.

Weight Problem Example  Timmy has a mass of 75 kg. What is his weight, in N?  Fg = mass*g = 75 kg * 9.8 N/kg = 735 N It’s important to know that mass does not change if the amount of gravity changes (like in space travel, but your weight does).

Natural Vs. Artificial Structures  Natural Structures – are structures which are the result of animals or natural forces. Eg. beaver dams, trees.  Manufactured Structures – are structures which are human made. Eg. the Eiffel Tower.

Types of Structures  There are three kinds of structures:  I. Mass Structures  II. Frame Structures  III. Shell Structures

Mass Structure  Is made by piling up materials into a shape or design. - Mountains, coral reefs are natural mass structures - Sand castles, dams and brick walls are manufactured mass structures. Advantages: held in place by its own weight, losing small parts often has little effect on the overall strength of the structure - mass structures are not always solid, but are layered and have hollowed out areas for specific functions. - a power dam and the Great pyramids of Egypt are a good examples

Frame Structure  Have a skeleton of strong materials, which is then filled and covered with other materials. Most of the inside part of the structure is empty space. - Load-Bearing Walls: these are the walls that support the load of the building. - Partition Walls: these are the walls that divide up the space inside the building. - are relatively easy to design and build, and cheap to build. The frame structure is the most common construction choice.

Shell Structures  use a thin, carefully shaped, outer layer of material to provide their strength and rigidity. The shape of a shell structure spreads forces throughout the whole structure, which means every part of the structure supports only a small part of the load, giving it its strength. Shell structures have two very useful features: - they are completely empty, so they make great containers - their thin outside layer means they use very little material Problems in building shell structures include: - A tiny weakness or imperfection on the covering can cause the whole structure to fail. - Flat materials are difficult to form into the rounded shell shape.

Combination Structures  Most structures are actually combinations of two or more structural types.  A house is an example a combination structure:  The walls and roof of a house are primarily a frame structure with wooden beams nailed together.  The foundation/basement of a house is a mass structure made up of concrete which is solid.