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Grade 7

Structures and communication

Module 22

Reinforcing doors and windows

REINFORCING, by using concrete and lintels over doors and window openings.


Structures are stabilized to prevent them from collapsing when forces or loads are put on them. To prevent a structure from falling down or collapsing we must try to make the internal forces bigger than the external forces under all conditions. How can you prevent a chair from collapsing when you sit on it? There are rails acting as ties at the bottom holding the legs together and preventing the legs from being pushed outwards.

A flagpole doesn’t need any supports under normal conditions. When the wind blows it can be supported by using guy ropes or ties that are flexible, but stop the poles from bending too far.

Different forces can act on different parts of a structure. Forces that structures have to stand up to are:

  • Compression – when we try to squash a structure ( a squashing force).
  • Tension – when we try to pull a structure apart from either end (a pulling force).

  • Bending – a force which is trying to bend a structure and it is in compression and tension simultaneously.
  • Torsion – a turning or twisting force like when you twist the top off a bottle.
  • Shear – a force created when two opposite forces try to cut a material; like when you use a pair of scissors to cut a piece of paper.

When one force is balanced by another they are said to be in EQUILIBRIUM.

When you build a structure, you do not always have to use solid material like wood or metal. Flexible material, such as wire, can be used for parts of the structure which will only be stretched or in tension. Wire can be used in tension (being stretched), but not in compression (be squashed).

Load from the top of the structure keeps the wire in tension and the structure remains rigid.

When the structure is loaded from the sides the wire is compressed (squashed) and it collapses. It is unstable.

Triangles are very useful when building structures. They can help to make a very strong and rigid structure. To make a rectangle more stable is to put in two more members from corner to corner. Four triangles are formed. This way of making structures more stable is called TRIANGULATION. It is often used when building bridges or cranes. It is not always necessary to put in two members, one would be enough. The second extra member is called the redundant member. When designing structures it is easy to use more members that are really needed. They will only add more weight to the finished structure and make it more expensive.

By making the base size of a structure heavier or wider, you can also make it more stable.

Assignment 1 (groupwork):

[lo 2.1]

Choice 1:

Design and make a horizontal structure using 25 paper straws and a glue gun. Your structure must support a marble as far out from the table as possible. Your structure must not come in contact with the floor or any of the other furniture in the room. It must support the marble for at least 30 seconds.

Choice 2:

Design and make a tower using 25 paper straws and a glue gun. Your tower must support a marble for at least 30 seconds as high as possible.

It is also important to get a structure’s centre of gravity so that it is able to balance. All objects have a point where they are held in balance by the force of gravity. This balancing point is called the centre of gravity, because it is the place where the whole weight of the object seems to centre. The balancing point of a rectangular shape, such as a square or a circle, is in the centre.

How can you find the balancing point of an irregular shape?

Where do you think is the best place for the heavy cargo to be stowed on a ship?

Gravity is the pull that objects have on other objects around them. Because the Earth itself is the largest object in our world, the pull of its gravity is the strongest we can feel. Gravity keeps everything resting on the ground. An object centre of gravity is found when it just balances on the edge of a table before it falls off.

Any object will balance on a pivot when its centre of gravity is low.

How can you change an object’s centre of gravity?

Assignment 2:

To make your own trapeze artist

[lo 2.1]

Requirements :

  • Thick white card
  • a drawing pin
  • a scissors
  • felt tip pens
  • strong glue
  • wire
  • 2 plasticine balls
  • a heavy bottle
  • a glue gun
  • pliers


  1. Draw the man shape onto some thick white card. Carefully cut out the shape with scissors.

1. Colour in the man to make him more human in the front and back using felt tip pens. Carefully glue a drawing pin to the bottom of the card.

  1. Cut of a length of wire using pliers. Glue it into place with the glue gun.

  1. Glue two plasticine balls to each end of the wire.

4. Carefully stand the balancing man on top of a bottle. He wobbles slightly, but should keep his balance.


Paper/card Glue, paper clips, a stapler, paper fasteners, washers, string or yarn, tape, pins
Textile Textile glue glues material onto material, PVA glue glues material to paper, card or wood, string , yarn, fasteners (buttons, velcro, zips, hooks and eyes, snap-fasteners, press-studs, strings, laces, ribbon)
Wood Nails, pins, screws, woodglue, joints
Metal Screws, bolts and nuts, soldering, glue, rivets
Plastic Screws, glue


Assessment standards (ASs)
We know this when the learner:
2.1 demonstrates knowledge and understanding of structures.
processing :2.2 demonstrates knowledge and understanding of how materials can be processed to change or improve properties (e.g. strength, fire resistance, waterproofing, taste, volume, texture).

Questions & Answers

what are two classical macroeconomics and what're their theories say about their equations?
what is the formula for calculating elasticity
aza Reply
what is elasticity of demand?
Rita Reply
Causes of economic growth
pierre Reply
What is elasticity of demand
What are the causes of economic growth
economic growth, establishment of industry, encourage of investor's, farm productivities, creation of institutions, construction of good road etc
elasticity of demand can be said to be the responsiveness of demand to a change in prices
impact of collusion in the economy referring to inefficiencies illustrated by means of graph
nondumiso Reply
The Factor price will determine the choice of techniques to produce.Expantiate
what is elasticity of demand?
Etta Reply
state and explain two types of demand
Institution involved in money market
Gande Reply
what is Economics
Kwame Reply
Economic is the study of scarcity
Economics is the study of a lot of things. It is split up into two areas of study, Microeconomics and Macroeconomics. Microeconomics is the study of an individual's choices in the economy and Macroeconomics is the study of the economy as a whole.
Economics is a science that studies human scarcity
What is Equilibrium price?
Equilibrium is the market clearing price. The point at which quantity demanded equals quantity supplied. The point at which the supply and demand curves intersect.
Equilibrium price*
Refers to the study of how producers use limited resources to satisfy human unlimited wants
why is economics important
Derrick Reply
What will you do as a consumer if you are not at equilibrium?
chukwu Reply
am new I will like to know about the graph relationship
Gloria Reply
comment on WTO principle on trading system. trade without discrimination
Omben Reply
optimize z=f(x,y)=6x²-9x-3xy-7y+5y²
Alex Reply
What is an indifference curve?
layla Reply
different levels of utilities of a person in a given set of bundles of goods
identify and quantify five social costs and social benefits of building a school
Mokgobo Reply
identify and quantity five social costs and social benefits of building a hospital
short run vs long run
state the law of diminishing return?
The Law of Diminishing (Marginal) Returns simply states that at some point in time a business/operation/etc.'s increased productivity will begin to decline.
For example, if a small pizza shop currently has 3 workers in the kitchen at any given time,and hiring 1 more worker will increase productivity, at some number of workers hired will the business see a decrease in productivity because the capital resources that the pizza shop has is not infinite.
Five social benefits of building a hospital, in my opinion and depending on where it's built, would be 1) Increased care for neighboring residents, 2) Potential jobs for individuals, 3) May decrease the travel time residents need to endure in order to reach the nearest hospital
4) May create work-study programs for individuals who aspire to be future Doctors, Nurses, Physicians, etc. 5) Assuming there are local pharmaceutical businesses nearby, the hospital may decide to purchase supplies local, increasing the business' sales. Thus, generating more income.
5 costs of building a hospital would be 1) Increased noise and waste pollution from service vehicles and hospital visitors, 2) May require large amounts of space, possibly jeopardizing nearby animal habitats, 3) May see an increase in traffic and possibly car accidents from frantic individuals
racing to see their injured friends, family members, etc. 4) Constructing a hospital and hiring staff is very expensive 5) To use funds, private or public, to finance the construction of a hospital cannot be used to fund any other projects. (The concept of opportunity costs.)
what is meant by inteference with the price mechanism operation?
We use a Supply and Demand graph to illustrate at what price level will the market for a certain good or service be at equilibrium. If the price for a good or service is set too high, consumers will be less inclined to buy that product Thus, creating a surplus.
This surplus will eventually drive the price back down to it's equilibrium point. Similarly, if a price for a good or service is set too low, individuals would be more inclined to buy more of a certain product, creating a shortage. This shortage will cause sellers to drive the price back up to the
equilibrium point.
in a comparison of the stages of meiosis to the stage of mitosis, which stages are unique to meiosis and which stages have the same event in botg meiosis and mitosis
Leah Reply
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Source:  OpenStax, Technology grade 7. OpenStax CNX. Sep 10, 2009 Download for free at http://cnx.org/content/col11032/1.1
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