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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

Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit 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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