5
Structures
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1.
Structures
In this section we deal with the concept of structure using some
concrete examples to focus students’ attention on its definition
in general terms. It is useful to bring up the issue from the start
so that the students can discuss what they understand by the
structure of a body, and also help make them aware of any pre-
existing knowledge they may have on the topic.
Having started working on the text, we deal with identifying, in
each example, which elements of the body in question belong
to the structure and which do not. The definition of structure
which is provided in the text is a simple definition which can
be expanded on by the students as they progress through the
various parts of the unit.
1.1.
Natural and man-made structures
We establish the difference between natural and man-made
structures by being able to study the latter within the field of
technology as a product of human activity.
Having identified structures we move on to an explanation of the
concepts of forces and loads.
PRESENTATION
FORCES AND LOADS
A summary using slides of the types of structures and the loads
that act on them.
2.
FORCES AND LOADS
We can begin by posing two small questions about forces and
structures. By doing this, we can introduce the concepts which
will be studied. We can also experiment by using the simple action
of pressing a rubber from different positions with our fingers and
observing what happens.
Although students have an intuitive idea of what a
force
is,
they tend to find it difficult to understand its definition and
systematisation because it is a relatively complex concept. The
definition of force which is provided in the text is the classic
one, based on recognition of its possible effects. To make this
more concrete and to identify the two possible effects (static and
dynamic) a simple activity is suggested in which the student must
distinguish what effect is produced by the application of different
forces on a variety of objects.
To expand on this point, the representation of stresses can be
achieved by means of vectors and explaining that in order to
define a force we must know its direction, magnitude and point
of application. We can demonstrate all these characteristics
through the force which we apply to a table, causing it to move.
Students can also be told that the effects of permanent loads
can be seen from a simple study of their actions on bodies.
However, the situation is more complex in the case of variable
ones because they can appear in different combinations,
sometimes counteracting each other (e.g. snow on a tree and
wind which pushes the branches upwards). In such cases, the
most unfavourable combinations must be taken into account.
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5. Structures
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www
Understand
1.
Can you clearly identify
which part of the bottle
forms its structure?What
parts of the bottle are
usedtomakeitstronger?
2.
FORCES AND LOADS
Physical bodies and objects need a structure to support the different forces that
act
on
6
them, likegravitywhichattractsobjects toEarthandcause them tohaveweight.
A
force
is anything that can deform a body (static effect) or make it move
or stop (dynamic effect).
1.
STRUCTURES
All physical bodies or objects have a
structure
. Its purpose is to maintain the
object’s shapeandnot tobreakwhena forceactson them.Anelephant’s skeleton
or the
grooves
1
on a plastic cup are examples of structures.
Sometimes the structure is easily identified in the body or object, like the bones of
vertebrates, the
frame
2
of a boat or the
columns
3
and
beams
4
in a building. For
other objects the structures are not easy to distinguish because the whole object
forms the structure.
Understand
2.
What structures canyou see in thesephotos?Are theynaturalorman-
made? Justify your answer.
There are two main types of loads:
❚
Fixed or permanent loads
are constant – they don’t
vary
8
over time. An
example would be the weight of the structure of a bridge and its component
parts (columns, road surface, etc.).
❚
Variable loads
vary over time, like the traffic going across a bridge or the
wind hitting it.
A
load
is a force that acts on a structure. It can be fixed or variable.
Loads produce
stress
on the structure of a body. The same load can cause
differenteffectsdependingon theshapeof thestructure, thepointofapplication,
the direction, and the trajectory.
Apply
7.
Illustrate the forces of
the actions in activity 6.
Todefine a force,wehave to
specify its intensity,directionand
thepointof application
direction
intensity
(proportional
to the length
of thearrow)
pointof
application
trajectory
(outwards)
❚
A structure is the set of
elements in a body that
preventabody frombreaking
ordeforming.
❚
Structures are natural or
man-made.
Key concepts
❚
A forcecandeformabody,
make it move or stop it.
❚
Astructurebears theeffects
of the forcesactingon it.
❚
A force that acts on a body
is called a load.
Key concepts
A structure is the set of elements in a body that are made to resist the effects
of the forces that act on it. A structure prevents a body from breaking or
deforming
5
.
Understand
3.
Name five natural and five man-made structures.
4.
Identify the structure of the following objects.
Analyse
5.
Find a photo of a Roman bridge and a suspension bridge. What
parts of each bridge forms their structure?
1.1.
Natural and man-made structures
Natural structures
are not made by man. They are in living beings, in their
composition, or the result of a geological process, for example, the shell of a crab,
a bird’s nest or a cave.
Man-made structures
are made by people to satisfy a need. Some examples are
the legs of a table or the case of a camera.
1
groove
:
long, narrow hollow space
on a surface
2
frame
:
hard structureof an object
3
column
:vertical stoneused to support
roof or arch
4
beam
:
horizontal straight piece of
wood/stone to support roof or floor
5
deform
:
change the natural shape
6
acton
:
have an effecton
7
squeeze
:press somethinghard,usually
to change shape
Understand
8.
Listen and say which concept is defined.
load
force
structure
Understand
6.
Say if the force doing the following actions produces a static or
dynamic effect: hitting a golf ball, sitting down on a sofa, pushing
a shopping trolley, your heart pumping blood, squeezing
7
a piece of
plasticine and giving your friend a hug.
Static effect
Dynamiceffect
8
vary
:
be different or change state or
condition
a pen
a plastic cup
a camera
a rocking chair
a bicycle