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Page 1

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Hooke's Law
The extension of an object
is directly proportional to
the applied force up to the
limit of proportionality
Fx
F=kx
k is the Spring Constant
for that object
(units: Nm-1)

Page 2

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Hooke's Law
F e
p


Gradient = k
Area = elastic
strain energy

d x
p = limit of proportionality
e = elastic limit dotted
line = unloading d =
permanent extension

Page 3

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Elastic deformation
The term "Elastic" in the
context of materials
means "Elastic behaviour"
i.e. up to the Elastic Limit
a material will return to its
original dimensions when
the deforming force is
removed.
So metals can behave
elastically over small
extensions.

Page 4

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Plastic deformation
A material which does not
return to its original
dimensions when unloaded
has deformed plastically.
Metals also exhibit plastic
behaviour for extensions
beyond their Elastic
Limit. A useful property
for making wires.

Page 5

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Plastic deformation
As a direct consequence of
plastic deformation, some
materials (usually metals)
are described as Ductile.
This means that they can
be drawn and stretched
into bars and wires by
tensile forces.

Page 6

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Brittle

Materials which do NOT
deform plastically before
breaking are described as
Brittle.

Page 7

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Ductile / Maleable
Materials which deform
plastically (and can be
stretched into wires)
before breaking are
described as Ductile.
Materials which can be
pressed or hammered into
different shapes are
Malleable.

Page 8

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Test
Define the following:
Hooke's Law
Limit of Proportionality
Elastic Limit
Ductile
Brittle

Page 9

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Test
If you were asked to
conduct a Hooke's Law
experiment, what
measurements would you
record AND repeat?
What graph would you plot
and how would the
gradient of this graph
relate to Hooke's Law (be
careful!)

Page 10

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Tensile Stress
The tensile force (tension)
per unit cross-sectional
area.


Tensile Stress = F/A

Units: Nm-2 or Pascals

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