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Objective
Describe how deformation is caused by a force in one direction and can be tensile or comprehensive
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sics Deformation
Can be caused by tensile or compressive forces
Tensile cause tension stretching forces
Compressive cause compression squeezing forces
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sics Deformation
two equal and opposite tensileforces stretching a wire
two equal and opposite compressiveforces squeezing a spring
Objective
Describe the behaviour of springs and wires in terms of force, extension, elastic limit, Hooke’s Law and the force constant – i.e. force per unit extension or compression
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sics Definitions
Force (F) applied to a spring or wire in tension or
compression Extension (x)
the change in length of a material when subjected to a tension, measured in metres
Elastic Limit the point at which elastic deformation becomes
plastic deformation
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sics Definitions
Elastic Deformation when the deforming force is removed, the object
will return to it’s original shape eg rubber band, spring (usually)
Plastic Deformation when the deforming force is removed, the object
will not return to it’s original shape eg Plasticine, Blutack
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sics Hooke’s Law
When tension is plotted against extension, a straight line graph denotes elastic deformation
This is summarised by Hooke’s Law:‘The extension of a body is proportional to the force
that causes it’
or as a formula: where F = Force
F = kx x = extensionk = force/spring constant
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sics Force/Spring Constant
F = kx Expressed in newtons per metre How much force is required per unit of
extension eg 5 N mm-1 means a force of 5 N causes an
extension of 1 mm Can only be used when the material is
undergoing elastic deformation
Objective
Determine the area under a force against extension (or compression) graph to find the work done by the force
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sics Work Done
Extension produced by tension F is x Work done to reach this extension is the area
under the graph
work done = area of triangle
= ½Fx
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sics Elastic Potential Energy
As work has been done to stretch the wire, the wire then stores Elastic Potential Energy
This also applies to compression forces For elastic deformation, the elastic potential
energy equals work done:
E = ½Fx
as F = kx then E = ½kx2
Objective
Define and use the terms stress, strain, Young modulus and ultimate tensile strength (breaking stress)
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sics Stretching Materials
One way of describing the property of a material is to compare stiffness
In order to calculate stiffness, two measurements need to be made: strain stress
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sics Stretching Materials
Strain is the fractional increase in the length of a material
Strain = extension (m) original length (m)
Stress is the load per unit cross-sectional area of the material
Stress (Nm-2) = force (N) cross-sectional area (m2)
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sics Young Modulus
To calculate stiffness, calculate the ratio of stress to strain:
Young Modulus (Nm-2) = stress
strain
or E = stress
strain
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sics Young Modulus
Limit of proportionality
strain
stress
Hooke’s Law Region
Elastic limit
gradient = Young modulus
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sics Ultimate Tensile Stress
Stiffness tells us about the elastic behaviour of a material (Young modulus)
Strength tells us how much stress is needed to break the material
The amount of stress supplied at the point at which the material breaks is called the ultimate tensile stress of the material
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sics Young Modulus Practical
markerwire
rule
load
wooden blocks
cardboard bridges
Young modulus practical
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sics Definitions
Elastic Deformation when the deforming force is removed, the object
will return to it’s original shape eg rubber band, spring (usually)
Plastic Deformation when the deforming force is removed, the object
will not return to it’s original shape eg Plasticine, Blutack
Objective
Describe the shapes of the stress against strain graphs for typical ductile, brittle and polymeric materials
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sics Ductile
Will stretch beyond it’s elastic limit
Will deform plastically Can be shaped by
stretching, hammering, rolling and squashing
Examples include copper, gold and pure iron
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sics Brittle
Will not stretch beyond it’s elastic limit
Will deform elastically
Will shatter if you apply a large stress
Examples include glass and cast iron
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sics Polymeric
Will perform differently depending on the molecular structure and temperature
Can stretch beyond it’s elastic limit Can deform plastically Can be shaped by stretching, hammering,
rolling and squashing Examples include polythene
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sics Polymeric
Cannot stretch beyond it’s elastic limit
Can deform elastically
Can shatter if you apply a large stress
Examples include perspex
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sics Summary
All materials show elastic behaviour up to the elastic limit
Brittle materials break at the elastic limit Ductile materials become permanently
deformed beyond the elastic limit Polymeric materials can show either
characteristics, depending on the molecular structure and temperature