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# 1.3.4 Behaviour of Springs and Materials - PowerPoint PPT Presentation

1.3.4 Behaviour of Springs and Materials. Objective. Describe how deformation is caused by a force in one direction and can be tensile or comprehensive. Deformation. Can be caused by tensile or compressive forces Tensile cause tension stretching forces Compressive cause compression

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## PowerPoint Slideshow about ' 1.3.4 Behaviour of Springs and Materials' - jaquelyn-albert

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Presentation Transcript

### Objective

Describe how deformation is caused by a force in one direction and can be tensile or comprehensive

Can be caused by tensile or compressive forces

• Tensile

• cause tension

• stretching forces

• Compressive

• cause compression

• squeezing forces

two equal and opposite tensile

forces stretching a wire

two equal and opposite compressive

forces squeezing a spring

### Objective

Describe the behaviour of springs and wires in terms of force, extension, elasticlimit, Hooke’s Law and the force constant – i.e. force per unit extension or compression

• 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

• 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

• 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 = extension

k = force/spring constant

F

Tension /N

x

Extension /mm

• 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

• 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

### Objective

Select and use the equations for elastic potential energy, E = ½Fx and ½kx2

• 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)

• 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

• 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)

• To calculate stiffness, calculate the ratio of stress to strain:

Young Modulus (Nm-2) = stress

strain

or E = stress

strain

Hooke’s Law Region

Elastic limit

Limit of proportionality

stress

strain

• 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

### Objective

Describe an experiment to determine the Young modulus of a metal in the form of a wire

Young modulus practical

### Objective

Define the terms elastic deformation and plastic deformation of a material

• 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

• 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

• 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

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

• Cannot stretch beyond it’s elastic limit

• Can deform elastically

• Can shatter if you apply a large stress

• Examples include perspex

• 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

• Physics 1 – Chapter 8

• SAQ 1 to 9

• End of Chapter questions 1 to 4