# Materials

What is density?
The mass per unit volume of a material or object
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What is the unit of density?
kgm^-3
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What is Hooke's law?
The extension of a stretched object is proportional to the force applied to it (up to the limit of proportionality)
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What is the formula for Hooke's law?
F = kΔL
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What does k represent?
Stiffness/spring constant
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What is an object's stiffness constant?
The force needed to extend the object by 1m
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Give two features of a force-extension graph which show that a material obeys Hooke's law
The graph will be a straight line which goes through the origin
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What does the gradient of a force-extension graph represent?
The stiffness constant, k
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What is the elastic limit?
The force/stress beyond which a material will be permanently stretched
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What is the limit of proportionality?
The point beyond which the force applied is no longer proportional to the extension of an object
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What is the limit of proportionality in relation to a force-extension graph?
The point beyond which the force-extension graph is no longer linear
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When does elastic deformation occur?
When a material returns to its original shape once the forces are removed (ie has no permanent extension)
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When does plastic deformation occur?
When a material is permanently stretched after the forces have been removed
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What is tensile stress?
The force applied to an object divided by its cross-sectional area
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What are the units of tensile stress?
Nm^-2 OR Pa
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What is tensile strain?
The change in length (or extension) divided by the original length of a material
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What are the units of tensile strain?
No units
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What is the breaking stress of a material?
The lowest stress that can be applied to break a material
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What is the ultimate tensile stress of a material?
The maximum stress that a material can withstand
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What is elastic strain energy?
The potential energy stored in a stretched material up to the elastic limit
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What does the area under a force-extension graph represent?
The elastic strain energy
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What is the formula for elastic strain energy?
E = 1/2FΔL
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Describe the energy transformation when a stretched mass-spring system is released
The stored elastic strain energy in the spring is transferred into kinetic energy (as the spring contracts) and gravitational potential energy (as the mass gains height)
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Describe how energy conservation can be applied to transport design
Cars are required to have crumple zones that deform plastically in a crash; this means that some of the car's kinetic energy goes into changing the shape of the car, so less is transferred to the people inside
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What is the Young's modulus of a material?
The tensile stress divided by the tensile strain of a material up to its limit of proportionality
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What does the gradient of a stress-strain graph represent?
The Young's modulus, E
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What does the area underneath a stress-strain graph represent?
The strain energy (or energy stored) per unit volume
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What is the yield point of a material?
The stress at which a large amount of plastic deformation takes place with a constant or reduced load
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What is the shape of a stress-strain graph for a brittle material?
A straight line through the origin until the breaking point (no curve so obeys Hooke's law)
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What makes a material brittle?
It must have a rigid structure
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## Other cards in this set

### Card 2

#### Front

What is the unit of density?

kgm^-3

### Card 3

#### Front

What is Hooke's law?

### Card 4

#### Front

What is the formula for Hooke's law?

### Card 5

#### Front

What does k represent?

## Comments

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Incredibly useful thank you

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