A measure of how much mass is contained in a given unit volume
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Equation for density?
p = m/v
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What is Hooke's law?
Force = spring constant x extension
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What is the spring constant?
A measure of how stiff a spring is; the higher the spring constant the stiffer the spring
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What is the limit of proportionality?
The limit, beyond which, when a wire or spring is stretched its extension is no longer proportional to the force that stretches it
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What is the elastic limit?
A point past which if you increase the force applied, the material will be permanently stretched
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What are compressive and tensile forces?
Compressive- squash the spring, Tensile- Stretch the spring
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What happens in elastic deformation?
The material returns to its original shape once the force is removed- no permanent extension
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Give an example of plastic deformation
When a metal is stretched beyond its elastic limit
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What happens in plastic deformation?
The material is permanently stretched after the force has been removed as some atoms move position and don't return to their original positions when the load is removed
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How do you find the work done in a force against extension graph for rubber?
The area under the loading curve
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What does the area between the loading and unloading curves represent?
The energy transferred to internal energy, due to which the rubber band becomes warmer
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What is tensile stress?
The force applied divided by the cross-sectional area. Units are Pa or Nm^-2
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What is tensile strain?
Change in length divided by the original length of the material (no units as it is a ratio)
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What is the effect of stress?
The atoms are pulled apart from each other
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What is breaking stress?
When th atoms separate completelyand the material breaks
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What is the UTS (ultimate tensile stress)?
The maximum amount of stress that a material can withstand
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What is elastic strain energy?
The amount of energy stored (as potential energy) in a material when it is stretched
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How is energy conserved in elastic deformation?
All work done is stored in the material as elastic strain energy and when the stretching force is removed, this stored energy is transferred into other forms
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How is energy conserved in plastic deformation?
Work is done to separate atoms and energy is not stored as strain energy- it's mostly dissipated as heat
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What is the Young Modulus a measure of?
How stiff a material is
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How do you calculate the Young Modulus?
E = tensile stress/tensile strain
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What does the gradient of a stress against strain graph show?
The Young Modulus
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What does the area under a stress against strain graph show?
The strain energy per unit volume
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What is the yield point on a stress/strain graph?
When the material suddenly starts to stretch without and extra load- a large amount of plastic deformation occurs here
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On a force-extension graph for a metal wire, where is the unloading line drawn?
Parallel to the loading line as stiffness constant k is still the same; the forces between the atoms are the same as they were during the loading
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Do brittle materials obey Hooke's Law?
Yes as the stress-strain graph is a straight line, showing it obeys Hooke's Law
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What is a brittle material?
One which doesn't deform plastically but snaps when the stress on it reaches a certain point
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What is a brittle fracture?
When a stress applied to a brittle material causes cracks at the material's surface to get bigger and bigger until the material breaks completely
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Give a useful exampleof plastic deformation
Crumple zones in cars- in the event of a collision, some of the car's kinetic energy goes into changing the shape of the vehicle's body, so less is transferred to the people inside
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