Section 4: Materials Flashcards
Scalar examples
Length/distance, speed, mass, temperature, time, energy
Vector examples
Displacement, velocity, force(including weight), acceleration, momentum
Difference between a scalar and a vector?
Vectors have a direction, scalars don’t have a direction.
Define the term ‘density’
Mass per unit volume
Give the formula for density
p=m/V
p-Density in Kgmˉ³
m-Mass in Kg
V-Volume in M³
Give the formula for Hooke’s law
F=kΔL
F-Force in N
k-Stiffness constant in Nm^-1
ΔL-Extension in m
What is meant by the elastic limit of a material?
The force (or load) beyond which a material will be permanently stretched
What is meant by the limit of proportionality for a material?
The force beyond which force is no longer proportional to extension
What does it mean if a material is deforming elastically?
A material that is deforming elastically returns to its original shape once the forces acting on it are removed
What does it mean if a material is deforming plastically?
A material that is deforming plastically is permanently stretched once the forces acting on it are removed
What is meant by tensile stress?
The force applied divided by the cross-sectional area
What is meant by tensile strain?
The change in length divided by the original length of the material
What is meant by breaking stress?
The smallest stress that’s enough to break a material
Explain how you would find he elastic strain stored by a stretched material using its force-extension graph
Work out the area underneath the curve up to the extension required
Briefly describe how energy is conserved as a stretched wire undergoes plastic deformation
Some energy is transferred to elastic potential energy, but some of the kinetic energy is used to separate the atoms and is dissipated as heat
Describe the energy changes in a spring as it is stretched and released
When a spring is stretched, kinetic energy is transferred to elastic strain energy and stored in the spring. When the spring is released the elastic strain energy is transferred to kinetic energy again
Give the derived formula of the Young modulus
E= tensile stress/tensile strain = F÷A/ΔL÷L = FL/AΔL E-Young modulus in Nmˉ² F-Force in N A-Cross-sectional area in m² ΔL-Extension in m L-Initial length in m
What does the gradient of a stress-strain graph tell you?
The Youngs modulus of the material tested
What does the area under a stress-strain graph give you?
The (strain) energy per unit volume stored in the material
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
What is shown by the area between the loading and unloading lines on a force-extension graph of a material undergoing plastic deformation?
The area between the two lines is the work done to permanently deform the material
What is a brittle material?
One which doesn’t deform plastically, but snaps when the stress on it reaches a certain point
Do brittle materials obey Hooke’s law? Explain your answer.
Yes. The stress-strain graph for a brittle material is a straight line, which shows it obeys Hooke’s law
Give an example of a brittle material.
Ceramics
What is a brittle fracture?
When a stress applied to a brittle material causes tiny cracks at the material’s surface to get bigger and bigger until the material completely breaks
Give the formula for calculating stress
Stress=F/A
Stress-Nmˉ² or Pa (pascals)
F-Force in N
A-cross-sectional area in m²
Give the formula for calculating strain
Strain=ΔL/L
Strain-No units as it is a ratio***
ΔL-Extension in m
L-Initial length in m
When are a pair of forces tensile?
If they stretch an object
When are a pair of forces compressive?
If they squash an object
Give the derived formula for work done when an object obeys Hooke’s law
Work done=½FxΔL
E=½FΔL
=½K(ΔL)²
How is the work done energy stored in an elastic deformation?
Elastic strain energy in the material
Give the overall equation for the energy changes in an oscillating spring
Change in kinetic energy= Change in potential energy
