How does a material obey hooke’s law?
The extension of the material is directly proportional to the applied force up to the limit of proportionality.
What is the equation for hooke’s law and what is each letter?
ΔF = k Δx
F = force (N)
k = spring constant (N m⁻¹)
x = extension (m)
What does spring constant measure?
Stiffness of a material.
The larger the spring constant the more stiff the material.
Define the limit of proportionality.
The point beyond which Hooke’s law is no longer true when stretching a material.
Define elastic limit.
The maximum amount a material can be stretched and still return to it’s original length.
In a force extension graph what does the gradient =
The spring constant.
Define stress.
The applied force per unit cross sectional area of a material.
What is a tensile force
They pull on an object and extend it.
What is a compressive force?
They push onto a object and compress it (squash).
What is the equation for stress and label each one and the units.
σ=F/A
σ = Stress (Pa) or (Nm⁻²)
F = Force (N)
A = Cross-Sectional Area (m²)
Define ultimate tensile strength.
The maximum force per original cross-sectional area a wire is able to support until it breaks.
Define strain.
Strain is the extension per unit length.
Strain is the ratio of the extension and the original length
What is the equation for strain and label each one with the units?
ε = ΔX/X
ε = Strain
X= Extension (m)
Bottom X = length (m)
Define young modulus
Is the measure of the ability of a material to withstand changes in length with an added load ie. how stiff a material is.
The ratio of stress and strain.
What is the equation for young modulus and give the units.
Draw a stress and strain graph and describe how to calculate young modulus from it and the link.
Stress and strain are directly proportional.
The gradient when linear is equal to Young Modulus.
Draw a simple force-extension graph showing the Hooke’s Law region, and the calculation to find k, the spring constant.
Define yield point.
The yield point beyond which the material continues to stretch (more strain is seen) even though no extra force is being applied to it (without additional stress)
Define Elastic deformation.
Elastic deformation where the material will return to its original shape when the load is removed
Name plastic deformation
Plastic deformation where the material will not return to its original shape when the load is removed.
What do stress strain curves show?
Stress-strain curves give an indication of the properties of materials such as
Up to what stress and strain they obey Hooke’s Law
Whether they exhibit elastic and/or plastic behaviour
The value of their Young Modulus
The value of their breaking stress
How to calculate elastic strain energy.
