What is the density of a material?
Its mass per unit volume. It is a measure of how compact a substance is.
What does Hooke's law state?
The extension is directly proportional to the force applied, given that the environmental conditions (i.e. temperature) are kept constant.
What is the limit proportionality?
The point after which Hooke's law is no longer obeyed.
What is the elastic limit?
The point (just after the limit of proportionality) where if you increase the force applied, the material will deform plastically (permanently stretched).
What is P and E?
P = limit of proportionality
E = elastic limit
What is the equation for Hooke's law?
F = Kx
F = force
K = spring constant
X = extension
What is tensile stress?
Force applied per unit cross sectional area.
Stress = F / A
What is tensile strain?
The extension caused by the tensile stress over the original length.
Strain = x / L
How can you calculate the elastic strain energy in a force-extension graph?
Calculate the area under the graph:
What is the breaking stress?
The value of stress at which the material will break.
This depends on conditions (temperature, humidity...)
What are the properties of a plastic material?
Large amount of extension as load is increased, especially beyond the elastic limit.
What are the properties of a brittle material?
The material will extend very little, and therefore will break at a low extension.
What would we see in a force-extension graph of a material beyond its elastic limit, being loaded and unloaded?
The unloading line will not return to the origin.
However, the lines would be parallel since K, the material stiffness is constant.
The area between the two lines is the work done to permanently deform the material.
How is energy stored in elastic and plastic stretches?
Elastic: all the work is stored as elastic strain energy,
Plastic: work done is used to move atoms, so energy is dissipated as heat.
How do the energy stores vary when a load is hung and removed from an elastic spring?
Kinetic (stretching) - elastic strain (stretched) - kinetic (unstretching) - gpe
What is the difference between stress-strain graphs and force-extension graphs?
Stress-strain graphs describe the behaviour of a material rather than a specific object.
What is UTS?
Ultimate tensile stress - highest point in a stress-strain graph as it shows the maximum stress the material can withstand.
What type of material is A?
What type of material is B?
What type of material is C?
What is Young modulus?
A value which describes the stiffness of a material.
Young modulus (E) = tensile stress / tensile strain
= F/A / X/L = F.L / X.A