4. Materials Flashcards
Define density.
The mass per unit volume of an object
What are the two possible units for density?
g/cm^3 OR kg/cm^3
State Archimedes’ Principle.
An object submerged in a fluid at rest has an upward buoynacy force (upthrust) equal to the weight of the fluid displaced by the object
What conditions are required for an object to float?
The weight of the object has to be equal to or less than the upthrust acting on it. Therefore the weight of the fluid displaced has to be greater than or equal to the objects weight.
What is the equation for viscous drag?
F = 6πηrv where F is viscous drag, η is the coefficient of viscosity of the fluid, r is the radius of the object and v is the velocity of the object.
Define viscous drag?
The frictional force between an object and a fluid which opposes the motion between the object and the fluid.
What are the conditions for Stoke’s Law?
Flow is laminar, object is small, object is spherical, motion between the sphere and the fluid is at a slow speed.
Define laminar flow.
Type of fluid in which the particles follow smooth paths in layers with each layer moving smoothly past the adjacent layers.
How can you change the viscosity of a fluid?
Viscosity is temperature-dependent and if you heat a fluid, its viscosity will decrease and vice versa.
State Hooke’s Law.
The extension of the material is directly proportional to the applied force (load) up to the limit of proportionality.
What is the equation used to represent Hooke’s law?
ΔF = kΔx where F is the applied force, k is the spring constant and Δx is the extension
How can you tell if a material obeys Hooke’s law by using a force extension graph?
There should be a straight line through the origin
Define the limit of proportionality.
The point beyond which Hooke’s law is no longer true when stretching a material (i.e. extension is no longer proportional to the applied force)
How can you identify the limit of proportionality on a force extension graph?
The point where the line on the graph beings to curve/no longer straight line through origin/begins to flatten out.
Define the elastic limit.
The maximum amount a material can be stretched and still return to its original length.
Define stress
The force applied per unit of cross sectional area. This includes tensile and compressive
What is the equation to find stress and what is its unit.
Stress = force / cross-sectional area, units: Pascals or Nm^-2
Define ultimate tensile stress
The maximum force per original cross-sectional area the wire can support unit it breaks
Define strain.
The extension per unit length
Define Young Modulus
The measure of the ability of a material to withstand changes in length with an added load. It is also defined as the ratio of stress and strain
What is the equation to find the Young Modulus of a material?
E = FL / Ax or stress/strain
How can you find the Young Modulus of a material from its stress strain graph?
The gradient of a stress-strain graph when it is linear is the Young Modulus
Describe the yield point.
The point where the material continues to stretch even though no extra force is being applied to it
What is elastic deformation?
The change of shape where the material will return to its original shape when the load is removed
What is plastic deformation?
The change of shape where the material will not return to its original shape when the load is removed
What is the equation to find elastic strain energy?
E = 1/2 F Δx where E is the elastic strain energy, F is the average force and Δx is the extension
What is meant when a material is described as brittle?
A material that fractures after its elastic limit has been exceeded
What is meant when a material is described as ductile?
A material that has the capacity to deform plastically, such as being drawn into a wire. Eg. steel, copper
What is breaking stress?
