Materials Flashcards
Define fluid
Any substance that can flow. Can be a liquid or gas, solids can sometimes behave as fluids - sand.
Define density
mass per unit volume.
Define upthrust
An upwards force caused by an object displacing fluid.
If the object is completely submerged, the mass of the fluid displaced = volume of object + density of fluid. m=Vp.
State Archimedes Principle
The upthrust on an object is equal to the weight of fluid displaced.
What is a Hydrometer?
An instrument used to determine density of a fluid.
Define Laminar Flow
Fluid moves in parallel layers. The velocity is constant over time.
What is the effect of fluid flowing through a pipe?
Friction between the outermost layer and the pipe means this layer moves slower.
Define Turbulent Flow
Layers mix. The fluid becomes chaotic and swirling eddies form. Velocity in a particular place changes over time.
Define Viscosity.
How resistant a fluid is to flowing.
How is flow of fluid and viscosity related?
The rate of flow of a fluid is inversely proportional to the viscosity of the fluid.
Stokes Law
F = 6πηrv
Only applies to small spheres at low speeds. Drag force is directly proportional to radius and velocity.
pressure=
=Force/Area
=density x g x height
Terminal Velocity
the velocity of a falling object when its weight is balanced by the sum of drag and upthrust acting on it.
Define Hookes Law
Force needed to extend a spring is proportional to the extension of the spring.
A material obeys Hooke’s Law
if it has not passed the limit of proportionality
State Hooke’s Law
Force Applied = spring constant x extension.
ΔF = kΔ x
Define elastic strain energy.
Work done in deforming a material sample before it reaches elastic limit.
ΔEel = 1/2 FΔ x
Compressive force
Squashes the material causing a negative extension.
Tension
A force acting within a material in a direction that would extend.
Define Stress
Stress σ = F/A
Define Strain
Strain ε = Δ x/x
=extension / original length
Youngs Modulus, E
E = σ/ε =stress/strain =Fx/AΔ x
E is the stiffness constant of a material,
Limit of proportionality
Beyond this point stress is not proportional to strain.
Elastic Limit
The material is permanently deformed beyond this point. Sample will not return to original size when stress is released.
Yield Point
Sudden increase in extension as atomic structure reorganised
Ultimate Tensile Stress
The highest stress value
Fracture stress
The value stress will be when the sample breaks.
Tough
can withstand dynamic loads / shocks / impacts / absorbs
a lot of energy before breaking
Ductile
can be pulled into a long thin shape
Brittle
breaks with plastic deformation/ shatters
What is meant by a strong
material?
Large force / stress required to break it
Upthrust =
pvg = density x volume x gravity
Explain the difference between elastic and plastic behaviour in a metal.
Plastic = permanent AND elastic = reversible
Elastic: bonds stretch but not broken / atoms move apart but then return
Plastic: bonds broken (when stressed) / atoms do not return to original position (after stress)
Explain the effect turbulence would have on the rate of flow
Reduces the speed / rate of flow
Because turbulence would disperse energy through the mixing/components of fluid follow longer path
Suggest how the elastic properties of the waistband help in keeping the trousers in
place.
Elastic tries to return to a smaller/original length. So will be in tension OR applies force /pull
