Topic 4 - Materials Flashcards
density =
mass / volume
archimedes pricipal is…
upthrust = weight of fluid displaced
fluid
a substance that can flow
density
is a measure of the mass per unit volume of a substance.
upthrust
is an upwards force on an object caused by the object displacing fluid
hydrometer
is an instrument used to determine the density of a fluid
when is viscous drag exerted?
when an object is in laminar flow
what does viscous drag depend on?
vicousity of fluid
speed of body
surface area
mass
the amount of matter in an object
Viscous drag
a resistive force acting on an object travelling through a fluid
stokes law
F = 6πrηv
units of coefficiant of viscosity, η
Nm^-2s or Pas
What does stokes law apply to
small spherical objects travellling at slow speeds with laminar flow
viscosity equation proof proof steps.
- sum of forces W = U + F
- W = m(s)g, U = m(f)g, F = 6πrηv
- m=ρV
- V = 4/3πr^3
- cancel like terms in each expression
- rearrange for η
uses for stokes law
alcohol testing, food, sports, car design, medicine, comestics
How to measure the volume of irregular objects
measure how much water it displaces
upthrust is…
weight of fluid displaced
Laminar flow properties
- streamlines
- no mixing of layers
Turbulent flow properties
- dissapation of energy
- eddy currents
- mixing of layers
What forces act when a material is stretched
- the stretching force
- interatomic forces
Hookes law
the force is directly proportional to the extension (and compression)
F = kx
Gradient of a force extension graph
the stiffness
limit of proportionality on a force extension graph
where the gradient stops being constant
yield point on a force extension graph
where the gradient levels out
elastic limit on a force extension graph
between the limit of proportionality and yield point
breaking point on a force extension graph
where the gradient drops off at the end
elastic deformation
material returns to its original shape once the deforming force has been removed
plastic deformation
material does not return to its original shape once the deforming force ha been removed
yield point
Where there is a large increase in extension for a small increase in force
The steeper the gradient of a force extension graph…
the stiffer the object
elastic potential energy
the work done to stretch a spring
calculate elastic potential energy from a force extension graph
area underneath the graph (up to the limit of proportionality)
elastic potential energy =
= 1/2Fx
properties of materials
- stiff/flexible
- elastic/plastic
- ductile/malleable
- tough/brittle
felxible
small force for large extension
ductile
can be drawn into wires, Produces a large plastic deformation under tension.
malleable
can be hammered into shape
tough
absorbs a lot of energy before they snap
brittle
can’t absorb energy, the crack/shatter immediately
How does area effect how much a wire stretches
- thin wires stretch more
- thick wires stretch less
tensile stress =
= force/area
stress unit
Pascal
difference between stress and pressure
- stress only happens to solids
- pressure applies to a surface stress occur throughout the solid
Ultimate tensile stress
the measure of strength. Its the stress when a material breaks or yields.
Tensile strain =
extension / original length
unit of strain
no unit (its a ratio/ fraction)
young modulus =
stress/strain
young modulus
a property of a material that measures how difficult it is to change the shape of a material
young modulus unit
Pascals
find the young modulus experiment
- measure the diameter of the wire. Measure the original length of the wire.
- attach the wire to the desk and thread it over a pulley with hanging masses on the end. Put a sticky label on the wire at the end of the metre ruler.
- Add the masses to the hanger and record the distance movement of the sticky marker.
- plot extension against the weight (mg). The gradient is equal to length/area*young modulus.
area under stress strain graph
the energy stored per unit volume
hard
resistant to indentation/scratching
Or
surface is resistant to plastic deformation
stiff
large force for small extension
high tensile strength
undergo a large stress/force before breaking.
linear relationship
increase in x is constant for fixed increases in y
what happens wire a wire passes its limit of proportionality
there is a large increase in extension for a small increase in mass. It will no longer return to its original shape as it has been plastically deformed.
how to make extension more accurate (5)
- use a pointer on the wire/ masses to make it easier to read
- read at eye level to avoid parallax
- use a set square to ensure the rulers vertical
- wait for extension to finish
- add masses gently
laminar flow
where the layers of flow are parallel and do not mix/cross, no abrupt change in speed or direction of flow.
turbulent flow
where the layers of flow mix forming eddy currents and causing energy to be dissipated. Random changes in speed or direction.
F in stokes law
viscous drag
spring constants in parallel
Kt = K1 + K2
spring constants in series
1/Kt = 1/K1 + 1/K2
viscosity equation
(2gr^2 (ρs - ρf))/9v
finding drag on an object
drag = +- upthrust +- weight
measure radius + mass of object & use known densities
area under stress strain graph
toughness (energy it can absorb before it snaps)
compressive strain vs tensile strain
compression decreases in length tensile increases in length
