materials Flashcards
what is density
measure of compactness of a substance
mass per unit volume
state what is meant by centre of mass [1]
the point through which (the line of action of) a force has no turning effect
State two necessary conditions for the beam to be in equilibrium [2]
resultant force = 0
sum of anticlockwise moments = sum of clockwise moments
State the law of conservation of energy [2]
energy cannot be created or destroyed
it can only be transferred from one form to another
density {}
ρ = m/v
Hooke’s law {}
F = K * change in L
what is hooke’s law
extension is directly proportional to force applied given that environmental conditions are kept constant
what is k
spring constant (Nm-1)
the stiffness of an object being stretched
tensile force
stretching
compressive force
squashing
what does a typical force-extension graph look like for a typical metal wire
straight line through origin
limit of proportionality
curves off
limit of proportionality
the point beyond which hooke’s law is no longer obeyed
the force is no longer proportional to the extension
elastic limit
the point beyond which the material suffers plastic deformation
In a force - extension graph what is the gradient
what is the area under the graph
k
since F = ke
spring constant (Nm-1)
work done to deform the material
how to investigate extension
set up clamp with spring
take original length
place load on spring and take final length
subtract to find extension
take repeats
plot a graph
what does it mean to say a deformation is elastic.
what does the graph look like?
the material returns to its original shape once the forces have been removed
aquafresh looking graph. returns to original length after extension
what does it mean to say a deformation is plastic
the material is permanently stretched after the load has been removed
materials stretched past their elastic limit show plastic deformation
what is meant by tensile stress
force applied per unit cross-sectional area resulting in stretch of the material
what is meant by tensile strain
a measure of how the material stretches
tensile stress {}
what are the units of tensile stress
force / cross-sectional area
Nm^-2 or Pa
tensile strain {}
what are the units of tensile strain
ΔL/L
extension / original length
no units - it’s a ratio
how would use a force extension graph to find the elastic strain energy stored by a stretched material
area under the graph
up to extension required
how is energy conserved as a stretched wire undergoes plastic deformation
some of the kinetic energy transferred to elastic potential energy
some of it is used to separate atoms and is dissipated as heat (thermal energy)
describe the energy changes as a spring is stretched and released
kinetic energy is transferred to elastic potential as the spring is stretched
elastic strain energy is stored in the spring until the spring is released when it its transferred back to kinetic energy
the greater the tensile force applied to a material …
the greater the stress on it
what is the effect of stress
atoms are starting to be pulled apart from one another
what is breaking stress on an atomic level
as stress is applied atoms are pulled apart from one another
eventually the stress becomes so great the atoms separate completely and the material breaks
(stress big enough to break a material)
what is ultimate tensile stress
and what is it also known as
maximum stress that a material can withstand before it fractures
breaking stress
what is elastic strain energy
stored energy in a material after work has been done to stretch it
work done {}
wd = 1/2 F * ΔL
(elastic strain energy - area under straight line graph)
Energy{}
E = 1/2 k * e^2
since F = ke
and E = 1/2 * F * e
why isn’t all work done stored as elastic strain energy when the deformation is plastic
some work is done to change the position of the atoms
not stored as strain energy
change in kinetic energy =
change in potential energy
( elastic + gravitational )
what are the units for young’s modulus
Nm^-2 or Pa
young’s modulus {}
tensile stress / tensile strain
gradient of a stress-strain graph
what are the four things you would have to measure in an experiment to find the young’s modulus of a wire
diameter of the wire
extension of the wire
original length of the wire
weight (load)
what does the area under a stress-strain graph tell you?
strain energy stored per unit volume in the material
how are stress and strain related
they are proportional to one another
( first cousin, twice removed on their mother’s side )
describe the young’s modulus experiment
wire
place varying loads and measure extension
measure diameter of wire to calculate cross-sectional area
plot stress strain graph
gradient = young’s modulus
REQUIRED PRACTICAL
what should the wire be like and why
thin and long
meaning it would extend more for the same force
decreasing uncertainty in measurements
REQUIRED PRACTICAL
what is assumed when calculating the stress
The cross-sectional area is circular
what is the difference between a material at its elastic limit and its limit of proportionality
past limit of proportionality the material no longer obeys hooke’s law but will still behave elastically and return to its original shape
past the elastic limit the material behaves plastically and will not return to its original shape when stress is removed
what is the yield point of a material
the point at which plastic deformation begins to occur
what is shown by the area between the loading and unloading curves on a f-e graph showing plastic deformation
the area is the work done to permanently deform the material
what does the f-e graph look like for a metal wire that has been stretched beyond its limit of proportionality
straight line through origin
curves off after limits (elastic and of proportionality)
unloading line parallel to loading ( since k remains constant )
returns to a longer length than original since permanent deformation has taken place
area between these lines is the work done to permanently deform the wire
what does the f-e graph look like for a brittle material
straight line
and then the material fractures
same for stress-strain
what is brittle fracture
when stress is applied to a material
cracks at the materials surface get bigger and bigger until the material breaks completely
what is a brittle material
one which doesn’t deform plastically but snaps when the stress acting on it reaches a certain amount
this is due to their rigid structure, atoms won’t move to prevent cracks getting bigger
do brittle materials obey hooke’s law, explain why
yes.
stress strain graph shows straight line through origin
give an example of brittle material
ceramics
what is the unit for density
Kgm^-3
using experimental methods to find the density of regular, irregular objects and liquid
regular
measure the mass
take appropriate measurements to calculate volume
density = mass / volume
irregular
measure the mass
immerse object on a thread in liquid in a measuring cylinder ( eureka can )
displacement = volume of object
density = mass / volume
liquid
measure mass
measure volume
how do you calculate the density of an alloy
use the densities of each material and the corresponding volumes of each to find a mass value for each metal
add these mass values up and divide by the total volume to find an overall density
1m^3 in cm^3
10^6 cm^3
if a weight is supported by the means of two springs in parallel to one another how is the extension of the springs affected
and the effective spring constant?
the weight is shared between the two springs so the overall extension is halved
the springs constant doubles as it has become more “stiff”
if a weight is supported by the means of two springs in series to one another how is the force needed affected
and the effective spring constant?
Force needed for the same extension would half
both springs experience the same tension it would singularly
therefore they both extend by the same amount
extension doubles
springs constant halves
what is young’s modulus a measure of
the stiffness of different materials
what is ultimate tensile stress a measure of
the strength of a material
what is a ductile material
a material that can be drawn into a wire
what is the difference between elastic and plastic deformation
elastic - when force is removed the object will return to its original shape
plastic - after load is removed it will not return to its original shape
how is the work done to stretch or compress a material stored
elastic strain energy
how is dissipation of energy in plastic deformation used to design safer vehicles
crumple zones
- kinetic energy transferred to plastic deformation
seat belts
stretch to convert passenger’s kinetic energy to elastic strain energy
