meterials (everything) (flash cards made while going through book learning topic)
what is density
mass per unit volume
give density equaiton and units for each
p(kgm^-3)=m(kg)/v(m^-3)
state hooks law
and give the equation
extension (ΔL) of a stretched object is proportionmal to the force (F) applied
F∝ΔL
therefore F=KΔL
dont forget about the reactionary force when drawing spring force diagra
what is K in Hooks law when deeling with springs
the spring constent (the stiffness of an spring)
- what happens to a spring when a tensile
or - compressive force acts on it.
- the spring stretches as a result of a tensile force.
- the spring squashes as a result of a comppressive force
- for a spring does K have the same value for compressive and tensile forces
- does this apply to all materials
- yes
- no
- draw a force over-extension diagram and describe it
- when force is released before the elastic limit is reached does the spring move back to its original position?
- when force is released after the elastic limit is reached does the spring move back to its original position?
- what is the area under the graph
- look up graph
- below the elastic limit the spring will contract to its original form.
- after the elastic limit, the spring will no longer contract to its original shape after the load is released.
- work done or if the elastic limit is not reached then the value of elastic strain energy as well
when does hooks law stop working
when foce becomes too great and the limmit of proportionality is reached
explain elastic deformation relative to the atoms within the material and why the original shape is kept after the load is released
a) when the meterial is put under tension, the atoms of the meterial are pulled apart from one another.
b) atoms can move small distences relative to there equalibrium positions, whithout actually changing position within the metarial
c) once load is removed, tha atoms return to there equalibrium distence apart.
explain plastic deformation relative to the atoms within the meterial and why the original shape is not kept after tension is released
a)when the meterial is put under tension, the atoms of the meterial are pulled apart from one another.
b) some atoms in the material move position relative to one another
c) when the load is removed the atoms don’t return to their original positions
d) plastic deformation occourse - the meterial does not return to its original positions
what happens when deformation is plastic
the meterial will not return back to its original shape (object is stratched past the elastic limit)
what happens when deformation is elastic
the meterial will return back to its original shape (object is not stratched past the elastic limit)
when a meterial is stretched, _______ has to be done in stretching the meterial
work
- what is enegy stored as during elastic deformation
- when this force is removed stored enegy is…
- elastic strain enegy within the meterial
- transfered to other forms
e.g an elastic namd is streted then fired across the room
- during plastic deformation work is done to _________ ______ and enegy is …
separate atoms
…not stored as strain enegy (its mostly disipated as thermal energy - think of blue stack heating up as it deforms)
stress equation and units
stress = F/A Nm^-2 or Pa
strain equation and units
strain = ΔL/L (no units, its just a ratio and is written as a number or percentage)
does it matter for the stress and strain equations if the forces producing the stress or strain are tensile or compressive
no - the stress and strain equations work with both tensile and compressive forces
the only diffrence is you tend to think of tensile forcese as positive (+) and compressive forces as negetive(-)
what is tensile stress on a meterial
force aplied on the meterial over and area
what is tensile strain
the change in lenth of the meterial
- draw the stress over strain graph labelling importend points on the graph
- explain the process of a material breaking as stress increases. go through all of the stages of the graph
- what is the gradient of the graph (before the limit of proportionality)
- what is the area under the stress over strain graph (before the limit of proportionality)
- as stress increses the force pulling atoms apart form each other increses. the stress increses to the ultimate tensile stress point (UTS). eventually the stress becomes so great that the atoms seperate completly, and the meterial breaks. this is the braking stress point (B).
- youngs modules
- enegy stored in the meterial per unuit volume
enegy per unit volume = 1/2 * stress * strain
what is UTS
ulitimate tensile stress. it is the maximan stress a meterial can withstand.
providong a meterial obeys hooks law what are the 2 equations for protential elastic strain enegy stored in the meterial
work done/E = 1/2 F * ΔL
subing in F=kΔL
E = 1/2 k * (ΔL)^2
when is stress and strain proportional
up to the elastic limit
(seems like it should be up to the limit of proportionality but if you think about plastic deformation only happens after the elastic limit)
(don’t need to think about it too hard just remember enegy is stored a elastic strain enegy up until the elastic limit and therefore before the elastic limit the material bounces back to the original position)
(you are not really going to be asked specifics about the paired between P and E)
young modulus equation and units
tensile stress / tensile strain
(F/A)/(ΔL/L)
=
FL/ΔLA
units = Nm^-2 ot Pa (same as stress since strain has no limits)
- what happens past the yeild point
- draw a stress over strain graph including the yirld point
- the meterial starts to stretch withought any load (large amount of plastic deformation takes place with a constent or reduced load)
2.look up or in revision book pg76
stress over strain graph for a brittle meterial and lable
stress over strain graph for a ductile meterial and lable
stress over strain graph for a plastic meterial and lable
- draw a force over extension graph with a unloading line where the meterial passes the elastic limit
- why is the unloading line parallel to the loading line
- what is the area between the 2 lines
- the stiffness K is the same (the forces between the atoms are the same as they were during the loading)
- the work done to permently deform the wire
strain equation
ΔL/L
what is the pricible of moments
when a body is balenced:
total clocwise moment = total anti clockwise moment about a point
