materials

Question 1

what are tensile forces

Answer 1

forces that produce extension
tensile deformation will occur

Question 2

what are compressive forces

Answer 2

forces that compress/shorten object
compressive deformation will occur

Question 3

what is the force extension graph

Answer 3

a straight line from the origin up to the elastic limit

Question 4

what is elastic deformation

Answer 4

the spring will return to its original length when the force is removed

Question 5

what is plastic deformation

Answer 5

permanent structural changes to the the spring occur and it does not return to its original length when the fore is removed

Question 6

what is hookes law

Answer 6

the extension of the spring is directly proportional to the force applied. this is true as long as the elastic limit of the spring is not exceeded

Question 7

what is the force constant equation

Answer 7

a spring obeying hookes law the applied force is directly proportional to the extension
f=kx
k is the force constant

Question 8

what is the unit of force constant

Answer 8

Nm^-1

Question 9

what does the force constant of a spring measure

Answer 9

measure of the stiffness of a spring
spring with large force constant is difficult to extend - stiff spring

Question 10

how do you find the spring constant from a force extension graph

Answer 10

the gradient of the linear region

Question 11

what is the hookes law proactical

Answer 11

attach spring at one end using a clamp, boss and clamp stand secured to the bench using a g clamp or a large mass
set up a meter ruler with a resolution of 1mm close to the spring
suspend slotted masses from the spring and as you add each one record the total mass added and the new length of the spring
improve accuracy of length measurements - use a set square and by taking reading at eye level to reduce parallax errors, can measure the mass of each added mass using a digital balance
obtain reliable results - take at lease six different reading and to repeat each on at least once

Question 12

what is the point called where a string stops obeying hookes aw

Answer 12

limit of proportionality

Question 13

what happens to the work done on a material when it hasn’t gone beyond its elastic limit

Answer 13

work done can be fully recovered

Question 14

what happens to the work done on a material when a material has gone through plastic deformation

Answer 14

some work done on the material has gone into moving its atoms to new permanent postions
energy is not recoverable

Question 15

how do you determine the energy stored (work done) in an elastic material using a force extension graph

Answer 15

work done by a force in extending the spring by a length
W=Fx
area under the force extension raph = work done

Question 16

what is the work done on the spring transferred to

Answer 16

elastic potential energy within the spring
energy is fully recoverable because of the elastic behaviour of the spring

Question 17

how can you derive the equation for elastic potential energy from the area under the force extension graph

Answer 17

E = area under graph = area of shaded triangle
E = 1/2 Fx where F is the force producing the extension x
spring obeys hookes law f=kx
E=1/2kx^2

Question 18

what does the leastic potential energy equation tell us

Answer 18

for a given spring E is directly proportional to the extension squared
doubling the extension would quadrupled the energy stored

Question 19

what is hysteresis loop

Answer 19

a loop shaped plot obtained when for example loading and unloading a material produce different deformations

Question 20

what does a force extension graph for metal wire look like

Answer 20

loading graph follows hookes law until the elastic limit of the wire
the unloading graph will be identical for forces less than the elastic limit
beyond the elastic limit unloading graph is parallel to the lading graph but not identical - the wire is permanently extended after the force is removed, it is longer than it was at the start as the wire has suffered plastic deformation

Question 21

what does a force extension graph look like for rubber

Answer 21

do not obey hookes law - rubber ban will return to its original length after the force is removed - elastic deformation - but the loading and unloading graphs are both curved and are different
forms a hysteresis loop - area under the graph is equal to the work done, more work is done when stretching a rubber band than is come when the extension decreases again
thermal energy is relased when the material is loaded and unloaded represented by the area inside the loop

Question 22

what does a force extension graph look like for polyetheme

Answer 22

does not obey hookes law
thin strips of polyethene are very easy to stretch and they suffer plastic deformation under relatively little force

Question 23

what is tensile stress

Answer 23

the force applied per unit cross sectional area of the wire

Question 24

what is the equation of tensile stress

Answer 24

tensile stress = force/cross sectional area
sigma = F/A

Question 25

what is the unit of tensile stress

Answer 25

pascals

Question 26

what is tensile strain

Answer 26

the fractional change in the original length of the wire

Question 27

what is the equation for tensile strain

Answer 27

tensile strain = extension/original length epsilon=x/L

Question 28

what is the unit for tensile strain

Answer 28

no units as is the ratio of two lengths can be written as a percentage

Question 29

what are the parts of a stress-strain graph

Answer 29

stress is directly proportional to strain from the origin to a point p - material obeys hookes law p - the limit of proportionality E - the elastic limit elastic deformation will occur up to the elastic limit. plastic deformation will occur beyond this limit Y1 - upper yeild point Y2 - lower yeild point between the yeild points is where the material extends rapidly UTS - materials ultimate tensile strength is the stress at this point, the maximum stress that a material can withstand when being stretched before it breaks, beyond this point the material may become longer and thinner at its weakest point - necking B - breaking point, where the material snaps the stress value at the point of fracture is known as the breaking strength of the material

Question 30

what properties does a strong material have

Answer 30

high ultimate tensile strength

Question 31

what is youngs modulus

Answer 31

the ratio of stress to strain for a particular material

Question 32

what is the equation for youngs modulus

Answer 32

youngs modulus = tensile stress/tensile strain E = sigma/epsilon E = (F/A)/(x/L) E=(F/A)x(L/x)

Question 33

what is the unit of youngs modulus

Answer 33

Nm^-2 pascals Pa

Question 34

how can you work out the youngs modulus from a stress strain graph

Answer 34

the gradient of the linear region

Question 35

what is the experiment to determine the youngs modulus of a wire

Answer 35

measure the diameter of the wire and apply various loads to it and measuring its length each time wire clamped securely at one end passed over a pullet and loaded with slotted masses at the other end wear eye protection in case the wire breaks diameter of wire can be measure using micrometer the cross sectional area of the wire can be calculated from A=pier^2 obtain a more accurate diameter by averaging measurements from several places along the wire the tensile forces acting on the wire calculated from the hanging mass using F=mg after applying each additional mass the extension is calculated x=extended length - original length improve accuracy by taking readings for at least six different masses and repeating them the stress and strain values for each load are calculated and can be used to plat a stress-strain graph youngs modulus determined from the gradient of the graph

Question 36

what would a stress strain graph look like for a brittle material

Answer 36

elastic behaviour up to its breaking point, without plastic deformation