Chapter 6 - materials

Question 1

what do tensile forces produce and what do compressive forces produce

Answer 1

tensile forces produce extension (tensile deformation) and compressive forces produce compression (compressive deformation)

Question 2

at what point on a force-extension graph does elastic deformation occur

Answer 2

the linear section

Question 3

outline a practical used to investigate Hooke’s law

Answer 3

hang a spring from a fixed point and measure its length
hang known masses on the spring and calculate their weight, measure the extension of the spring when acted upon by these weights
plot a force-extension graph

be more specific than this in an exam in terms of wording e.g. using two set markers and measuring the distance between them using vernier callipers

Question 4

what can be done following loading to investigate plastic deformation

Answer 4

the weights added can then be removed one at a time and another line is plotted on the graph, the difference in the x intercepts of the loading line and the unloading line is the plastic deformation

Question 5

state Hooke’s law

Answer 5

“for forces less than the elastic limit of the spring, the extension of the spring is directly proportional to the force applied”

Question 6

equation for Hooke’s law

Answer 6

F = kx

Question 7

what represents the work done on a force-extension graph

Answer 7

the area under the graph

Question 8

equations for work done/elastic potential energy

Answer 8

E = 1/2 x force x extension
E = 1/2 x kx^2

Question 9

how can knowing the work done help calculate the possible kinetic energy if a spring is released

Answer 9

set them equal

from this velocity can be calculated

Question 10

why might a loading and unloading curve of a force-extension graph be different

Answer 10

the material has undergone plastic deformation

Question 11

how does a metal wire react to having forces applied/removed to/from it

Answer 11

it follows Hooke’s law up until its elastic limit when it undergoes plastic deformation
the unloading line will be parallel but different

Question 12

how do rubber bands react to loading/unloading

Answer 12

rubber bands do not follow Hooke’s law, they are elastic (to a point) so will return to their original shape but their loading and unloading curves are different.
they follow a hysteresis loop, more work is done loading than unloading

Question 13

what is the name of the shape of the force-extension graph for rubber and how is most of the energy lost

Answer 13

a hysteresis loop and through heat

Question 14

how does polythene react to loading and unloading

Answer 14

polythene does not obey Hooke’s law and easily undergoes plastic deformation

Question 15

two things occur under tensile forces, what are they

Answer 15

tensile stress and tensile strain

Question 16

calculating tensile stress (lowercase sigma)

Answer 16

tensile stress (lowercase sigma) = force/cross-sectional area

Question 17

calculating tensile strain

Answer 17

strain (epsilon) = extension (x) / original length

Question 18

what does a stress-strain graph look like for a ductile material

Answer 18

increases linearly until limit of proportionality, then hits elastic limit then curves round forming an n shape where you have yield limits, then increases under necking to hit UTS then breaks

Question 19

what does a stress-strain graph look like for a brittle material

Answer 19

a straight line then nothing, the steeper the line the more brittle the material

Question 20

what is the young modulus and how is it calculated

Answer 20

it can be thought of as a measure of ‘brittleness’

young modulus = stress/strain

Question 21

how can you calculate the young modulus from a stress-strain graph

Answer 21

the gradient of the linear part

Question 22

what is a strong material

Answer 22

one with a high ultimate tensile strength UTS

Question 23

what is the yield point of a material

Answer 23

the point at which the material starts to extend rapidly as stress increases

Question 24

what is plastic deformation

Answer 24

a permanent change in shape which remains after the load is removed

Question 25

what is the ultimate tensile stress of a material

Answer 25

the maximum force (stress) a material can withstand before it breaks

Question 26

what is a typical magnitude for young modulus

Answer 26

10^9 —> 10^12

Question 27

what are some things which you should always mention when describing the properties of materials

Answer 27

• brittle or polymeric or rubber
• undergoes plastic/elastic deformation
• obeys or doesn’t obey Hooke’s law

Question 28

what can we say about the breaking points of two objects of the same material

Answer 28

they are likely to both break at the same stress

this does not mean the same force

Question 29

outline a practical which can be done to find the young modulus of a material

Answer 29

• fix a wire at one end and where it runs over a pulley at the other end
• measure diameter (at different points) and original length of the wire from the clamp to the marked point
• add masses (of a known mass) and measure extension (difference in distance from clamp to marked point) until the wire snaps
• plot a force- extension graph or stress-strain graph
• the young modulus = gradient / (a/l) or just gradient if stress-strain

Question 30

what is something to remember when calculating forces for F = kx and then F = ma to calculate acceleration when something is hanging vertically

Answer 30

• if a mass is causing the extension you will have to consider NET force so
  F(net) = kx - mg or something similar

Question 31

what is the best way to lay out questions involving the young modulus and f = kx questions

Answer 31

write at the top all the quantities you know with their symbols so you don’t get confused

Question 32

what two things to always mention if talking about how graphs show Hooke’s law

Answer 32

• straight line/linear

- passes through origin (if force-extension graph)

Question 33

why might a rubber material be good when you don’t want elastic collisions e.g. tyres

Answer 33

• Rubber has a hysteresis loop shape for its force-extension graph
• hence energy is lost through heat when it is repeatedly loaded and unloaded
• hence collisions won’t be elastic

Question 34

what is an important thing to watch for in strain measurements

Answer 34

• strain should be as a fraction/decimal not percentage but it is sometimes given as a percentage

Question 35

what are some safety precautions and reliability points to make when doing the young modulus practical

Answer 35

Safety:
- wear goggles/ stand behind a screen when doing it in case the wire snaps

reliability:

• take 3 measurements of diameter and find an average
• put a small initial load on the wire to ‘take up slack’
• use long wire (to give larger extension values)