materials

Question 1

what is density?

Answer 1

a measure of the compactness of a substance. it relates to the mass of the substance to how much space it takes up. its mass per unit volume

Question 2

for a solid object to float on a liquid, what does its density need to be?

Answer 2

lower density than the fluid

Question 3

whats the density of water?

Answer 3

1g cm^-3
or
1000kg m^-3

Question 4

what is Hooke’s law?

Answer 4

the extension of a stretched wire is proportional to the load of force F

Question 5

what is the elastic limit?

Answer 5

when the force becomes great enough, this is reached which causes a force-extension graph to plateau, increasing the force past this will lead to a permanent stretch of the material

Question 6

how long do metals obey Hooke’s law for?

Answer 6

up till the limit of proportionality which is just before the elastic limit

Question 7

what is the limit of proportionality?

Answer 7

the point beyond which the force is no longer proportional to the extension.

Question 8

how could you investigate how the extension of an object varies with the force used to extend it?

Answer 8

use a clamp stand and a clamp the object at a measured height
then add weights one at a time to the object
calculate the extension by new length - original length
plot a graph to see result

Question 9

how can you tell if a deformation is elastic?

Answer 9

the material will return to its original shape once forces are removed, for a metal this occurs as long as Hooke’s law is obeyed

Question 10

how can you tell if a deformation is plastic?

Answer 10

the material is permanently stretched, occurs for metals stretched past their elastic limit

Question 11

what is tensile stress?

Answer 11

the force applied divided by the cross sectional area. this causes strain

Question 12

what are tensile forces?

Answer 12

if the forces stretch a material

Question 13

what are compressive forces?

Answer 13

if the forces squash the material

Question 14

what is tensile strain?

Answer 14

the change in length

Question 15

what is the breaking stress?

Answer 15

when stress becomes so large that atoms separate completely and the material breaks

Question 16

what is ultimate tensile stress?

Answer 16

the maximum stress that a material can withstand

Question 17

what is elastic strain energy?

Answer 17

before the elastic limit, all the work done in stretching is stored as potential energy in the material. this stored energy is called elastic strain energy

Question 18

what does the area under a force-extension graph give you?

Answer 18

elastic strain energy

Question 19

how is energy conserved in elastic deformation?

Answer 19

work is done while stretching which is stored as elastic strain energy this then gets transformed into other forms eg. kinetic in an elastic band

Question 20

how is energy conserved in plastic deformations?

Answer 20

work is done to separate atoms
mostly dissipated as heat

Question 21

how is energy conserved in a vertical spring?

Answer 21

elastic strain energy is stored, when mass is released this transfers to kinetic and gpe. spring then starts to compress and KE is transferred back to elastic strain energy
change in KE= change in potential energy (elastic+gpe)

Question 22

what is the Youngs modulus?

Answer 22

a measure of how stiff a material is

Question 23

the Youngs modulus experiment RP4

Answer 23

-set up a bench and clamp a wire fixed at one end with a pulley at the other which you will hang weights off
-test wire should be thin and as long as possible
-first find cross sectional area of the wire using a micrometer for the diameter in several places and take an average x pi r^2
-start with the smallest weight necessary to straighten the wire
-measure the distance between the marker and fixed end of wire- this is the un stretched distance
-then increase weight in 100g intervals recording the marker each time to get extension
-then use results to calculate stress and strain and plot a stress-strain graph

Question 24

how can you find the Youngs modulus from a stress-strain graph?

Answer 24

its the gradient

Question 25

what does the area under a stress-strain graph give you?

Answer 25

the strain energy (energy stored) per unit volume

Question 26

if hooke’s law is obeyed, what should a stress-strain graph look like?

Answer 26

straight line

Question 27

how do you find the limit of proportionality on a stress-strain graph?

Answer 27

the point where the graph starts to bend

Question 28

how would you find the elastic point on a stress-strain graph?

Answer 28

after the limit of proportionality, past this the material will no longer be able to go back to its original size and shape

Question 29

how do you find the yield point on a stress-strain graph?

Answer 29

point just before a sudden drop in stress- material starts to stretch without any extra load

Question 30

what are force-extension graphs dependant on?

Answer 30

dimensions

Question 31

what does the area between loading and unloading line on force-extension graph give you?

Answer 31

work done to permanently deform the wire

Question 32

how is a stress-strain graph different for brittle materials?

Answer 32

it doesn’t curve, just straight lines
straight line till fractures

Question 33

what is brittle fracture?

Answer 33

when stress is applied to a brittle material any tiny cracks at the materials surface get bigger and bigger until it completely breaks