0:06 so far looking at Springs what we can
0:08 see is that you apply a force and the
0:10 spring gets longer but this is very much
0:12 just specific to this particular spring
0:14 I could make a different spring maybe
0:17 wind it tight to make the metal thicker
0:19 and we'd have very different properties
0:20 what it's sometimes more useful to look
0:21 at is rather than the actual physical
0:23 thing that we've made is the actual
0:25 material that it's made from and this is
0:26 where we can extend the ideas of force
0:30 and extension to stress and strain as
0:31 you prepare for exams you might feel at
0:32 times that you're under a lot of stress
0:34 there's a lot of hard work and then you
0:36 feel the strain but in engineering and
0:38 physics terms these two words mean very
0:40 different things so first of all stress
0:43 what's that well basically if you apply
0:46 a force to an object then it will deform
0:48 so we saw the Springs how you apply a
0:50 force the whole thing will move and
0:51 basically if you have a force that
0:54 causes this causes some kind of
0:56 deformation then we said there's a
0:58 stress applied and we can talk about the
1:01 force per unit
1:04 area and the symbol for stress is a bit
1:08 of a weird one it's a sigma so stress is
1:12 equal to the force per unit area you
1:14 have something which is being stressed
1:16 then it will change shape and strain
1:18 tells us how much something changes
1:20 shape compared to its original length
1:23 and if we look at strain uh we can basic
1:25 uh give the symbol for strain this kind
1:28 of Epsilon and that's equal to the
1:32 extension over the original
1:35 length just be aware that sometimes this
1:38 extension is also given symbol Delta L
1:40 uh and sometimes the length might be l n
1:42 to it might be a capital l it doesn't
1:43 really matter about the symbols but this
1:45 is effectively how much something got
1:47 bigger compared to its original length
1:50 now in terms of units well if something
1:53 uh is a force divided by area then the
1:56 units of stress are going to be given in
1:58 Newtons per square
2:01 meter and if we look at strain well
2:04 strain because it's a meter divided by a
2:06 meter it has no units it's a
2:09 dimensionless quantity so is stress
2:11 related to strain good question it is
2:13 the more Force you apply to an object
2:15 the longer it's going to get so there is
2:17 a relationship between the stress that
2:20 you apply to an object and also the
2:22 strain that it experiences now this
2:23 really does depend on the metal if you
2:25 apply the same Force to a piece of
2:27 copper as to a piece of aluminium or
2:29 steel then they will all change by a
2:31 different amount and we can look at this
2:34 ratio of stress to strain and this gives
2:36 rise to something that we call Young's
2:39 modulus now Young's modulus has a symbol
2:42 e uh and it's equal to the ratio of the
2:44 stress applied to the
2:46 strain if we look at mild steel for
2:50 example the value of e is about
2:52 210 gigap
2:55 pascals the reason it's pascals is
2:56 because effectively One Pascal which is
2:58 the unit of pressure is the same as 1
3:01 Newton per square meter which we can
3:02 also measure stressing so we can measure
3:05 in Newtons per square meter or pascals
3:06 and because this is a unitless quantity
3:08 for strain that means the units for
3:09 stress are also the units for Young's
3:13 modulus and it takes a lot of force to
3:16 get metal to the to the form so that's
3:18 why we have a giga here so about 210
3:21 gigap pascals is a kind of uh kind of
3:22 value that we all typically get for
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