# 2 Quick questions (M1 and exams)

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#1
1. In M1, why do we assume that the strings are light and inextensible? I've been told that assuming that it's inextensible guarantees common acceleration but I can't see why. As for light, I've seen two different explanations that both make sense to me one is that it being light lets you treat the mass term in F=ma as zero, meaning that the string must be in equilibrium, making tension problems far easier. The other explanation I've seen is that if the string is light, we don't have to worry about it's mass causing anything to accelerate.

2. I wanna quickly check the whole thing with modules being swapped between maths and further maths, can modules which are ladled as A2 be swapped with modules that are ladled as AS? I've been told that to get an A* in further, you need to average 90% over 3 A2 modules and get a A overall but what would happen if say for example you say FP2, M2 and M3 but didn't score highly enough for either an A or an A*? Could they then swap something like M1 from normal maths to give you an A in further? (of course, this assumes that the swap doesn't effect your maths grade and that the M1 mark is sufficient, and I'm under the impression that FP2 cannot be swapped).
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5 years ago
#2
(Original post by M1Player)
1. In M1, why do we assume that the strings are light and inextensible? I've been told that assuming that it's inextensible guarantees common acceleration but I can't see why. As for light, I've seen two different explanations that both make sense to me one is that it being light lets you treat the mass term in F=ma as zero, meaning that the string must be in equilibrium, making tension problems far easier. The other explanation I've seen is that if the string is light, we don't have to worry about it's mass causing anything to accelerate.
Light so that we can ignore the effect of its mass

In extensible so that we can assume that length remains constant and that tension is constant through the string
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#3
Why does the a constant length matter? And what's keeping the tension the same?
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5 years ago
#4
(Original post by M1Player)

2. I wanna quickly check the whole thing with modules being swapped between maths and further maths, can modules which are ladled as A2 be swapped with modules that are ladled as AS? I've been told that to get an A* in further, you need to average 90% over 3 A2 modules and get a A overall but what would happen if say for example you say FP2, M2 and M3 but didn't score highly enough for either an A or an A*? Could they then swap something like M1 from normal maths to give you an A in further? (of course, this assumes that the swap doesn't effect your maths grade and that the M1 mark is sufficient, and I'm under the impression that FP2 cannot be swapped).
Cs have to be in maths
FPs have to be in FM

Applications ... A 2 can only be in maths if the related 1 is there as well

After that they are distributed to give the best grade in maths (with the lowest possible score)

Only things with a 2 or higher (not C2) can count as A2 units for the A* in FM
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5 years ago
#5
(Original post by M1Player)
Why does the a constant length matter? And what's keeping the tension the same?
If the length is variable we get a whole new set of rules involved (wait for springs to appear)

When looking at connected bodies you use the fact that tension is the same at both ends
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#6
^Yeah... I thought that it might have an energy based explanation, is there a more-M1 style solution?
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5 years ago
#7
(Original post by M1Player)
^Yeah... I thought that it might have an energy based explanation, is there a more-M1 style solution?
What do you mean by "an M1 style solution"?

The idea behind all the Mechanics modules is that you're simplifying reality so that you can deal with it using the mathematics you know! If a string is extensible then by definition when you apply a force to it, some of the energy is consumed extending the string rather than just accelerating it. If you go on to study module M3 then you will see how to model extensible strings.
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#8
^As far as OCR's concerned (and I assume, most M1 course), energy doesn't appear until M2, so I was hoping for an explanation that does not involve energy. I'd already somewhat familiar with spring stuff from the mechanics I've done in physics but I'm just wary in case it appears on an M1 exam.
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5 years ago
#9
(Original post by M1Player)
1. In M1, why do we assume that the strings are light and inextensible? I've been told that assuming that it's inextensible guarantees common acceleration but I can't see why. As for light, I've seen two different explanations that both make sense to me one is that it being light lets you treat the mass term in F=ma as zero, meaning that the string must be in equilibrium, making tension problems far easier. The other explanation I've seen is that if the string is light, we don't have to worry about it's mass causing anything to accelerate.

2. I wanna quickly check the whole thing with modules being swapped between maths and further maths, can modules which are ladled as A2 be swapped with modules that are ladled as AS? I've been told that to get an A* in further, you need to average 90% over 3 A2 modules and get a A overall but what would happen if say for example you say FP2, M2 and M3 but didn't score highly enough for either an A or an A*? Could they then swap something like M1 from normal maths to give you an A in further? (of course, this assumes that the swap doesn't effect your maths grade and that the M1 mark is sufficient, and I'm under the impression that FP2 cannot be swapped).
If the string is inextensible, the two particles are always the same distance apart (round the pulley) so their speed is always the same so their (magnitude of ) acceleration is always the same, i.e. inextensible = same acceleration for the two particles.

Light string means each little bit of string is taken to have zero mass, so m x a = 0 so the forces on the little bit of string must cancel out, so the tension above that little bit of string must equal the tension below it, i.e. the tension in the string is the same all the way along the string.

While we're at it, a smooth pulley means that the tensions on both sides of the pulley are equal.

The bold bits are the answers you need for M1 questions of the type "how have you used the fact that ......
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5 years ago
#10
(Original post by M1Player)
^As far as OCR's concerned (and I assume, most M1 course), energy doesn't appear until M2, so I was hoping for an explanation that does not involve energy. I'd already somewhat familiar with spring stuff from the mechanics I've done in physics but I'm just wary in case it appears on an M1 exam.
But what are you expecting to be "explained"?

You don't have the mathematics to deal with energy problems in M1, so you assuume that all strings are inextensible and then you don't have to worry about energy at al!
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