# OCR MEI Mechanics 4 (M4)

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Hi, this thread is for anyone taking the OCR MEI M4 exam to talk about the content and problems in the module, and to talk generally about maths of course

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How are people feeling for the exam next week? I've still got to get my head around the "dynamics of a rotating body" chapter a little more but aside from that it should be good

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#3

im feeling pretty good about this one! I am actually sitting M1 and M2 at the same time

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#4

same, the dynamics of rotating bodies is the hardest one for me! feeling pretty good about it though, been getting As and A*s on past papers so far so you're doing M1, M2 and M4 in the same session? is that even allowed?

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#5

apparently you are allowed to sit up to 6 hours of exams on each day, which is just enough for me It felt really weird doing M3 first though, and man it was so boring sitting in M1 lessons; so easy it defies belief

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#6

(Original post by

How are people feeling for the exam next week? I've still got to get my head around the "dynamics of a rotating body" chapter a little more but aside from that it should be good

**B Jack**)How are people feeling for the exam next week? I've still got to get my head around the "dynamics of a rotating body" chapter a little more but aside from that it should be good

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I've noticed with all the M4 past papers, for variable forces there has never been a question based largely on gravitational energy (GMm/r^2) or work done/ impulse of force integrals, I reckon it might be worth just making sure you're good with those parts of variable forces because all the other past papers have basically been just mdv/dt, mvdv/dx or Pv. It's only a suggestion but based on how the questions in the S4 paper were quite different from previous years I wouldn't be surprised if it were the same with M4 too..

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#10

The point when you realise that actually, it may have been worth revising for M4... :/ I didn't get the last part of question 3 - how can you find a point of equilibrium when you don't actually know the value of lambda - if lambda>2mg, then lambda could be infinite, in which case there are no points of equilibrium for the given range of 2theta (though thinking about it I guess pi/3 would be stable... but then in another case when lambda isn't infinite it wouldn't be...)

Anyway, apart from that I don't think I did too badly... I hope :/

Anyway, apart from that I don't think I did too badly... I hope :/

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#11

For the last part of 3 you had to show that for dv/d(theta) = 0, either cos(theta) = 0 (leads to pi/2) or sin(theta) = lambda/(2lambda-2mg). Given lambda > 2mg, this fraction is always less than one, so you can arcsin it without worrying about domains, so there is a point of equilibrium at

theta = arcsin(lambda/(2lambda-2mg)). A bit of mucking about with the same inequality showed that the second derivative was always positive for lambda > 2mg, so it was stable for all lambda > 2mg. (pi/2 was unstable).

Unless that's all wrong. Wouldn't be that surprised...

theta = arcsin(lambda/(2lambda-2mg)). A bit of mucking about with the same inequality showed that the second derivative was always positive for lambda > 2mg, so it was stable for all lambda > 2mg. (pi/2 was unstable).

Unless that's all wrong. Wouldn't be that surprised...

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#12

How did you do the first part of question 2? I only got the answer they got when I ignored gravity.

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#13

(Original post by

For the last part of 3 you had to show that for dv/d(theta) = 0, either cos(theta) = 0 (leads to pi/2) or sin(theta) = lambda/(2lambda-2mg). Given lambda > 2mg, this fraction is always less than one, so you can arcsin it without worrying about domains, so there is a point of equilibrium at

theta = arcsin(lambda/(2lambda-2mg)). A bit of mucking about with the same inequality showed that the second derivative was always positive for lambda > 2mg, so it was stable for all lambda > 2mg. (pi/2 was unstable).

Unless that's all wrong. Wouldn't be that surprised...

**cwt255**)For the last part of 3 you had to show that for dv/d(theta) = 0, either cos(theta) = 0 (leads to pi/2) or sin(theta) = lambda/(2lambda-2mg). Given lambda > 2mg, this fraction is always less than one, so you can arcsin it without worrying about domains, so there is a point of equilibrium at

theta = arcsin(lambda/(2lambda-2mg)). A bit of mucking about with the same inequality showed that the second derivative was always positive for lambda > 2mg, so it was stable for all lambda > 2mg. (pi/2 was unstable).

Unless that's all wrong. Wouldn't be that surprised...

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#14

(Original post by

How did you do the first part of question 2? I only got the answer they got when I ignored gravity.

**james22**)How did you do the first part of question 2? I only got the answer they got when I ignored gravity.

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#15

Yup, should definitely have revised a little for this exam At least this is going to be the one module that doesn't count as all towards overall grades for me...

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#16

(Original post by

Yup, should definitely have revised a little for this exam At least this is going to be the one module that doesn't count as all towards overall grades for me...

**Beth1234**)Yup, should definitely have revised a little for this exam At least this is going to be the one module that doesn't count as all towards overall grades for me...

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I thought the exam went well, the question 3 part iii took a while to find the nature of the equilibrium point that wasn't pi/2, but in the end I managed to show it was stable. Other than that I thought it was a really nice paper.

For question two where you had to find v in terms of x, how did you explain why the velocity was the negative root rather than the positive root? (v=-sqrt((k/m)((a^2)-(x^-2))) )

For question two where you had to find v in terms of x, how did you explain why the velocity was the negative root rather than the positive root? (v=-sqrt((k/m)((a^2)-(x^-2))) )

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#19

(Original post by

I thought the exam went well, the question 3 part iii took a while to find the nature of the equilibrium point that wasn't pi/2, but in the end I managed to show it was stable. Other than that I thought it was a really nice paper.

For question two where you had to find v in terms of x, how did you explain why the velocity was the negative root rather than the positive root? (v=-sqrt((k/m)((a^2)-(x^-2))) )

**B Jack**)I thought the exam went well, the question 3 part iii took a while to find the nature of the equilibrium point that wasn't pi/2, but in the end I managed to show it was stable. Other than that I thought it was a really nice paper.

For question two where you had to find v in terms of x, how did you explain why the velocity was the negative root rather than the positive root? (v=-sqrt((k/m)((a^2)-(x^-2))) )

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