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# OCR-MEI M3: January 27th. Watch

1. Both M1 and M3 are on the 27th (and I'm sitting them both).

How is everybody with M3? Personally I need to do a bit more practice on my weaker areas, such as simple harmonic motion and situations involving Hooke's law. The centres of mass and dimensional analysis really aren't too bad though.
2. Hey im doing M1 and M3 as well (and chemistry OCR A in the morning lol) (*SNAP*)

as for M3, centre of mass and dimensional analysis should be easy 36 or so marks.

Simple harmonic motion isn't too bad 9yet more equations to memorise YAWN easy enough)

my weakest area is more circular motion but not impossible. Hopefully, i'll get the 90%+ so that i can secure an A* in further maths.

anyways good luck with it; i'll probably come back to this forum and discuss questions (hehe MOAN)

craig

p.s. looking good for ur offers but i didn't like the look of warwick lol so i applied to UEA for the G103 as well

only need AAB with an A in maths which should be easy lol

subjects:
biology
chemistry
physics
further maths
maths

AS grades: 4A's (im taking the whole a level further maths this year xD)
3. (Original post by craigp100)
AS grades: 4A's (im taking the whole a level further maths this year xD)
That's what I'm doing!

As for M3, I'm having a few problems with Hooke's law and simple harmonic motion.
4. (Original post by craigp100)
as for M3, centre of mass and dimensional analysis should be easy 36 or so marks.
Are you doing the same exam board as us?

I think we'd have to be exceptionally lucky to get two whole questions on these.

But anyway good luck
5. (Original post by OL1V3R)
That's what I'm doing!

As for M3, I'm having a few problems with Hooke's law and simple harmonic motion.
I keep forgetting which way up the fraction is for .

Isn't the correct form:

EDIT: I just thought of a way of remembering:

And:

Therefore:

And then the above thing sort of follows from this:

But I need a way of remembering it easily during the exam!
6. (Original post by placenta medicae talpae)
I keep forgetting which way up the fraction is for .

Isn't the correct form:

EDIT: I just thought of a way of remembering:

And:

Therefore:

And then the above thing sort of follows from this:

But I need a way of remembering it easily during the exam!
Try and make up a mnemonic for the part. For example, just keep thinking "omega squared is the mass under the stiffness"; quite literally, if the mass is hanging, then the mass is actually under where the stiffness is (the string!). Likewise in the fraction, the mass is the denominator!
Also, when making a formula for solving SHM, and you get the form $x=asin(\omega&space;t+\varepsilon&space;)$ and the start of the graph is going the wrong way (e.g. if it's going up when you know it should be moving downwards) do you always add $\pi&space;/2$ to your value of $\varepsilon$?
And how would you show that a string becomes slack by using an energy method (e.g. if you've got two strings at fixed ends with a particle joining them, and then the system is pulled and released)? Would you need to show that at one point the extension is zero?
7. (Original post by OL1V3R)
T
And how would you show that a string becomes slack by using an energy method (e.g. if you've got two strings at fixed ends with a particle joining them, and then the system is pulled and released)? Would you need to show that at one point the extension is zero?
for horizontal motion its much better if you dont use enrgy conservation unless stated..use F=ma
usually the question would require you to show its moving in SHM..
Since this is a string depending on whether its moving when one has an extension the other one can go slack ..
if you could post a question with the problem your having..it would be better.
8. Here's the question sheet: it's question 2 (iii) I couldn't quite get. You might need a login, but if you're doing MEI, you might likely have one.
9. (Original post by OL1V3R)
Here's the question sheet: it's question 2 (iii) I couldn't quite get. You might need a login, but if you're doing MEI, you might likely have one.
I dont do MEI , im doing edexcel ...post the question perhaps?
10. (Original post by rbnphlp)
I dont do MEI , im doing edexcel ...post the question perhaps?
Yep, sure thing

Each of two light elastic strings, AB and BC, has modulus 20 N. AB has natural length 0.5 m and BC has natural length 0.8 m. The strings are both attached at B to a particle of mass 0.75 kg. The ends A and C are fixed to points on a smooth horizontal table such that AC = 2 m. Initially the particle is held at the mid-point of AC and released from rest.

This is what the diagram roughly looked like:

A-----------------------B----------------------C
|------------------------ 2m --------------------|

(i) Find the tension in each string before release and calculate the acceleration of the particle immediately after it is released. [5]

The particle is now moved to the position where it is in equilibrium. The extension in AB is
e m.

(ii) Calculate e. [4]

The particle is now held at A and released from rest.

(iii) Show that in the subsequent motion BC becomes slack. Calculate the furthest distance of the particle from A. [6]

11. (Original post by OL1V3R)
Yep, sure thing

Each of two light elastic strings, AB and BC, has modulus 20 N. AB has natural length 0.5 m and BC has natural length 0.8 m. The strings are both attached at B to a particle of mass 0.75 kg. The ends A and C are fixed to points on a smooth horizontal table such that AC = 2 m. Initially the particle is held at the mid-point of AC and released from rest.

This is what the diagram roughly looked like:

A-----------------------B----------------------C
|------------------------ 2m --------------------|

(i) Find the tension in each string before release and calculate the acceleration of the particle immediately after it is released. [5]

The particle is now moved to the position where it is in equilibrium. The extension in AB is
e m.

(ii) Calculate e. [4]

The particle is now held at A and released from rest.

(iii) Show that in the subsequent motion BC becomes slack. Calculate the furthest distance of the particle from A. [6]

yh I think as you said would need to show that the tension in the string BC is 0 ..when B is 1.2m from A
12. I feel so hardcore just coming on this thread. Can somebody tell me which of the m3 past papers were the hardest? Don't have much time because I have chem bleh.
13. (Original post by DevilsJackass)
I feel so hardcore just coming on this thread. Can somebody tell me which of the m3 past papers were the hardest? Don't have much time because I have chem bleh.
Oops, I think I got here too late!
In any case, I could probably only answer based on grade boundaries and my personal experience.
14. Hey,
well as promised im back! as for the mei exam M3 *yawn*

lol sorry but i felt that the exam couldn;t have gone any better even if i wrote the paper myself!

Don't know why but i found the questions super easy thsi time round which can only be a good thing (unless my results ssay different...)

How did everyone else find it?

Craig
15. yeah i think it was easier than usual - well that's what most people in my class thought
just really annoyed as i fell into their trap on the last part of question 1 as the string is stretched when the mass is above O and i didn't take this into account...
16. (Original post by Mr Nonsense)
yeah i think it was easier than usual - well that's what most people in my class thought
just really annoyed as i fell into their trap on the last part of question 1 as the string is stretched when the mass is above O and i didn't take this into account...
That question was about an inextensible string wasn't it?
Oh dear ...
17. (Original post by craigp100)
Don't know why but i found the questions super easy thsi time round which can only be a good thing (unless my results ssay different...)
I think they also changed the format from the usual quite a bit.

It was quite a surprise to be given centre of mass stuff in question 2!
18. Seemed ok, was pushed for time and made some silly mistakes, but overall think I did fine.
19. I think the paper was quite good, there were no really cryptic questions. But I know I made a few mistakes, such as the tension in the two strings with the hanging lamina (for some reason I added both horizontal and vertical moments of the centres of mass about B, now I know I shouldn't have . The SHM question was also quite reasonable but I'm not sure if I got the right amplitude. The dimensional analysis and circular motion questions were very nice but I didn't quite understand why v² involved "+ cosθ" because surely the height is 1.25 - 1.25cosθ. Ah well, I tweaked the algebra a bit and got the result I wanted.

I'm thinking I might get an A for this paper; would like 90% though

And M1 was also really good.
20. It was E.k(vertically below O) = E.k(at angle theta) - mgh. So it was -(1.25-1.25cos(theta)) = 1.25cos(theta) - 1.25.

I thought it was tough but just about doable. I thought Questions 3 was by far the hardest, part i and iv I was reasonably sure on but parts ii and iii confused me a bit. SHM was tricky as well but once I got the answers I'm pretty sure they're right because they fitted with the other parts of the question. Would be disappointed with less than 90%..

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