OCR MEI M2 - 15 June 2010 - Page 2

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Student9876543210

In all calculations on the framework tensions/compressions I always used 30 degrees instead of 60 degrees and vice versa.

Anyone got any ideas how many marks I'd get for doing that (out of 9)? I still got consistent corresponding tensions/compressions across the shape but obviously wrong values. And I even crossed through a few of the values properly worked out - stupid change of mind!!

Possibly 4/9 for stating whether each was a tension/compression. I find it hard to be see how you could have got more than that I'm afraid :frown:

Reply 21

Isabobble

Donor

Possibly 4/9 for stating whether each was a tension/compression. I find it hard to be see how you could have got more than that I'm afraid :frown:

But don't they normally have clauses in the beginning of the mark scheme allowing for consistent errors? Like 'only punish for wrong angle once' or something? I can't remember exactly what it says :confused:

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Reply 22

Bee291

XShmalX

Can anyone remember what the set-up of the stand and string on the final part of question 2 was? I wasn't 100% sure what they were describing.

Nor me I kind of drew it from A towards the bottom right corner of the page.

But i didnt really understand how that made it tip. I resolved horizontally but then forgot to resolve vertically!

Has anyone got a markscheme? I want to work out the extent of my failure!

Reply 23

Isabobble

Bee291

I think that's how it was supposed to be - there was a horizontal component acting around the bottom right corner, and this was making it tip, but I don't think I took moments correctly in the end :s-smilie:

Reply 24

Donor

For the tilting, was it:

PQ: arctan(22/68)

RQ : (arctan 15/68)

RS: (arctan (40-22)/68) = (arctan 18/68)

So clearly the angle of RQ is the smallest.

Also the last part was moments about RS? and seeing as the distances were in cm, divide your centre of masses by 100. Edit: they cancel out anyway so no need to do this. I think I've made another mistake here by not converting 10 to 0.1. Oh dear oh dear.

20g(0.4-0.175) = 0.1cos50xP

so P = (20g x 0.225)/0.1cos50

I don't think I put the g in so I got it wrong. Ahhh more marks dropped :frown:

Reply 25

Student9876543210

Isabobble

...

Yeah I prefer this idea :smile:

, cos I did use the correct method consistently - so at least should get some kind of method marks. What do people think A boundary will be?

Reply 26

Isabobble

Student9876543210

Yeah I prefer this idea :smile:

, cos I did use the correct method consistently - so at least should get some kind of method marks. What do people think A boundary will be?

It had better be low, I don't think I'll be able to live with myself if I don't get into my Uni :o:

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Reply 27

Jamie23

what did people get for the tension in the string in Q2?
i got 34.49N, dont have a clue if its right though

Reply 28

Straightpath

Here's the paper.

I totally messed up the last question :mad:

Donor

Would UI lose marks for not changing the units? I just used 17.5, 50g, etc all together?

Also, I was thinking what you were, but then I thought, maybe, the sin(50) componant of the weight is acting to help stop the thing topple as well, isn't it?

for the last part of Q4:
Do you do 1/2*m*1.5^2 = work done by P (which is Pcos(35)*distance, right?) - work done by friction (which you have to work out again with a different value of P?) - work done by S, right?

Reply 30

username219321

I think I got 1,4 completely right, lose a couple of marks on last bit of 2 cos of misinterpeting the angle thing (I ended up with it at 50 degrees to the UPWARDS vertical), lose a couple of marks on 3 maybe for the explanation bit, think I've definitely got an A for the paper overall though. Was my last maths module and I'm pretty confident I've got A*A* or A*A in maths and further maths now :smile:

Reply 31

futuredoc77

Donor

i used the same method, except i subtracted all these angles from 90 degrees, because that is the angle they have to turn by... or that's what i thought at least.. and then you get the RS one to be the smallest... it makes sense if you design the experiment mentally... i don't know...

Reply 32

username219321

futuredoc77

The diagram I drew for each was the line of action of the weight going through the point it was about to tip through, then marking on the angle in a place where you could inverse tan the lengths to find it

Reply 33

Tallon

matt2k8

how did you do Q4 and toppling on Q2 (just look at my earlier post) ta.

Reply 34

futuredoc77

matt2k8

ya that's what I did, but when you turn the stand, its by 90-the angle...

think of it this way, if the centre of mass has a very small y-value, then the value arctan gives you would be a small angle as well, but in that particular case, the object is more stable, thus it needs to be turned by a bigger angle to topple... therefore, the bigger the angle found by the arctan of lengths, the smaller the required angle to turn it by to get it to topple... so you use the 90-angle...

i hope i'm making sense.

Reply 35

username219321

Tallon

how did you do Q4 and toppling on Q2 (just look at my earlier post) ta.

Q4: i) resolve, F = uN etc.
ii) work done = f *d
iii) put into s = 1/2(u + v)t, show inconsistency
iv) find change in KE, WD by friction and WD by the force. then work done against S = wd by force - wd by friction - change in ke

Q2 ill draw up my diagrams and scan them in

Reply 36

dylan_chatterton

Q2 ... part iv .... find the tension in the string at 50deg to the verticle downwards..?? .... how is that done ... i tried taking moments about the right edge with the horizontaL AND verticle components of the weight and tension ...what did people get ??

Reply 37

NewAccount77

DeanT

Did anyone get 6.3J for the last question? I did

210cos35 - W_f - \Delta E_k = S

Pretty sure I got something similar.

Reply 38

Tallon

matt2k8

what I mean is, for Q2, you need to use the sin50 and cos50 componants don't you? the sin(50) heps it stay where it is and the cos(5) topples it, no?

I didn't think about using SUVAT for 4. There was a way with power though wasn't there? Forgot what I did, but I think it made sense at the time. stuff about F =MA and Power = FV I think.