OCR MEI M3 May 2016 - Post Exam discussion Watch

StrangeBanana
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#41
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(Original post by -Gifted-)
Oh my one arithmetic slip two marks dropped. Oh also @StrangeBanana how many marks for the part after that do u think i will drop cos i used thst value of k to find the change in tension.
And also i made a stupid mistake and forgot to add weight force for the shm show that sk i coulsnt get the exact show that. Will thst be like 3 marks dropped ?
Maybe just 1. :P I don't think they'll be very harsh on this question, considering the veritable carnage it has caused. Yeah for the one after, I dunno; you might get all the marks for error carried forward, you might lose 1. I don't think you'd lose more than that though, if your method was right.

Ah, bad luck. Probably drop 2, maybe 3.

(Original post by HFancy1997)
16/9 here too, I went through it with two of my physics teachers and we all got 16/9
Are you sure you've told them that there is a collision at the bottom? It's definitely not 16/9, because to get that you have to conserve energy, which isn't valid.
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pepperben
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#42
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(Original post by StrangeBanana)
I remember the speeds of the combined particle right after the collision was:
\frac{5}{k+1}\sqrt{\frac{ag}{3}}

And using conservation of energy got you a (k+1)^2 = 25/9, which you solve to get 2/3.



You do, in the collision.



I'm sure it's 2/3. I think this question tripped up most people.
did the question give wherethe particles collided ? if so i missed this aghhhhh
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WhiteBison
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Screwed up most of the 9 marker, but as the rest was really easy I feel like the UMS won't be that low unfortunately. About half the people I know got K right. I think the average for M3 is about 64 raw for an A and I don't think it'll be much different this year.

Edit: If 16/9 is incorrect then correction - nobody got it right. Not at my school anyway.
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DraigGoch
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(Original post by pepperben)
did the question give wherethe particles collided ? if so i missed this aghhhhh
At the bottom of the circle
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Wunderbarr
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#45
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(Original post by StrangeBanana)
x
Well done Ed.
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cbricknell
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#46
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Guys, the answer to the last question was actually 6.
I asked my teacher and shes got a Doctorate and she said no one in the class she knows of got it right.
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StrangeBanana
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(Original post by pepperben)
did the question give wherethe particles collided ? if so i missed this aghhhhh
Yeah, at the bottom. Bad luck! A lot of people didn't see it, I think.

(Original post by DraigGoch)
Most further maths didn't do M3, there were 3 of us doing additional further and 1 who bravely decided on M3 instead of D1
Ah, I see. Lol in terms of enjoying the exam I'd much rather do more mechanics than D1, but as for maximising UMS I don't think so.
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StrangeBanana
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(Original post by Wunderbarr)
Well done Ed.
:yy: Who are you, lol?

(Original post by cbricknell)
Guys, the answer to the last question was actually 6.
I asked my teacher and shes got a Doctorate and she said no one in the class she knows of got it right.
For k, or the change in tension?

People probably aren't telling her the question accurately. If you'd like to know, my teacher has a doctorate as well
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captainstripes
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(Original post by StrangeBanana)
I'll chip in

1ai) alpha = -1, beta = 1, gamma = -1
ii) 0.05 grams
bi) 17.5m
ii) 2520N

2i) Show that k = 2
ii) Show that CM is (0.625, 0)
iii) angle was 72.6 degrees

3i) Show that the diff. equation holds
ii) amplitude = 0.2m, period = 2pi/sqrt(10)
iii) velocity = 0.384ms^-1 downward

4i) cos(alpha) = 1/3
ii) Show speed is sqrt(ag/3)
iii) k = 2/3
iv) -(8/3)mg

Please add in/correct as necessary ^^



Yeah, you get -14/3 if you don't take into account that the mass has changed after the collision



Conservation of energy to get the speed of P just before collision, then conservation of momentum to get the speed of the combined mass just after the collision (in terms of k). Then conservation of energy again, because you know its speed when it gets to A. Quite an involved question!
Thank you, i got all those answers. And yes K is 2/3. The only thing is i did the substraction the wrong way around in the last part of q4, so i got the difference as ∆T=(20/3)mg. So i should get 3/4 marks for that. The only part i couldnt do was to prove cos(alpha) =1/3. But someone told me that you needed to use conservation of mechanical energy at A & B to get this result. Oh well. Overall very pleased with this paper, I reckon A grade will definitely be in the low 60's
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sras97
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I did the 16/9 method- do you think I will get ANY method marks at all for that
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¡Muy bien!
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(Original post by sras97)
I did the 16/9 method- do you think I will get ANY method marks at all for that
I think you will because you used conservation of energy to find the speed of p before the collision. That was a necessary step for the the momentum method.
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izzy21
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Hi, does anyone remember the actual questions or the amount of marks available for each question? Trying to work out how many I dropped. Thanks
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¡Muy bien!
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(Original post by izzy21)
Hi, does anyone remember the actual questions or the amount of marks available for each question? Trying to work out how many I dropped. Thanks
I remember in question 4: part 3 was 9 marks and part 4 was 4 marks.
The SHM question had a 4 mark part for amplitude and time period.
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Bunderwump
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#54
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Somehow I managed to get alpha = 3, beta = -1 and gamma = 1 for the first question. For some reason I always do the worst on dimensional analysis questions. I have everything else right, so hopefully that's still well into the 90s for UMS. Although I may have messed up 1 ii) as well. Can anyone remember what questions 1 i) and 1 ii) were?
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izzy21
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(Original post by ¡Muy bien!)
I remember in question 4: part 3 was 9 marks and part 4 was 4 marks.
The SHM question had a 4 mark part for amplitude and time period.
Cheers
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¡Muy bien!
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(Original post by StrangeBanana)
I'll chip in

1ai) alpha = -1, beta = 1, gamma = -1
ii) 0.05 grams
bi) 17.5m
ii) 2520N

2i) Show that k = 2
ii) Show that CM is (0.625, 0)
iii) angle was 72.6 degrees

3i) Show that the diff. equation holds
ii) amplitude = 0.2m, period = 2pi/sqrt(10)
iii) velocity = 0.384ms^-1 downward

4i) cos(alpha) = 1/3
ii) Show speed is sqrt(ag/3)
iii) k = 2/3
iv) -(8/3)mg

Please add in/correct as necessary ^^



Yeah, you get -14/3 if you don't take into account that the mass has changed after the collision



Conservation of energy to get the speed of P just before collision, then conservation of momentum to get the speed of the combined mass just after the collision (in terms of k). Then conservation of energy again, because you know its speed when it gets to A. Quite an involved question!
There was also a question in SHM where a = -10 (x-4) and you had to say what's the value of x at the equalibrium position? Or was it in a pastpaper and I'm already mixing things up?
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WhiteBison
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(Original post by Bunderwump)
Somehow I managed to get alpha = 3, beta = -1 and gamma = 1 for the first question. For some reason I always do the worst on dimensional analysis questions. I have everything else right, so hopefully that's still well into the 90s for UMS. Although I may have messed up 1 ii) as well. Can anyone remember what questions 1 i) and 1 ii) were?
You should be okay, because if I remember correctly, alpha was absorbed into a constant along with k in the following dimensional questions. You've probably just lost a couple of marks.
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StrangeBanana
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(Original post by ¡Muy bien!)
There was also a question in SHM where a = -10 (x-4) and you had to say what's the value of x at the equalibrium position? Or was it in a pastpaper and I'm already mixing things up?
Oh, yeah, there was! I'll edit that in.
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oswim
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(Original post by cbricknell)
Guys, the answer to the last question was actually 6.
I asked my teacher and shes got a Doctorate and she said no one in the class she knows of got it right.
As in 6mg? Because that's what I got for the last question! I got k = 2/3 and agree with strangebannanas answers for everything else. I thought it was a really nice exam overall - lots of show thats. Hopefully I got the 90 ums i need
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mohdstudent
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Can anyone remember what the very last part of the last question asked for exactly?

Because i didnt get what you guys got. All i did was find tension when P is attached then I found the tension P+Q are attached and found the difference and i got a positive number. Will I get any marks for this??

Also I am getting both M3 and M2 full papers tomorrow so if anyone is interested in having a copy let me know.
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