OCR MEI M3 May 2016 - Post Exam discussion Watch

VlAd x
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#21
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#21
(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.
But was it a Inelastic collision tho?
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DraigGoch
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#22
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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 ^^
I trust the strangebanana I got all the same apart from a made a silly mistake in the direction of the velocity of the shm. What do you think about grade boundaries?
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Davi6336
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#23
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#23
(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.
How do you get K+1 all squared, as if this is a mass what equations contain m^2 in them?
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DraigGoch
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#24
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#24
(Original post by Davi6336)
How do you get K+1 all squared, as if this is a mass what equations contain m^2 in them?
The square of the momentum equation.
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Davi6336
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#25
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#25
(Original post by DraigGoch)
The square of the momentum equation.
I wouldn't have thought you use impulse,

All energy was conserved so I just took the energy at one point when mass was just M, and took energy at the other point where mass was m(k+1) as you have all the angles, do you know what be wrong with that
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StrangeBanana
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#26
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#26
(Original post by VlAd x)
Did the question say this is a I elastic collision
Well it can't be, because the particles coalesce, i.e e = 0. So energy isn't conserved.

(Original post by imsoanonymous123)
you know you're in for a good mark when all your answers are the same as strangebanana's

also I reckon this years grade boundaries might be reasonably high simply because I felt like this paper was unusually short for an M3 paper (4 marks for writing amplitude and period LOL and 7 marks for the part (i) show that in question 3), so there won't be enough people running out of time to keep the grade boundaries down. In terms of the difficulty of the questions hard to say whether it is any different from the average though.
:awesome:

I don't know though, I only know 1 other person in my school who got k right (and even he messed up the last one about the change in tension), so I think question 4 was significantly harder than usual. I'd bet that brings down the boundaries, at least a bit
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-Gifted-
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#27
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#27
Ohh StrangeBanana i think did thst i had a slip instead of (25/9) square rooted take away 1 i think i did 25/3 swuare rooted take away 1. How many marks do u reckon i dropped ?
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StrangeBanana
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#28
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#28
(Original post by Davi6336)
I wouldn't have thought you use impulse,

All energy was conserved so I just took the energy at one point when mass was just M, and took energy at the other point where mass was m(k+1) as you have all the angles, do you know what be wrong with that
Energy wasn't conserved, though, because of the inelastic collision.

(Original post by -Gifted-)
Ohh @StrangeBanana i think did thst i had a slip instead of (25/9) square rooted take away 1 i think i did 25/3 swuare rooted take away 1. How many marks do u reckon i dropped ?
Well you'd lose 1 accuracy mark and maybe 1 method mark if they're being harsh?
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VlAd x
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#29
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#29
(Original post by StrangeBanana)
Well it can't be, because the particles coalesce, i.e e = 0. So energy isn't conserved.



:awesome:

I don't know though, I only know 1 other person in my school who got k right (and even he messed up the last one about the change in tension), so I think question 4 was significantly harder than usual. I'd bet that brings down the boundaries, at least a bit
Damn. I thought about momentum but then I thought it couldn't be that......
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DraigGoch
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#30
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#30
(Original post by Davi6336)
I wouldn't have thought you use impulse,

All energy was conserved so I just took the energy at one point when mass was just M, and took energy at the other point where mass was m(k+1) as you have all the angles, do you know what be wrong with that
its not impulse just the normal mv before = mv after and you square it because of the squared velocities in the energy equation.
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DraigGoch
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#31
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#31
(Original post by StrangeBanana)
Well it can't be, because the particles coalesce, i.e e = 0. So energy isn't conserved.



:awesome:

I don't know though, I only know 1 other person in my school who got k right (and even he messed up the last one about the change in tension), so I think question 4 was significantly harder than usual. I'd bet that brings down the boundaries, at least a bit
How many people do it in your school, only 4 did in mine?
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pepperben
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#32
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#32
Where did the particles collide? If this was given in the question then I misread it arghhhbb
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-Gifted-
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#33
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#33
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 ?
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Davi6336
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#34
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#34
(Original post by DraigGoch)
its not impulse just the normal mv before = mv after and you square it because of the squared velocities in the energy equation.
Doesn't that working out look a bit on the small side for 9 marks though
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username2640523
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#35
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#35
16/9 here too, I went through it with two of my physics teachers and we all got 16/9

Posted from TSR Mobile
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DraigGoch
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#36
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#36
(Original post by Davi6336)
Doesn't that working out look a bit on the small side for 9 marks though
there was also the finding the speeds at the bottom of the circle so all it all it seems about right
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StrangeBanana
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#37
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(Original post by VlAd x)
Damn. I thought about momentum but then I thought it couldn't be that......
It was an unusual one, for sure; applying conservation of energy twice, and momentum in between, all in just one part of a question.

(Original post by DraigGoch)
How many people do it in your school, only 4 did in mine?
16, I think. Are there only 4 people in your school doing further maths, or could some people choose a different module? All the further maths people in my school did M3.
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-Gifted-
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#38
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Definitely the answer is 2/3, i made a slip, but if u did correct PCLM and correct energy conservation making sure you are using (m+km) as the mass after, then you should get it. 16/9 definitely isn't right.

Boundaries maybe 60 for A?
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DraigGoch
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#39
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(Original post by StrangeBanana)
16, I think. Are there only 4 people in your school doing further maths, or could some people choose a different module? All the further maths people in my school did M3.
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
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-Gifted-
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#40
(Original post by StrangeBanana)
It was an unusual one, for sure; applying conservation of energy twice, and momentum in between, all in just one part of a question.



16, I think. Are there only 4 people in your school doing further maths, or could some people choose a different module? All the further maths people in my school did M3.
please read my previous post, I am really scared about how many marks i will drop... thanks.
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