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Edexcel Mathematics: Mechanics M3 6679 01 - 17 May 2017 [Exam Discussion]

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Original post by Orlandothefraser
I made the same mistake as you. You think we'd only lose 4 marks? There was a part b, calculating the angle, surely we would lose marks on there as well.


Im not sure how many marks we'd lose on part a it depends on the mistake but for part b i think youd just lose one as long as you get the correct method?
I made a mistake in the elastic string question where I presumed it was vertical, and another one in the hollow cylinder where I said that Tension=4mg>0. How many overall marks would I have lost?
Besides that, it was a decent paper. I'd say just a bit harder than last year due to these tiny details.
Original post by xyz9856
Dont panic, I dont think the grade boundaries will be that high for an A grade. It should be around 64/75 - A and 70/75 - A*

You have to consider that many people that use this site are of the best, if you break down the exam you can see where the average candidate could lose marks:

Q1 - Volume of revolution by accident
Q2 - This was stupidly easy for 8 marks
Q3 - Easy to make a mistake here, I used the M4 method of creating an equation of motion and then using vdv/dx
Q4 - The amount of people that will get this question wrong, mainly due to misreading the question (thinking it was solid) or not including the base of the cylinder when calculating centre of mass of hollow cylinder (many people would have assumed this was 4a/2)
Q5 - Again, people thinking this was vertical, not considering the EPE when the string was at AB, using the wrong length etc.
Q6 - Fairly easy overall, however I lost a mark here for considering the reaction to be R>/=0, which is not true, it should be R>0
Q7 - This question would be unavailable to a weak candidate, proving the length isn't too bad, proving the SHM is easy if you know what to do but you have to be careful, finding the time overall provides a lot of room for error.

Overall I think I have scored 72 or 73

Good luck for your other exams, I'm hoping to get an A* with that combo also, M2M3FP2

hehe, i did volume of reveloution by accident , even checked my answers and didnt realise it was just a lamina 😭🔫 .my teacher said should only drop a couple marks there though as the rest of the integration using that formula was right.
Original post by -Phoenix-
I made a mistake in the elastic string question where I presumed it was vertical, and another one in the hollow cylinder where I said that Tension=4mg>0. How many overall marks would I have lost?
Besides that, it was a decent paper. I'd say just a bit harder than last year due to these tiny details.

I dont think they gonna penalize for saying tension instead of reaction, and the other question i assume 2 to 3 marks if you write down everything but put mgh in by mistake
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83457
4
Ah s**t, totally glossed over the final bit where it says the string snaps. I knew something was fishy, given that the question was 8 marks long.

How many marks do you guys think calculating the time period from C to D will be be? I'm guessing 3ish, and then 5 for the final bit where the string snaps.
Original post by Student_2357
If you made a mistake and got the wrong value for the distance of the C.O.M in part a) then in part b) you would only lose 1 A mark in calculating the angle provided your method was correct.


Hey there I did solid hemisphere by accident, how many marks would I lose ofr that question?
Original post by anhkiet1903
Yeah that question was a joke. When i saw they give you omega, i was expecting its gnna be tricky since you dnt usually get that. But then they give you tension aswell so it was hella easy


omega was also conveniently 6g :laugh:
Original post by anhkiet1903
I dont think they gonna penalize for saying tension instead of reaction, and the other question i assume 2 to 3 marks if you write down everything but put mgh in by mistake


Ah okay :redface: . Thank you :h: . Hopefully it all goes well
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83457
4
Original post by anhkiet1903
Hi, if I did part 6c wrong where I put in mgh as well when finding the speed, i wrote EPE=KE+mgh+EPE. So i got a lower speed. How many marks would i lose ?


If you were to get marks it would be for:
Realising there is still EPE in the string when it's horizontal
Calculating the two EPEs

Since your formula is wrong in this context, i'm not sure if you'd get method marks for that.
So from my judgement, possibly 2 max, maybe 3.
Original post by 83457
Ah s**t, totally glossed over the final bit where it says the string snaps. I knew something was fishy, given that the question was 8 marks long.

How many marks do you guys think calculating the time period from C to D will be be? I'm guessing 3ish, and then 5 for the final bit where the string snaps.


Probably the other way round. It's highly unlikely that doing v=s/t is worth 5 marks. I'd say you would lose at most 2 or 3 marks
How many marks would be lost for using volumes instead of areas in question 4? :cry2: :coma: :nah: :facepalm:
is this an acceptable method for the circle proof question?
IMG_20170517_165922.jpg485.3KB
if on the centre of mass question instead of putting 2(pi)a^2for mass of shell i used 4/3(pi)a^2 but then all the working using that value and the next part using the obtained c of m were correct. how many marks would i lose?
Original post by -Phoenix-
I made a mistake in the elastic string question where I presumed it was vertical, and another one in the hollow cylinder where I said that Tension=4mg>0. How many overall marks would I have lost?
Besides that, it was a decent paper. I'd say just a bit harder than last year due to these tiny details.


I got the same thing for that question, I just called it the reaction force, so I don't know if you would loose any marks
Original post by -Phoenix-
I made a mistake in the elastic string question where I presumed it was vertical, and another one in the hollow cylinder where I said that Tension=4mg>0. How many overall marks would I have lost?
Besides that, it was a decent paper. I'd say just a bit harder than last year due to these tiny details.


Well, the tension was 4mg which is greater than zero so that's right...

Oh ignore me I see what you mean - should've said normal reaction force instead of tension. Still harsh to remove more than one mark for that type of error.
(edited 6 years ago)
Original post by xyz9856
Dont panic, I dont think the grade boundaries will be that high for an A grade. It should be around 64/75 - A and 70/75 - A*

You have to consider that many people that use this site are of the best, if you break down the exam you can see where the average candidate could lose marks:

Q1 - Volume of revolution by accident
Q2 - This was stupidly easy for 8 marks
Q3 - Easy to make a mistake here, I used the M4 method of creating an equation of motion and then using vdv/dx
Q4 - The amount of people that will get this question wrong, mainly due to misreading the question (thinking it was solid) or not including the base of the cylinder when calculating centre of mass of hollow cylinder (many people would have assumed this was 4a/2)
Q5 - Again, people thinking this was vertical, not considering the EPE when the string was at AB, using the wrong length etc.
Q6 - Fairly easy overall, however I lost a mark here for considering the reaction to be R>/=0, which is not true, it should be R>0
Q7 - This question would be unavailable to a weak candidate, proving the length isn't too bad, proving the SHM is easy if you know what to do but you have to be careful, finding the time overall provides a lot of room for error.

Overall I think I have scored 72 or 73

Good luck for your other exams, I'm hoping to get an A* with that combo also, M2M3FP2


Hi
What did you get for your centre of mass for the cylinder then? Since we assume it's a thin hollow cylinder, isn't it just the geometrical centre?
And if we were meant to derive it separately given that the thickness can't be said to be 0, that would definitely be worth around 4-5 marks by itself rather than be in a 6 mark question.
Original post by 83457
If you were to get marks it would be for:
Realising there is still EPE in the string when it's horizontal
Calculating the two EPEs

Since your formula is wrong in this context, i'm not sure if you'd get method marks for that.
So from my judgement, possibly 2 max, maybe 3.


heyy thanks for replying :biggrin:, hopefully I will only lose 2 marks which is the better scenario obv and could be the different between an A and A* for me :biggrin:
Original post by dididid
if on the centre of mass question instead of putting 2(pi)a^2for mass of shell i used 4/3(pi)a^2 but then all the working using that value and the next part using the obtained c of m were correct. how many marks would i lose?

I honestly don't think we should lose more than 5 marks in total for that whole question. Having said that, 5 marks could easily make the difference in our grades:frown:
Original post by Manboobsmatthew
is this an acceptable method for the circle proof question?


I think that should be fine. V needs to be sufficiently large to keep R>0. You did prove it in the end but you started off with the condition required.
Original post by Major-fury
Hey there I did solid hemisphere by accident, how many marks would I lose ofr that question?


Hi
Check out post #398 :smile:

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