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

Original post by JJSinha

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:

highly doubt, that id lose 5 marks for putting 4/3 down instead of 2. thats very harsh . i thought it would be like 1 or 2 anybody think otherwise?

(edited 6 years ago)

Reply 402

xyz9856

Original post by JJSinha

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.

The object in question is a hollow cylinder, with one end having a 'base' of 2 dimensions

Consider the base of the cylinder having com (a,0) and mass p(a^2)pi
Consider the surface curved area of the cylinder to have mass 2pi(a)*(4a), where 4a is the height, having com (a,2a)

therefore

\bar{y}*(9\pi a^{2}\rho ) = ((\rho\pi a^{2})*0 ) + (8\pi a^{2}) *(2a))

\bar{y}=\frac{16a}{9}

This is obviously very nearly 4a/2, however the base will change the COM of the cylinder

(edited 6 years ago)

Reply 403

xyz9856

Original post by JJSinha

Hi
that would definitely be worth around 4-5 marks by itself rather than be in a 6 mark question.

Sorry I didn't see this bit, marks on this exam were very far off. In my eyes, question two should have been about 5 marks and the COM question should have been 8 marks.

Reply 404

Original post by xyz9856

Sorry I didn't see this bit, marks on this exam were very far off. In my eyes, question two should have been about 5 marks and the COM question should have been 8 marks.

100% , Q2 was ridiculously easy and for 8 marks . smh

Reply 405

xyz9856

Original post by dididid

100% , Q2 was ridiculously easy and for 8 marks . smh

Did the question twice lol, it shocked me and made me question my answer many times

Reply 406

dilhan11

Can someone pls help, i did the paper today and stressing about how many marks I have lost.
Question 3 regarding the hemisphere and cylinder, i read wrong and took it as two solids being joined together and found that centre of mass. (6 marks) i believe my method was correct for part b and part a.
I think Q6 (the string on a horizontal table) i forgot about it having EPE even when in line with points a and b, and just simply converted the total EPE into KE (6marks). My value was 0.4 m/s off

Reply 407

dilhan11

any predicted grade boundaries, i think itll be around 66 for an A and 70 for an A*

Reply 408

Tiarnancf

10

Original post by rachstanley

The mass of hemispherical shell was on the c1 formula sheet!

AGHHHH THAT IS GOING TO ANNOY ME FOR MONTHS!!! I went straight to the Mechanics section of the formula sheet and when nothing was there I didn't think to look at C1 (why was it under C1 in the first place we didn't do that in core 1?)

Reply 409

anhkiet1903

Original post by dilhan11

Can someone pls help, i did the paper today and stressing about how many marks I have lost.
Question 3 regarding the hemisphere and cylinder, i read wrong and took it as two solids being joined together and found that centre of mass. (6 marks) i believe my method was correct for part b and part a.
I think Q6 (the string on a horizontal table) i forgot about it having EPE even when in line with points a and b, and just simply converted the total EPE into KE (6marks). My value was 0.4 m/s off

i think you lose quite a lot of mark for calculating it as solid instead as shells, prob 4-5 marks. And forgetting EPE would be around 2-3 marks. I could be wrong for the com question so dont take me on that and hope for the best :biggrin:

(edited 6 years ago)

Reply 410

dilhan11

Original post by anhkiet1903

i think you lose quite a lot of mark for calculating it as solid instead as shells, prob 4-5 marks. And forgetting EPE would be around 2-3 marks

They would deduct 5/6 for that?

Reply 411

FlatearthFTW

15

Original post by dididid

highly doubt, that id lose 5 marks for putting 4/3 down instead of 2. thats very harsh . i thought it would be like 1 or 2 anybody think otherwise?

Oh no, I meant for the whole question. With finding the angle part too. And I was suggesting 5 as an upper limit. To get mass and centre of mass right for one object in those columns gets you like a 1 mark. So that's the one we would've lost. But I'm worried we might lose a few more A1s.

Reply 412

AlainaJayde

1

Original post by ccurnock

Would I lose marks for using T instead of R for question 6?

I wouldn't have thought so, maybe one mark at the most

Reply 413

AlphaArgonian

12

I thought the paper was alright, but could someone explain 4b to me? That's the only question that still confuses me.

Reply 414

Original post by JJSinha

Oh no, I meant for the whole question. With finding the angle part too. And I was suggesting 5 as an upper limit. To get mass and centre of mass right for one object in those columns gets you like a 1 mark. So that's the one we would've lost. But I'm worried we might lose a few more A1s.

theres generally follow through marks and the second part was only 3 marks and probably lose 1 there and 1/2 on the first bit.

Reply 415

Original post by AlphaArgonian

I thought the paper was alright, but could someone explain 4b to me? That's the only question that still confuses me.

the c of mass of the shape had to line up with the final point of contact of the shape withe the plane. and drawing a good diagram you can see that tan(theta)=(C of M)/a and then the a's will cancel out and you just arctan both sides :smile:

(edited 6 years ago)

Reply 416

FlatearthFTW

15

Original post by dididid

the c of mass of the shape had to line up with the final point of contact of the shape withe the plane. and drawing a good diagram you can see that tan(theta)=(C of M)/a and then the a's will cancel out and you just arctan both sides :smile:

Yes that was fine but using the wrong formula for the mass would give the wrong centre of mass. Hence, a wrong answer for part b as well. So M marks given but A marks lost right?

Reply 417

Shinji7

1

In the vertical circle question i simply calculated v and stated v>0 at the top, instead of R>0. Anyone know how many marks I'd lose?

Reply 418

Tiarnancf

10

Original post by JJSinha

Yes that was fine but using the wrong formula for the mass would give the wrong centre of mass. Hence, a wrong answer for part b as well. So M marks given but A marks lost right?

Yeah I think so? I'd say you'll probably get 3/9 marks (I was looking at last year's question on statics of rigid bodies to find out how many marks I'd get and it was roughly 3)

Reply 419