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Wednesday 15th AS Mei Maths and Mech discussion

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Yeh I did a similar thing to u Brandon but I think solving the quadratic to find one solution seems better tbh
Original post by jsjsjsjs333
Least I don't feel like the only one feeling so conscious of the fact it was 7 marks that I felt the need to draw the graph haha :smile:


Original post by BrandonShulver
Yeah I found dy/dx, set =0 to find stationary which was x=1 by solving x^3/2=1, then plugged it in to get y=-3. Then I found d^2y/dx^2 and plugged in x=1 to show a minimum, and then did +- 0.1 and plugged into dy/dx to show gradients at each point to the left and right to show minimum. Then drew the graph😂 don’t really know how it was 7 marks really, was confused on what to include to get the full marks so just tried everything really
Reply 41
Avatar for frashid
frashid
6
Original post by BrandonShulver
Yeah I found dy/dx, set =0 to find stationary which was x=1 by solving x^3/2=1, then plugged it in to get y=-3. Then I found d^2y/dx^2 and plugged in x=1 to show a minimum, and then did +- 0.1 and plugged into dy/dx to show gradients at each point to the left and right to show minimum. Then drew the graph😂 don’t really know how it was 7 marks really, was confused on what to include to get the full marks so just tried everything really


OMG i forgot to deferentiate twice 😱😱😱
I don’t know if it’s right I honestly just tried every differentiation method I learnt to show it was a minimum at (1,-3) because I was so confused how it was 7 marks (and they gave us 2 pages??)
Original post by frashid
OMG i forgot to deferentiate twice 😱😱😱
I reckon something like 1 marks to differentiate, a mark to set to zero, two marks to prove minimum (one to differentiate, one for conclusion), mark for y co ordinate, and some marks for proving it was only one...

Seems a slight stretch though
Original post by BrandonShulver
I don’t know if it’s right I honestly just tried every differentiation method I learnt to show it was a minimum at (1,-3) because I was so confused how it was 7 marks (and they gave us 2 pages??)
I don’t know how you’d get a quadratic though? Unless you make a substitution? And would that be 7 marks?
Original post by sdhasdjg
Yeh I did a similar thing to u Brandon but I think solving the quadratic to find one solution seems better tbh
Original post by BrandonShulver
I don’t know if it’s right I honestly just tried every differentiation method I learnt to show it was a minimum at (1,-3) because I was so confused how it was 7 marks (and they gave us 2 pages??)


I differentiated twice to show that it was greater than 0 which = a minimum (Is that right idek pahah)
Yeah I did that, I got it was 3 and hence a minimum but idk does that really prove it’s a stationary point ??
Original post by sdhasdjg
I differentiated twice to show that it was greater than 0 which = a minimum (Is that right idek pahah)
Interestingly enough, one last years stats paper there is a similar question worth 8 marks, and it's more guided too (Q10, pure and stats). It's split into parts with prompts too (e.g find dy/dx etc), so maybe they are just being lenient with marking?
Yeah, stationary point is proven by dy/dx=0, second part is just proving what type.
Original post by BrandonShulver
Yeah I did that, I got it was 3 and hence a minimum but idk does that really prove it’s a stationary point ??
What did we have to do for that inversely proportional question at the end?
Ah yeah I remember that question actually, yeah it found dy/dx then d^2y/dx^2 for 4 marks then 5 marks prove it was stationary I guess by solving dy/dx=0 and plugging x coordinate into d^2y/dx^2 to prove minimum/max as I don’t know how else you would do it really
Original post by jsjsjsjs333
Interestingly enough, one last years stats paper there is a similar question worth 8 marks, and it's more guided too (Q10, pure and stats). It's split into parts with prompts too (e.g find dy/dx etc), so maybe they are just being lenient with marking?
It was inverse proportion, so y=k/x, then sub in some numbers. I got k at about 10.5 I think
Original post by sdhasdjg
What did we have to do for that inversely proportional question at the end?
Reply 52
Avatar for frashid
frashid
6
Original post by jsjsjsjs333
You got a quadratic? I don't think I did, which was my issue. I also solved it like a normal equation, ending up with x^3/2=1, and then just said there was only one solution for that?

On reflection, factorising sounds like a much more correct answer!


Sorry not a quadratic and cubic it was something like 10x^2(x^2-2x+3)=0 something along those lines
What did everyone put for the vectors? The one on about its weight? i put 0i-2.5gj, but wasn't sure. For the next park I added the forces then divided them by 2.5 (F=ma), but my vectors knowledge is pretty bad. Happy there wasn't a bigger question on them ngl
k good, I thought I did something wrong
Original post by jsjsjsjs333
It was inverse proportion, so y=k/x, then sub in some numbers. I got k at about 10.5 I think
I used the formula m=k/t as it was inversely proportionate then pluggedin values for m and t given to get k=10.5, then use new model and plug in values given and solve 2 simultaneous equations to get a=-0.042 or something and b=2.31 I think. Then just comment why some models aren’t suitable or something. For the last part for finding the time for the mass to be 0, I accidentally put 55 seconds instead of hours... will I be deducted a mark?😂
Original post by sdhasdjg
What did we have to do for that inversely proportional question at the end?
yeh I did -2.5gj but actually multiplied it by 9.8 idk if it matters if I did that or not - then for the second part I did exactly the same
Original post by jsjsjsjs333
What did everyone put for the vectors? The one on about its weight? i put 0i-2.5gj, but wasn't sure. For the next park I added the forces then divided them by 2.5 (F=ma), but my vectors knowledge is pretty bad. Happy there wasn't a bigger question on them ngl
I doubt you'll loose the mark, because I feel like you'd get it for without the units (if that makes sense). I feel like they'd have specified give units, or it will just say ignore units in markscheme.
Original post by BrandonShulver
I used the formula m=k/t as it was inversely proportionate then pluggedin values for m and t given to get k=10.5, then use new model and plug in values given and solve 2 simultaneous equations to get a=-0.042 or something and b=2.31 I think. Then just comment why some models aren’t suitable or something. For the last part for finding the time for the mass to be 0, I accidentally put 55 seconds instead of hours... will I be deducted a mark?😂
Nah, it won't matter you multiplied it out - same thing! And thats good, starting to feel more confident about the paper.
Original post by sdhasdjg
yeh I did -2.5gj but actually multiplied it by 9.8 idk if it matters if I did that or not - then for the second part I did exactly the same
I put that I thought it was logical as the mass doesn’t move horizontally and acts downwards with 2.5g. Yeah next part I just done a resultant then divide the i and j by 2.5 to get 0.6i+8.4j or 0.8i+6.4j (not sure but something like that)
Original post by jsjsjsjs333
What did everyone put for the vectors? The one on about its weight? i put 0i-2.5gj, but wasn't sure. For the next park I added the forces then divided them by 2.5 (F=ma), but my vectors knowledge is pretty bad. Happy there wasn't a bigger question on them ngl
(edited 4 years ago)

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