Mechanics 1 - January 2012 query..

Original post by Kdot

Well basically I asked for my script to be sent back to me because I think I had done better than what my results were telling me, however I have had mixed answers on what I should do in terms of remarking from several teachers and friends...

I would like to see what everyone thinks about the answered question in regards to the mark scheme obviously, and what they would give out of the available 11 marks, I have attached both pages of the question and the mark scheme.

Appreciate your views, regardless of how harsh they are...

I would like to add that I have tested this method on many other different questions of the same style and they have all successfully lead to the correct answer as this question did...

Page 1 - http://i40.tinypic.com/24mxmz5.jpg

Page 2 - http://i43.tinypic.com/e88p6b.jpg

Markscheme for the question - http://i41.tinypic.com/f24lz9.jpg

How many marks did you scored in this question? Do they tell the question wise marks when you take your script?

Can you please explain, what are you trying to do in part 'c', second part. It looks very confusing, and you would have probably dropped the marks there.

Actually part 'c' wasn't as difficult as you made it look, i am struggling to understand how are you getting the right answer with it.

Reply 2

iamben

Original post by raheem94

How many marks did you scored in this question? Do they tell the question wise marks when you take your script?

Can you please explain, what are you trying to do in part 'c', second part. It looks very confusing, and you would have probably dropped the marks there.

Actually part 'c' wasn't as difficult as you made it look, i am struggling to understand how are you getting the right answer with it.

He's using the quadratic equation formula in order to solve T, as he's got a quadratic? He's made it more complicated than needed but I'd give you 11 marks out of 11 I think. But I'm struggling to see why you'd add the two values of T? I don't get the reasoning behind it. Apart from that it all looks perfect.

Reply 3

Original post by iamben

He's using the quadratic equation formula in order to solve T, as he's got a quadratic? He's made it more complicated than needed but I'd give you 11 marks out of 11 I think. But I'm struggling to see why you'd add the two values of T? I don't get the reasoning behind it. Apart from that it all looks perfect.

It does give the right answer by adding the two values but why?

I know he is using the quadratic formula, but i don't get the reason behind making that equation and why does it give the right answer?

Can you understand the making of his quadratic. He is considering the downward motion, finding the time it takes to reach A.

Reply 4

iamben

Original post by raheem94

It does give the right answer by adding the two values but why?

I know he is using the quadratic formula, but i don't get the reason behind making that equation and why does it give the right answer?

Can you understand the making of his quadratic. He is considering the downward motion, finding the time it takes to reach A.

Yeah, I thought he'd have to calculate the final velocity when hitting 6.6m on the way down and then work out the time. I'm not too sure, I don't understand why it works and it looks like a fluke to me. I would have done it the way in the mark scheme. Everything apart from that looks right though. If someone can explain it then that would be helpful.

EDIT: Is T2 the time coming down? Therefore T1 + T2 is the overall time.

(edited 12 years ago)

Reply 5

IMG_20120330_200938.jpg471.2KB

Original post by iamben

Yeah, I thought he'd have to calculate the final velocity when hitting 6.6m on the way down and then work out the time. I'm not too sure, I don't understand why it works and it looks like a fluke to me. I would have done it the way in the mark scheme. Everything apart from that looks right though. If someone can explain it then that would be helpful.

EDIT: Is T2 the time coming down? Therefore T1 + T2 is the overall time.

Original post by Kdot

Well basically I asked for my script to be sent back to me because I think I had done better than what my results were telling me, however I have had mixed answers on what I should do in terms of remarking from several teachers and friends...

I would like to see what everyone thinks about the answered question in regards to the mark scheme obviously, and what they would give out of the available 11 marks, I have attached both pages of the question and the mark scheme.

Appreciate your views, regardless of how harsh they are...

I would like to add that I have tested this method on many other different questions of the same style and they have all successfully lead to the correct answer as this question did...

Page 1 - http://i40.tinypic.com/24mxmz5.jpg

Page 2 - http://i43.tinypic.com/e88p6b.jpg

Markscheme for the question - http://i41.tinypic.com/f24lz9.jpg

Here is the mistake:

The OP's answer looks to be a fluke. The OP has probably gone wrong here.

Reply 6

4ever_drifting

Original post by raheem94

The OP's answer looks to be a fluke. The OP has probably gone wrong here.

It should have been a minus sign from the start since g is acting downwards.

I can see why it does work, though it is definitely not the most straight forward answer. OP has calculated the speed of the particle at 6.6m up, then gone from there but the new s should be 0, giving 4.9t^2-13.3t=0 so t=0 or 2.7.

However, OP has effectively calculated the time at which the particle will be 6.6m above the 6.6m point, i.e. 13.2m up - see the diagram for why this works though it is not conventional. The method works in this case.... but you could end up with some imaginary numbers if you're not careful... It would work out eventually only if you have a calculator that deals with complex numbers.

Reply 7

Original post by 4ever_drifting

It should have been a minus sign from the start since g is acting downwards.

I can see why it does work, though it is definitely not the most straight forward answer. OP has calculated the speed of the particle at 6.6m up, then gone from there but the new s should be 0, giving 4.9t^2-13.3t=0 so t=0 or 2.7.

However, OP has effectively calculated the time at which the particle will be 6.6m above the 6.6m point, i.e. 13.2m up - see the diagram for why this works though it is not conventional. The method works in this case.... but you could end up with some imaginary numbers if you're not careful... It would work out eventually only if you have a calculator that deals with complex numbers.

Thanks and +rep to you.

Do you think OP will score full marks for it?
I think he should only score full, if he indicates that it is by symmetry.

And also do examiners take so much time to understand a candidates response, the OP's response is very confusing(i have now understood it) and takes lot of time to understand.

And it is probably a fluke.

(edited 12 years ago)

Reply 8

4ever_drifting

Original post by raheem94

Thanks and +rep to you.

Do you think OP will score full marks for it?
I think he should only score full, if he indicates that it is by symmetry.

And also do examiners take so much time to understand a candidates response, the OP's response is very confusing(i have now understood it) and takes lot of time to understand.

And it is probably a fluke.

I honestly don't know - it seems as though one examiner has decided it doesn't deserve full marks already and it did take me some time to understand the solution myself - as you suggest examiners will likely not be so patient given they have 100s of scripts to mark

My personal response is that it demonstrates a lack of understanding by using the 6.6m twice where the 2nd one should be 0, the method used would not work in all cases (assuming complex numbers are to be avoided), and OP's method is not well explained - there is no explanation why the 2 times are added and it is confusing unless you take the time to work out what OP was working out which is not what an examiner should have to do.

Reply 9

Original post by 4ever_drifting

I honestly don't know - it seems as though one examiner has decided it doesn't deserve full marks already and it did take me some time to understand the solution myself - as you suggest examiners will likely not be so patient given they have 100s of scripts to mark

My personal response is that it demonstrates a lack of understanding by using the 6.6m twice where the 2nd one should be 0, the method used would not work in all cases (assuming complex numbers are to be avoided), and OP's method is not well explained - there is no explanation why the 2 times are added and it is confusing unless you take the time to work out what OP was working out which is not what an examiner should have to do.

In my opinion, the OP might not know how to do this, so he tried a fluke.

Who will try to do this question in this way? I have no idea, i have never seen anyone doing such questions in this way.

And that's correct, the examiner will probably not have so much time to work out the OP's method. It takes a long to understand what he has done.

Reply 10

OP

I can hardly see how something like this can be fluked, regardless, my teacher went through multiple methods of working around questions this style and just before the exam I went over it with another student, similar question on the june 2011 paper, think it was q1..

I can only say I repeated what I had been taught and what had previously consistently worked for me..
There is two ways around it, at least I think there is, by taking different starting points and depending on the values used you either add the two times, or take away one from the other.

The 4.9 is the correct part, g should have been negative because its moving against gravity, another mistake :rolleyes:

.
Its hard to explain everything in detail about an exam which I took 3 months ago, but it seems to me it has worked out perfect even if there was mistakes in the process, just checked again the answer is correct to 10 s.f which is no where near lucky...

Overall 7/11 marks. Personally I think thats beyond harsh, yet that could still be biased because its coming from me...

(edited 12 years ago)

m1 pt1 marks.jpg223.5KB

Reply 11

OP

Original post by 4ever_drifting

It should have been a minus sign from the start since g is acting downwards.

I can see why it does work, though it is definitely not the most straight forward answer. OP has calculated the speed of the particle at 6.6m up, then gone from there but the new s should be 0, giving 4.9t^2-13.3t=0 so t=0 or 2.7.

However, OP has effectively calculated the time at which the particle will be 6.6m above the 6.6m point, i.e. 13.2m up - see the diagram for why this works though it is not conventional. The method works in this case.... but you could end up with some imaginary numbers if you're not careful... It would work out eventually only if you have a calculator that deals with complex numbers.

Thats pretty much it, dont think I can add to that...

Reply 12

tiny hobbit

15

In part c, the signs in your first equation under "2nd" made me think that you were going to find the time from 6.6m down to the ground (which would be the same as the time from the ground upto 6.6m, i.e. 0.42.. secs). The calculation would then be 3 4/7 - the two times, which is not what you did.

I think that a brief explanation of your intentions would have helped, as there is no evidence that you actually intended the sign on the 4.9 to be negative.

Symmetry is obviously allowed, since that's how you and the mark scheme did part a, so the quickest way to do part c is to just find the 0.42... as you did and then subtract 2 lots of it from the total time. That isn't one of the versions offered in the mark scheme, but it would clearly be allowed.

I think that it is crucial in this sort of question to explain which time you think you are finding, since there are so many possibilities. The same goes for projectile questions in M2.

(edited 12 years ago)

Reply 13

Original post by Kdot

I can hardly see how something like this can be fluked, regardless, my teacher went through multiple methods of working around questions this style and just before the exam I went over it with another student, similar question on the june 2011 paper, think it was q1..

I can only say I repeated what I had been taught and what had previously consistently worked for me..
There is two ways around it, at least I think there is, by taking different starting points and depending on the values used you either add the two times, or take away one from the other.

The 4.9 is the correct part, g should have been negative because its moving against gravity, another mistake :rolleyes:

.
Its hard to explain everything in detail about an exam which I took 3 months ago, but it seems to me it has worked out perfect even if there was mistakes in the process, just checked again the answer is correct to 10 s.f which is no where near lucky...

Overall 7/11 marks. Personally I think thats beyond harsh, yet that could still be biased because its coming from me...

The main problem here is that you have done a very confusing method and you have made a mistake(sign of g) while doing which confuses it even more.

You should have got full in it, if you wouldn't have done that mistake(sign of g) and would have explained your method clearly. Because this method does take a long time to understand.

I got 100/100 in M1 last year, i was good enough to solve an M1 paper in less then 2/3 of the time, but my paper finished at the last minute, because i tried to explain each and every step to ensure that i don't drop a mark. Another thing is you should always indicate which direction you are taking as positive, this makes it easier for the reader to understand your intention. e.g. If you would have indicated the directions, then it would have been easily figured out that you have made a mistake in the sign of 'g' so it wouldn't have caused a confusion.

I still don't understand why your teacher taught such a confusing way, i have never seen it before.

Reply 14

OP

He showed us all the different ways to do it, but this one came to mind just because I had just explained it to a friend literally 30 mins before the exam. My issue is that I self marked every paper I did and if I got the final answer I would give full marks, as the mark scheme is a bit confusing.

Kind of disappointed with what I got, because walking out that exam I thought it was going to be a 90 ums in the bag.. unfortunately not :s-smilie:

However I still think there are marks to be gained, just want to see what everyone thinks about it? Deserved 7/11 marks,more or even less?...

Reply 15

Original post by Kdot

He showed us all the different ways to do it, but this one came to mind just because I had just explained it to a friend literally 30 mins before the exam. My issue is that I self marked every paper I did and if I got the final answer I would give full marks, as the mark scheme is a bit confusing.

Kind of disappointed with what I got, because walking out that exam I thought it was going to be a 90 ums in the bag.. unfortunately not :s-smilie:

However I still think there are marks to be gained, just want to see what everyone thinks about it? Deserved 7/11 marks,more or even less?...

Ask a teacher such as TenOfThem, they may have better idea of marking.

But the problem is that you have done very confusing way, which the examiner might have failed to understand, you should have explained it.

Reply 16

OP

Was hoping there would be a few people on here that have previously marked papers and see what their opinion is.

Reply 17

Original post by Kdot

Was hoping there would be a few people on here that have previously marked papers and see what their opinion is.

Try to get the attention of Arsey, he will have complete knowledge of the marking system.

Reply 18

OP

"Arsey has chosen not to receive private messages or may not be allowed to receive private messages. Therefore you may not send your message to him/her.

If you are trying to send this message to multiple recipients, remove Arsey from the recipient list and send the message again."

Umm didn't manage to find anywhere where I could drop a message either :/

Reply 19