Mark-Scheme Error for 4729/01 OCR (not MEI) M2 14th Jan 2013

Hi everyone,

For Q3 part iii) on this past paper, I get by conservation of momentum $-0.5*9.8+2m=0.5*3.72-m$. Solving yields $m=\frac{6.76}{3}=2.5333333kg$.
This is the correct answer as the mark-scheme agrees.

However, the expression they use for obtaining the answer makes no sense they have $0.5*2gsin30-2m=m-0.5*3.72$.

What do they think they're doing with the $2gsin30$ bit?
As clearly $2sin30=1$ so why even bother putting it in?

If i'm missing the point here could someone please care to explain it to me? As until someone does, its an error.

with kind regards,


thanks,

Illusion

I patiently explained what you were doing wrong yesterday and you seemed satisfied. What has changed since yesterday? 2gsin30 is there for the benefit of the examiner not you. I think you have now just picked the 'sense' of the directions differently - your positive is their negative and so on.

I was satisfied yesterday for your explanation that $m=2.25$ and not $m=3.04$ like i suggested. My reason for asking further is because the particles are on a slope and i didn't know if $2gsin30$ had something to do with that.

regards

Illusion.

Hi everyone,

For Q3 part iii) on this past paper, I get by conservation of momentum $-0.5*9.8+2m=0.5*3.72-m$. Solving yields $m=\frac{6.76}{3}=2.5333333kg$.
This is the correct answer as the mark-scheme agrees.

However, the expression they use for obtaining the answer makes no sense they have $0.5*2gsin30-2m=m-0.5*3.72$.

What do they think they're doing with the $2gsin30$ bit?
As clearly $2sin30=1$ so why even bother putting it in?

If i'm missing the point here could someone please care to explain it to me? As until someone does, its an error.

with kind regards,


thanks,

Illusion

It is just carried forward from the first part. If the candidate had calculated it incorrectly there they can still get the credit in the last part.

Markscheme is fine.

They have taken down the plane as the positive direction, whereas you've taken up the plane as the positive direction.

If you multiply both sides of your equation by -1, you end up with their equation. It's the same thing, essentially.


As to the 2gsin30 - IMO it's far better to put the details in the original equation, prior to any working out. When you come to check it later it's obvious what's going on, whereas if you'd already performed some mental arithmetic, you're relying on spotting the same thing when you check to understand how you got to that equation.