Ratio's, do AQA do them in a really weird way?

Hi peeps.

I'm revising for AQA M2 and have come across the lovely question 7Bi in the Jan 2012 exam.

On a vertical circle point A is at the top, B at the bottom. The ratio of tensions at Points A and B are 2:5. It then asks you to find the equation for U in terms of G and A.

I'm perfectly fine with all of the equations and re arranging however AQA seem to do the opposite of everything I have ever seen or been taught with the ratios. I assumed that the the tension at A would be 2 when the tension at B would be 5. This corresponds to logic as surely when at the top point the tension is less due to slower v and minus MG.

However AQA have written the tension to be 5 at A and 2 at B. They justify this by writing A/B = 2/5 in the mark scheme, however if you apply that logic to any other ratio (even really basic stuff you learn in year 6) then you get the opposite. For example if the ratio of boys to girls is 2:3 then you have 2 boys and 3 girls, not B/G=2/3 which would give 3 boys and 2 girls??????????

Really bloody fustrating as I can do all of the actual mechanics stuff in this question but they throw in a really messed up ratio, please help.

Can you provide a link to (or screenshot) the exact question and markscheme?

I don't know whether it's the exam board or your interpretation at fault because you seem to have got yourself confused in what you've written!

If A:B = 2:5 then A/B = 2/5
If B:G = 2:3 then B/G = 2/3

However, the first statement isn't consistent with A = 5 and B = 2.

I think we need to see more detail of the official AQA question and answer!

There are 5 pups, 2 are boys, and 3 are girls

Part-to-Part:
The ratio of boys to girls is 2:3 or 2/3

Question Paper
http://filestore.aqa.org.uk/subjects/AQA-MM2B-QP-JAN12.PDF

Mark Scheme.
http://filestore.aqa.org.uk/subjects/AQA-MM2B-W-MS-JAN12.PDF

Question 7Bi


What confuses me is wherever I look on the internet about basic ratios I get examples like this




Where the ratio refers to the items in the order they are mentioned, so tensions A and B ratio of 2:5, means that the 2 refers to the A and the 5 to the B. But it doesn't??

I don't see what the problem is!

If T1 is the tension at A and T2 is the tension at B then the question tells you that T1:T2 = 2:5.

The mark scheme writes this in the format T1/T2 = 2/5 which is perfectly correct.

I don't know why you think this means that T1 = 5 and T2 = 2 - that doesn't make sense!

...

I understand what notation that AQA is using now, however too me it isn't consistent with ANYTHING i have seen before.

If you say the ratio of boys to girls is 48:52, that would mean 48%boys, 52% girls, generally the numbers are in the order that they were mentioned in the prior sentence. That's what is confusing me, why is it suddenly the opposite the way AQA are writing it?

The same holds for the example though.

It's not the "opposite" - it's completely the same as all the examples you've given!

Suppose the tension at A is 2 Newtons and the tension at B is 5 Newtons.

Then (tension at A) : (tension at B) = 2 : 5

and (tension at A) / (tension at B) = 2 / 5

How is this different from 2 boys, 5 girls giving boys : girls = 2 : 5 or boys / girls = 2 / 5?

Oh feck, just realised how I'm doing it wrong, I'm taking the ratio and subbing it directly into the equation where in reality you need to do the opposite to them to make them equal to each other.

So the ratio 2:5 you need to times the left side by 5 and the right side by 2, thus giving the 5Ta = 2Tb for them to be equal.