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# C2 Help pleeeeease Tweet

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1. C2 Help pleeeeease
L = Rθ - rθ

K = .5(R^2θ - r^2θ)

Find an expression for r in terms of R, K and L

I manipulated the first equation to get:
r = R - L/θ

and the 2nd one to get:
r^2 = R^2 - 2K/θ

I then divided the second equation by the first one to get:
r = R - 2K/L

However, the mark scheme says the answer is r = 2K/L - R

Would really appreciate it if someone could explain to me why my method is wrong. Got C2 at 9am!
2. Re: C2 Help pleeeeease
are you doing OCR?
3. Re: C2 Help pleeeeease
(Original post by peachesandcream77)
are you doing OCR?
Nope, WJEC
4. Re: C2 Help pleeeeease
(Original post by MikeySwansea)
L = Rθ - rθ

K = .5(R^2θ - r^2θ)

Find an expression for r in terms of R, K and L

I manipulated the first equation to get:
r = R - L/θ

and the 2nd one to get:
r^2 = R^2 - 2K/θ

I then divided the second equation by the first one to get:
r = R - 2K/L

However, the mark scheme says the answer is r = 2K/L - R

Would really appreciate it if someone could explain to me why my method is wrong. Got C2 at 9am!
you're doing

What makes you think that = R - 2K/L

You can't just divide one part by the part below it.

that's like saying which obviously is not true.
5. Re: C2 Help pleeeeease
(Original post by MikeySwansea)
L = Rθ - rθ

K = .5(R^2θ - r^2θ)

Find an expression for r in terms of R, K and L

I manipulated the first equation to get:
r = R - L/θ

and the 2nd one to get:
r^2 = R^2 - 2K/θ

I then divided the second equation by the first one to get:
r = R - 2K/L

However, the mark scheme says the answer is r = 2K/L - R

Would really appreciate it if someone could explain to me why my method is wrong. Got C2 at 9am!

Now divide equation

I gave a full solution because the OP's exam is tomorrow.
6. Re: C2 Help pleeeeease
(Original post by hassi94)
you're doing

What makes you think that = R - 2K/L

You can't just divide one part by the part below it.

that's like saying which obviously is not true.
Thank you Hassi

Edit: you to Raheem
Last edited by MikeySwansea; 17-05-2012 at 23:57.
7. Re: C2 Help pleeeeease
(Original post by raheem94)

Now divide equation

I gave a full solution because the OP's exam is tomorrow.
P.S The last 3-4 steps are redundant. You multiplied by L and then divided by L.
8. Re: C2 Help pleeeeease
(Original post by hassi94)
P.S The last 3-4 steps are redundant. You multiplied by L and then divided by L.
I was being foolish.

I have an S1 exam tomorrow, so i was revising for it, didn't noticed what i am writing.
9. Re: C2 Help pleeeeease
(Original post by raheem94)
I was being foolish.

I have an S1 exam tomorrow, so i was revising for it, didn't noticed what i am writing.
Aha it's no problem I just thought I'd let you know. Good luck with S1 I'm sure you'll nail it mate
10. Re: C2 Help pleeeeease
(Original post by hassi94)
Aha it's no problem I just thought I'd let you know. Good luck with S1 I'm sure you'll nail it mate
Hoping for a simple probability question, other chapters are fine.

11. Re: C2 Help pleeeeease
(Original post by raheem94)
Hoping for a simple probability question, other chapters are fine.

A month today so quite a while away (Physics Unit 5).
12. Re: C2 Help pleeeeease
(Original post by hassi94)
A month today so quite a while away (Physics Unit 5).
Physics with which board?
You would be looking forward for an A* in physics, right? How many marks do you need for it?
13. Re: C2 Help pleeeeease
(Original post by raheem94)
Physics with which board?
You would be looking forward for an A* in physics, right? How many marks do you need for it?
AQA. I'd hope so but I didn't do great in January so it will be very hard. I need about 112-114/120 (depending on what my ISA mark scales to UMS-wise). While I could probably do this given enough effort - I don't want to put in the effort if it means compromising a STEP grade.