M2 January 2011 Exam question help!

Hey guys could somebody help me with the last 9-mark question on the OCR (not mei) M2 january 2011...
i got the 2 equations and 1 inequality and do not know what to do next...

1. 0.2a + 0.7b = 0.36
2. b - a = 1.8e
3. a > b

i do not know where to go next :frown:

i try to solve simultaneously but still get stuck, could someone show me how they did it explaining each part please and when i look at the mark scheme i still have no idea.

thanks a lot. :smile:

Reply 1

Indeterminate

Original post by Maphs

i try to solve simultaneously but still get stuck, could someone show me how they did it explaining each part please and when i look at the mark scheme i still have no idea.

thanks a lot. :smile:

Solve 1 and 2 simultaneously to get a and b in terms of e.

Once you've done that, set a > b and solve the inequality for e.

Reply 2

Maphs

Original post by Indeterminate

Solve 1 and 2 simultaneously to get a and b in terms of e.

Once you've done that, set a > b and solve the inequality for e.

I did that and i got...

1. a = 0.4 - 1.4e
2. b = 0.4 + 0.4e

than when i put it into " a > b " it all cancels and i still do not get the answer.
could you please show me how you would do it using my equations. thanks.

Reply 3

Maphs

Original post by Maphs

ANYBODY??

Reply 4

ghostwalker

Original post by Maphs

ANYBODY??

Well it works out to e<0, which from what you're saying isn't correct.

However that's what your equations give, so there must be something wrong with the equations.

Without seeing the original question, I can't tell.

Reply 5

Indeterminate

Original post by Maphs

ANYBODY??

Having read through the problem, I can tell you that your equations are wrong.

Given that the speed of A is greater than that of B after the collision, what can you say about the direction A travels in?

By the way, I get

e>\frac{4}{5}

Reply 6

Maphs

Original post by Indeterminate

e>\frac{4}{5}

I am quite sure that the direction could be going in either way...
1. if the speed of a is greater than another collision would occur between
a and b.

but i thought that if you get the direction wrong than the answer would be right but be a negative answer???

oh and here is the question paper ...
http://www.ocr.org.uk/Images/63594-question-paper-unit-4729-mechanics-2.pdf

it is the last question.

can someone please show me how they did it please.

Reply 7

Maphs

Also for the second part, it says that the collision between b and c is a perfectly elastic, so that means that b MUST be moving to the left as e = 1 so just bounces off.
if so than how is there 2 masses.

oh and just a reminder that i am resitting M2 so anyone for the first part. help

anybody please.

Original post by Maphs

anybody please.

Assuming A,B,C are in order from left to right.

All your initial equations were correct, apart from a > b.

a,b are vectors pointing to the right - towards C.

We are told that the speed of A is greater than the speed of B, but since A is to the left of B, this must mean that A is now going in the opposite direction.

a will be negative (and b positive).

So for it's magnitude to be greater than b, we required:

-a > b

And this gives the desired answer.

Reply 10

Maphs

i thought that if you get the direction wrong then the answer will be negative but still the correct magnitude.?

Reply 11

ghostwalker

Original post by Maphs

i thought that if you get the direction wrong then the answer will be negative but still the correct magnitude.?

Nope. If you get all the directions wrong, then depending on the question, you will get the correct magnitude, just the wrong direction, but if you have a mixture, then anything is possible.

Edit: If you're working out a velocity and get the direction wrong, then, yes, you can get a negative value with the correct magnitude. But that's not what's happening here.