AQA A2 MM2B Mechanics 2 – 27th June 2016 [Exam Discussion Thread]

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Yes it's 2root gl definitely

I'm genuinely tired of people strongly asserting that incorrect corrects are 'definitely' correct. Did you even consider why it might be wrong? It's wrong, so no idea what your definition of 'definitely' is. Look up conditions for full revolutions and be amazed.

Reply 181

forestspirit

Original post by tanyapotter

for the expression of r in the earlier question, was c= 5i-5j?

wasn't it c = 3i - 3j

Reply 182

IrrationalRoot

Original post by C0balt

I swear it was root5lg, there was a similar question to that in one of past papers, I think it was after finding expression for T and we had to make T=0

It was

. Dw about the people that disagree.

Reply 183

C0balt

Did everyone get some weird stuff for the vertical circular motion before the 2rootlg or root5lg question? The tension ones

Reply 184

Jm098

Original post by forestspirit

wasn't it c = 3i - 3j

Yeah

Reply 185

Jm098

Original post by C0balt

Did everyone get some weird stuff for the vertical circular motion before the 2rootlg or root5lg question? The tension ones

Yeah some strange answers 😂 Can't imagine how frustrated the examiners must be marking it

Reply 186

IrrationalRoot

Original post by Jm098

Yeah some strange answers 😂 Can't imagine how frustrated the examiners must be marking it

Those m,g,u,l expressions were ugly as hell lol.

Reply 187

tanyapotter

Original post by forestspirit

wasn't it c = 3i - 3j

oh yeah i think i wrote that, i just remember them both being the same and one being positive and the other negative

when t=0, what did it say the displacement was? do you remember?

Reply 188

Martin9808

Original post by IrrationalRoot

Lool mate , it's 2 root gl I'd love to see the look on your face when you realize this

Reply 189

steph-carys

Original post by tanyapotter

oh yeah i think i wrote that, i just remember them both being the same and one being positive and the other negative

when t=0, what did it say the displacement was? do you remember?

I think 3i - 5j... pretty certain

Reply 190

jtebbbs

Thought it was 2root(ag) as well, it's definitely (I think) root(5ag) cause if you put 2root(ag) into the equation for centripetal force at the top, it gives you a negative tension which is impossible, realising this leads you to root(5ag).

Reply 191

forestspirit

Original post by tanyapotter

oh yeah i think i wrote that, i just remember them both being the same and one being positive and the other negative

when t=0, what did it say the displacement was? do you remember?

8.20 metres rings a bell

Reply 192

AmazingAl

Obligatory Mechanics post

https://www.youtube.com/watch?v=DqPNK4wE7jw

Reply 193

IrrationalRoot

Is this the method that anyone else used for Q8? Not sure where I went wrong.

No KEs to consider first of all. Now by conservation of energy,

GPE+EPE before = GPE+EPE+Work after

Taking x to be the distance from A when next at rest, height for GPE is x/2. (xsin30.)
This is taking the 0 GPE level to be at the point when next at rest.

EPE is for one string, since extension 0 for one string, extension 5 for the other so EPE from that string.

Because of our choice of GPE level, GPE=0 after.

EPE after is total of the two EPEs for both strings, so with extension of x in one of them and extension of x-6 in another.

Work found by x multiplied by coefficient of friction multiplied by reaction force which is mgcos30.

Then I got two answers, chose the larger one, added 4 (got 7.05m).

Reply 194

forestspirit

Original post by steph-carys

I think 3i - 5j... pretty certain

that was the initial position, so when you integrate the velocity to get the displacement and set t=0, you get 3i - 5j = -2j + c, so c = 3i - 3j

Reply 195

sam_97

Original post by IrrationalRoot

Just ignore it, people are bound to believe that what they have done is correct. I got

u > 2\sqrt{gl}

in the exam but have since realised that this expression is wrong as the tension at the top of the circle could be negative.

(edited 7 years ago)

Reply 196

IrrationalRoot

Original post by Jm098

Oh ****

I think they'd ecf it though

You think? Well, that'd be nice but given the way the mark schemes work I'm likely not to get any marks :frown:

. Hoping for some sympathy from the examiner...

Reply 197

jjsnyder

Original post by IrrationalRoot

Extension would be 6-X in the other string, but shouldn't affect your answer I don't think.

Posted from TSR Mobile

Reply 198

IrrationalRoot

Original post by Martin9808

Lool mate , it's 2 root gl I'd love to see the look on your face when you realize this

Yep, you're one of those people. Think you're always right even in the face of a clear proof that you're incorrect. I'm expecting you to mysteriously stop posting when you understand the correct method :wink: