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Edexcel - M3 - 18th May 2016

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Original post by physicsmaths
Yeh take moments works out in a line.

Forst you need to find Fr.

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Why did you take moments and not use general geometry instead? I tried to do it without taking moments but I couldn't.
Original post by oShahpo
Why did you take moments and not use general geometry instead? I tried to do it without taking moments but I couldn't.


It seemed very obvious since R reaction goes straight through O hence no moment about O. Plus it is so many marks it won't be 7 marks of geometery in a M3 paper.


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Original post by physicsmaths
It seemed very obvious since R reaction goes straight through O hence no moment about O. Plus it is so many marks it won't be 7 marks of geometery in a M3 paper.


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I have been avoiding using Moments in M3 and it has been working right until this problem :s-smilie: Damn it.
Original post by Euclidean
Mass is proportional to surface area so yeah

In theory we can say that mass and SA are the same as we remove the constant of proportionality when we calculate the COM

It's good practice to write mass as k x surface area, and this helps when we have two solids which have different densities

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Yeah, thank you :biggrin:


Hopefully the moments question isn't too bad - they're the one's I still always get wrong :s-smilie:
Original post by Terminatoring
image.jpegGuys how the hell do I do (c)?


where is this question from??
Original post by physicsmaths
It seemed very obvious since R reaction goes straight through O hence no moment about O. Plus it is so many marks it won't be 7 marks of geometery in a M3 paper.


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i did it using the sine rule and got the right answer would that be allowed?
Original post by 1asdfghjkl1
i did it using the sine rule and got the right answer would that be allowed?


Probably but how?


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Original post by physicsmaths
Probably but how?


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I knew it could be done, screw moments!
Sin (180 - (90-alpha) ) / r = sin(theta) / (3/8 r)
Original post by oShahpo
I knew it could be done, screw moments!
Sin (180 - (90-alpha) ) / r = sin(theta) / (3/8 r)


Hmm ok, I still wouldn't bother. Moments is the way to go with these as it is the intended way one would guess.
Plus you don't have to delve long into it.
I probably should have tried a geometric way and probably could have got it if moments had not have worked.
A mechanics way is guaranteed to work in the M3 exam however geometry methods won't so kind of a hit and miss if it doesn't work out. I would do moments if i was u.


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Original post by physicsmaths
Hmm ok, I still wouldn't bother. Moments is the way to go with these as it is the intended way one would guess.
Plus you don't have to delve long into it.
I probably should have tried a geometric way and probably could have got it if moments had not have worked.
A mechanics way is guaranteed to work in the M3 exam however geometry methods won't so kind of a hit and miss if it doesn't work out. I would do moments if i was u.


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Is this generally the case with all of those balancing problems? Does taking moments always work with them?
Original post by oShahpo
Is this generally the case with all of those balancing problems? Does taking moments always work with them?


Oh no,It depends on the aim you see, if its a 6 marker you know 100% that they don't want a 6 mark geometrical solution and intention is to use mechanics. In most of them you want to use a
i) COM(then use a right angle triangle) ii) find new COM then do
iii) or moments
How do you do the questions with masses on the ends ?



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Original post by physicsmaths
Oh no,It depends on the aim you see, if its a 6 marker you know 100% that they don't want a 6 mark geometrical solution and intention is to use mechanics. In most of them you want to use a
i) COM(then use a right angle triangle) ii) find new COM then do
iii) or moments
How do you do the questions with masses on the ends ?



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Yea I understand. I used to do the new CoM method but I think I will start using moments instead.
Original post by physicsmaths

How do you do the questions with masses on the ends ?

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I mostly find the new centre of mass and work from there.
In the textbook they say you should learn/memorise how to derive/prove some of the centre of mass formulae for some shapes, I'm talking about the ones which start of with delta x. Do we actually have to bother learning these proofs? - I haven't seen a past question which asks you to do a proof yet.
Has anyone done June 2014 (R) Question 4b? I'm not really sure how to approach the question
Original post by JustDynamite
Has anyone done June 2014 (R) Question 4b? I'm not really sure how to approach the question


Ive got my completed paper but im eating atm. Give me 20 minutes.


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Any really hard January M3 papers?
Or any hard questions ?
Run out of stuff to do.


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Original post by JustDynamite
Has anyone done June 2014 (R) Question 4b? I'm not really sure how to
Attachment not found
approach the question


Here you goimage.jpg
Hopefully it is readable.
image.jpgimage.jpg
Attachment not found
(edited 7 years ago)
image.jpg381.2KB
image.jpg389.6KB
I'm hoping the paper we get isn't heavy on finding angles and geometry etc.

That kills me every time.
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Rtdsv
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M3 loooool even M1 kills me

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