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Edexcel - M3 - 18th May 2016

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Original post by ememoville
For the lamina question I found the equation of the line without flipping it do you think this will be okay?


Yep thats fine.


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Original post by Student403
thanks for the solutions

just concerned b/c i put T>0 not T>=0

I thought in the book it said T had to be > 0


My teacher proved to me mathematically that T > 0 and T=> 0 is equivalent and that even if T = 0 it will complete a circle.
Original post by oShahpo
My teacher proved to me mathematically that T > 0 and T=> 0 is equivalent and that even if T = 0 it will complete a circle.


Okay that's good news. So hopefully everyone's right?
Original post by oShahpo
My teacher proved to me mathematically that T > 0 and T=> 0 is equivalent and that even if T = 0 it will complete a circle.


How did he manage that. T>0 and T>=0 can never be equivalent.


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Original post by physicsmaths
How did he manage that. T>0 and T>=0 can never be equivalent.


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Equivalent in the sense that the motion of the particle following would be the same. He used differentials to prove that the tension would immediately increase enough to raise T above zero and bring back the string to being taut.
Original post by oShahpo
Equivalent in the sense that the motion of the particle following would be the same. He used differentials to prove that the tension would immediately increase enough to raise T above zero and bring back the string to being taut.


i am interested, could you send it?
Original post by Student403
Okay that's good news. So hopefully everyone's right?


I thought with the inequality, T>0 r<l/2 so therefore AB>= l/2 otherwise you'd not cover the range of lengths if that makes sense
Original post by physicsmaths
i am interested, could you send it?


He found an equation of T in theta, then found an expression of T everywhere else in terms of T at the top, then took the limit of T everywhere as Ttop approaches zero and showed that T would remain above zero everywhere else. He also showed the the difference between when the string is slack on top and when it's almost slack makes no difference to the length of the string and thus the motion continues all the same.
Original post by ememoville
I thought with the inequality, T>0 r<l/2 so therefore AB>= l/2 otherwise you'd not cover the range of lengths if that makes sense


It's not like probability in that sense. Think about it if AB = l/2, then r is also = l/2
Original post by Yua
anyone get 87.3 for 5b? my head of maths says its correct


Original post by Student403
IAL?


Not IAL. Is this for M3 or a different exam?
Original post by tiny hobbit
Not IAL. Is this for M3 or a different exam?

What was the question about? I don't remember seeing that number.
Original post by oShahpo
What was the question about? I don't remember seeing that number.


I think you're quoting the wrong person. I haven't a clue what it's the answer to either.
Original post by ememoville
I thought with the inequality, T>0 r<l/2 so therefore AB>= l/2 otherwise you'd not cover the range of lengths if that makes sense


This doesnt work.
Equality holds at r=l/2.
So if r<=l/2 multiply through by -1 and add l to both sides and we get LHS is AB.


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Seems the book lacks consistency.
Screen Shot 2016-05-19 at 18.46.10.png12.6KB
Original post by tiny hobbit
Not IAL. Is this for M3 or a different exam?


5b was a proof that T < or > pi sqrt(2l/g) or something lol... Definitely not UK
Original post by Student403
5b was a proof that T < or > pi sqrt(2l/g) or something lol... Definitely not UK


I know, but thanks anyway.
Original post by tiny hobbit
I know, but thanks anyway.


LOL. Seems like sarcasm, but it probably ain't.
Original post by tiny hobbit
I know, but thanks anyway.


Whoops yeah! Any idea on the proof of AB <= 1/2 l? Whether T should have been set to < 0 or <=0?
Original post by Student403
Whoops yeah! Any idea on the proof of AB <= 1/2 l? Whether T should have been set to < 0 or <=0?


reverse all ur inequality signs
Original post by physicsmaths
reverse all ur inequality signs


whoops lol

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