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Something that's been bugging me (OL maths)

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  • 03-06-2008 3:00pm
    #1
    boobookitty
    Closed Accounts Posts: 232 ✭✭


    2005 Paper 1 Q7 part c.

    (Twelve T minus 3 over 2t squared)
    h = 12t - 3/2t(2)

    Find speed. Now I know for speed you differentiate once. However I keep getting 12 - 3t and the answer is 12.

    This is embarassing as Differentiation is supposed to be my second best question. :(


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    #2
    Cokehead Mother
    Closed Accounts Posts: 773 ✭✭✭Cokehead Mother


    12 - 3t is the speed at the t seconds.

    Does the question ask for say the initial speed, which is the speed at 0 seconds?


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    #3
    boobookitty
    Closed Accounts Posts: 232 ✭✭boobookitty


    Cokehead Mother wrote: »
    12 - 3t is the speed at the t seconds.

    Does the question ask for say the initial speed, which is the speed at 0 seconds?

    Oh, sorry let me write the full question.

    "The distance of the car from p, after t seconds, is given by
    s = 12t - 3/2t(2)
    where s is in metres.

    Find the speed of the car as it passes p.


    *facepalm*

    All I have to do is add 0 in for t.

    God, I'm such an idiot.


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    #4
    conroy221
    Closed Accounts Posts: 14 ✭conroy221


    are you sure thats the answer, what year is it, ill have a look at the question and answer


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    #5
    Timans
    Registered Users Posts: 2,370 ✭✭✭Timans


    Just looking at the question now on examinations.ie. The diagram shows that p is right in front of the car, therefore it would pass p at 0 seconds because it is already at p.

    So, sub 0 in for p and you get 12.

    I'm not sure though, let Cokehead make sure, I ain't great at maths.

    Link - http://examinations.ie/archive/exampapers/2005/LC003GLP100EV.pdf


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    #6
    Cokehead Mother
    Closed Accounts Posts: 773 ✭✭✭Cokehead Mother


    s is the distance from p, so when the car is at p, s will equal 0 and so will t.

    You know speed = 12 - 3t

    so at t = 0, speed = 12 - 3(0) = 12.


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    #7
    boobookitty
    Closed Accounts Posts: 232 ✭✭boobookitty


    Cokehead Mother wrote: »
    s is the distance from p, so when the car is at p, s will equal 0 and so will t.

    You know speed = 12 - 3t

    so at t = 0, speed = 12 - 3(0) = 12.

    Thanks very much guys. I musn't be thinking very straight at all. :(


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    #8
    cHaTbOx
    Registered Users Posts: 8,011 ✭✭✭cHaTbOx


    boobookitty wrote: »
    Oh, sorry let me write the full question.

    "The distance of the car from p, after t seconds, is given by
    s = 12t - 3/2t(2)
    where s is in metres.

    Find the speed of the car as it passes p.


    I don't do ordinary but i think t=0 at p. ds/dt = 12-3t so 12-3(0)=12

    and then to find time to stop is ds/dt=0 so 12-3t=0 3t=12 t=4

    and just sub t=4 into s to get distance


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    #9
    conroy221
    Closed Accounts Posts: 14 ✭conroy221


    makes it clearer when you look at the diagram, thats what the leaving does to you, makes you think backwards :P


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    #10
    blue-army
    Registered Users Posts: 1,301 ✭✭✭blue-army


    boobookitty wrote: »
    2005 Paper 1 Q7 part c.

    (Twelve T minus 3 over 2t squared)
    h = 12t - 3/2t(2)

    Find speed. Now I know for speed you differentiate once. However I keep getting 12 - 3t and the answer is 12.

    This is embarassing as Differentiation is supposed to be my second best question. :(
    Cokehead Mother wrote: »
    12 - 3t is the speed at the t seconds.

    Does the question ask for say the initial speed, which is the speed at 0 seconds?
    Believe it or not I did the exact same question yesterday....and spent 10mins trying to work that out! haha:D


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    #11
    blue-army
    Registered Users Posts: 1,301 ✭✭✭blue-army


    boobookitty wrote: »
    2005 Paper 1 Q7 part c.

    (Twelve T minus 3 over 2t squared)
    h = 12t - 3/2t(2)

    Find speed. Now I know for speed you differentiate once. However I keep getting 12 - 3t and the answer is 12.

    This is embarassing as Differentiation is supposed to be my second best question. :(
    by the way.....If you weren't sure in the exam.....and did it twice.....with answers 12 and 9......You would still get full marks ,yeah?


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    #12
    conroy221
    Closed Accounts Posts: 14 ✭conroy221


    i presume they would have to, because you got the right answer, they cant dock you marks for doing the other bit, because your told not to cross out any work you do


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    #13
    boobookitty
    Closed Accounts Posts: 232 ✭✭boobookitty


    Sweet, I can do maths really quick so all I have to do if I'm unsure if do like 3 copies of the question in 3 different methods. :P


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    #14
    Peleus
    Registered Users Posts: 784 ✭✭✭Peleus


    NO! if you do a question twice, ONE of them must be crossed out. They still give you marks if the crossed out one is right but if you write down two answers you don't get it right. or something like that.

    just put a line through it so that its still readable. You are only allowed one answer. not two. if you cross out the right answer you still get full marks.


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    #15
    blue-army
    Registered Users Posts: 1,301 ✭✭✭blue-army


    Peleus wrote: »
    NO! if you do a question twice, ONE of them must be crossed out. They still give you marks if the crossed out one is right but if you write down two answers you don't get it right. or something like that.

    just put a line through it so that its still readable. You are only allowed one answer. not two. if you cross out the right answer you still get full marks.
    That's a bit strange tbh.....can anyone confirm this?

    I heard that they correct both and include the higher marking of the two...


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    #16
    Alvis
    Registered Users Posts: 183 ✭✭Alvis


    blue-army wrote: »
    That's a bit strange tbh.....can anyone confirm this?

    I heard that they correct both and include the higher marking of the two...

    According to the OL marking scheme general guidelines, it states
    8. Unless otherwise indicated in the scheme, accept the best of two or more attempts – even when attempts have been cancelled.

    So I'd say you're right


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    #17
    conroy221
    Closed Accounts Posts: 14 ✭conroy221


    'just put a line through it so that its still readable. You are only allowed one answer. not two. if you cross out the right answer you still get full marks'

    Peleus, how could that possibly be true, your saying that you cant give two answers, but then you say that if you cross out the correct answer and leave the wrong answer uncrossed, you still get full marks? How could you possibly get full marks, because you are officially giving two answers, and if you cross out the right one, you are telling the examiner that you believe the other answer to be the right answer.

    Put it this way, with maths, there are very few hit and miss questions, which means, if you think its not the right answer, but do believe that you have made just a small slip in the working out of the answer, or are on the right track to the answer, then you should get most of the marks.


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