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AQA Physics PHYA4 - Thursday 11th June 2015 [Exam Discussion Thread] Watch

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    (Original post by CD223)
    Usually I'm okay with these questions, but this one has stumped me:

    Jan 2013 - any takers?

    Attachment 401113


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    How about this?

    First pair of equations are given ratios.
    Second pair of equations are the relevant equations, replacing the other constants with k and rearranging for k to form the next equations nicely.
    The last pair of equations are from making the k equations equal which is true since the original ratios are substituted in at the same time.
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    Guys I'm looking for this specific revision material which was uploaded to dropbox or some other site. It had revision materials containing both As and A2 physics with topic heading and such, anybody know where I can find it?
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    (Original post by Mehrdad jafari)
    It was meant to be something like this even thought i used my own results which give me an inaccurate mass of the DVD. X is the mass of the DVD
    Attachment 401149


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    If you take my expression and your expression with a mass of 200g you get the same answers.

    In my answer I used both the 3.1% increase and 3.3% increase which both ended up with answers ~13g.

    The 3.1% equation produced:
    12.5922g

    The 3.3% equation produced:
    13.4178g

    Averaging the two results gives a mass of the DVD of 13.005g.


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    (Original post by Somniare)
    using M = Mass of moon, m = mass of earth, R = radius of moon and r = radius of earth

    Gm/(r^2) = 6 GM/(R^2) (gravitational field strength)

    0.6 m/(4/3*pi*r^3) = M/(4/3*pi*R^3) (density)

    We can then cancel out to get

    m/r^3 = 0.6 M/R^3
    m/r^2 = 6 M/R^2

    Dividing the second set by the first set gives

    r = 6/0.6R
    r/R = 10
    Ah but it says the answer is 3.6 :/


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    (Original post by JayCS)
    How about this?

    First pair of equations are given ratios.
    Second pair of equations are the relevant equations, replacing the other constants with k and rearranging for k to form the next equations nicely.
    The last pair of equations are from making the k equations equal which is true since the original ratios are substituted in at the same time.
    Thank you!


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    (Original post by CD223)
    Ah but it says the answer is 3.6 :/


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    Oh dear >< again I don't know what I'm doing wrong...Although JayCS' method seems unassailable.
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    (Original post by CD223)
    If you take my expression and your expression with a mass of 200g you get the same answers.

    In my answer I used both the 3.1% increase and 3.3% increase which both ended up with answers ~13g.

    The 3.1% equation produced:
    12.5922g

    The 3.3% equation produced:
    13.4178g

    Averaging the two results gives a mass of the DVD of 13.005g.


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    Oh right. Then yeah. I though of doing that too in the exam but i scrabbled so much that i ended up having no space left for the answer, lol


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    (Original post by Somniare)
    Oh dear >< again I don't know what I'm doing wrong...Although JayCS' method seems unassailable.
    I made a mistake too thinking it was 10. I'd have never have got the correct answer in the real thing.


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    (Original post by Mehrdad jafari)
    Oh right. Then yeah. I though of doing that too in the exam but i scrabbled so much that i ended up having no space left for the answer, lol


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    Ah I was in a similar position Hahha!

    Be interesting to see how the MS does it if there were multiple methods.


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    (Original post by CD223)
    Ah I was in a similar position Hahha!

    Be interesting to see how the MS does it if there were multiple methods.


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    Yeah exactly!


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    Maybe they will accept different approaches for the DVD question. After all, each of the approaches on here seem to get the correct answer of 13g.
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    (Original post by Somniare)
    using M = Mass of moon, m = mass of earth, R = radius of moon and r = radius of earth

    Gm/(r^2) = 6 GM/(R^2) (gravitational field strength)

    0.6 m/(4/3*pi*r^3) = M/(4/3*pi*R^3) (density)

    We can then cancel out to get

    m/r^3 = 0.6 M/R^3
    m/r^2 = 6 M/R^2

    Dividing the second set by the first set gives

    r = 6/0.6R
    r/R = 10
    Hi there.

    Your method works; I think you just made a mistake on the first equation you formed from cancelling out.

    I get that it is m/r^3=5M/3R^3 i.e. 1/0.6 not 0.6 on the RHS.

    The rest worked out from there.
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    (Original post by JayCS)
    Hi there.

    Your method works; I think you just made a mistake on the first equation you formed from cancelling out.

    I get that it is m/r^3=5M/3R^3 i.e. 1/0.6 not 0.6 on the RHS.

    The rest worked out from there.
    Oh yes of course! Thank you! =D
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    (Original post by JayCS)
    Hi there.

    Your method works; I think you just made a mistake on the first equation you formed from cancelling out.

    I get that it is m/r^3=5M/3R^3 i.e. 1/0.6 not 0.6 on the RHS.

    The rest worked out from there.
    Do you use the same approach for all ratio questions?
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    (Original post by CD223)
    Do you use the same approach for all ratio questions?
    Hey.

    I use this format for questions that require more than one ratio to be used -- in this case, for gravitational field strength and density.

    For other ratio questions, I just form one ratio equation such as below.

    Name:  Capacitance Question.png
Views: 112
Size:  22.2 KB

    I don't use either method that often though since these questions don't seem to be that common; I end up using proportions for multiple choice most of the time.
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    CD223,

    Here's the set of questions =D Some are past questions, some are ones they think are likely to come up.
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    (Original post by JayCS)
    Hey.

    I use this format for questions that require more than one ratio to be used -- in this case, for gravitational field strength and density.

    For other ratio questions, I just form one ratio equation such as below.

    Name:  Capacitance Question.png
Views: 112
Size:  22.2 KB

    I don't use either method that often though since these questions don't seem to be that common; I end up using proportions for multiple choice most of the time.
    Cool! thanks. How are you doing on past papers?
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    (Original post by Somniare)
    CD223,

    Here's the set of questions =D Some are past questions, some are ones they think are likely to come up.
    Thanks for sharing!


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    (Original post by CD223)
    Cool! thanks. How are you doing on past papers?
    PHYA4 papers are good to do since PHYA4 was the first unit we learned. I suspect it's the same for most people. I've only just starting doing PHYA5 papers though since there aren't that many to do.

    Yourself?
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    (Original post by JayCS)
    PHYA4 papers are good to do since PHYA4 was the first unit we learned. I suspect it's the same for most people. I've only just starting doing PHYA5 papers though since there aren't that many to do.

    Yourself?
    Yeah me too - I feel the lack of PHYA5 papers will make me very nervous and unprepared come the exam.


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