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

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    (Original post by Paul Dirac)
    I would say B: Clearly momentum doesn't change. For the kinetic energy, we know that the collision will slow down their overall speed (clearly they can't travel together at more than or equal to the speed of the first car!). But their mass is increased. kinetic energy is proportional to mv^2, where m gets bigger and v gets smaller, as we just showed. But v is squared, so v getting smaller causes more affect overall, so mv^2 get smaller overall. Hence the kinetic energy decreases. So: momentum is unchanged, kinetic energy decreases --> option B.
    Omg I remember thinking this when I first did this question 😁😁

    Thanks for reminding me!! I'm sure there will be a more logical way of doing it but I'm happy with that


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    It's quite confusing if you do it in a mechanics maths way. That's obviously why they don't use numbers with it


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    (Original post by gcsestuff)
    It's quite confusing if you do it in a mechanics maths way. That's obviously why they don't use numbers with it


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    I am good with numbers i dont like ratii and when something is proportional to tbe other etc

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    (Original post by a.a.k)
    I am good with numbers i dont like ratii and when something is proportional to tbe other etc

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    Both come from the same thing. I prefer neither. Before the mathematics of it is derived you have got to use your reasoning then to derive the mathematics of it. Unfortunately the education goes the other round


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    (Original post by Mehrdad jafari)
    Both come from the same thing. I prefer neither. Before the mathematics of it is derived you have got to use your reasoning then to derive the mathematics of it. Unfortunately the education goes the other round


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    Well i am not good with reason but facts

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    (Original post by a.a.k)
    Well i am not good with reason but facts

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    Facts are obtained by the process of reasoning


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    (Original post by a.a.k)
    I am good with numbers i dont like ratii and when something is proportional to tbe other etc

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    (Original post by gcsestuff)
    It's quite confusing if you do it in a mechanics maths way. That's obviously why they don't use numbers with it


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    Not too hard actually, I just found a way to do it.

    Let the mass of the first truck be m and let their combined mass be M. Let the speed of the first truck before the collision be v, and let their speed together after the collision V.

    Momentum is conserved so mv=MV, i.e. V=mv/M.

    Kinetic energy of first truck before collision: 0.5mv^2

    Kinetic energy of both trucks together: 0.5MV^2=0.5M(mv/M)^2=0.5m^2v^2/M.

    We want to find out which of these kinetic energies is larger.

    First one is 0.5v^2*m, second is 0.5v^2(m^2/M), so we can ignore the factors of 0.5v^2 and concentrate on comparing the sizes of m and m^2/M. But M>m, because M=combined mass of m and the other truck. Therefore m^2/M<m^2/m=m. So the second kinetic energy is less than the first one, i.e. kinetic energy decreases.
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    (Original post by Mehrdad jafari)
    Facts are obtained by the process of reasoning


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    Did i say before i wasnt good with words. I think did. Si plz excuse me i dont know what i am saying

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    (Original post by Paul Dirac)
    Not too hard actually, I just found a way to do it.

    Let the mass of the first truck be m and let their combined mass be M. Let the speed of the first truck before the collision be v, and let their speed together after the collision V.

    Momentum is conserved so mv=MV, i.e. V=mv/M.

    Kinetic energy of first truck before collision: 0.5mv^2

    Kinetic energy of both trucks together: 0.5MV^2=0.5M(mv/M)^2=0.5m^2v^2/M.

    We want to find out which of these kinetic energies is larger.

    First one is 0.5v^2*m, second is 0.5v^2(m^2/M), so we can ignore the factors of 0.5v^2 and concentrate on comparing the sizes of m and m^2/M. But M>m, because M=combined mass of m and the other truck. Therefore m^2/M<m^2/m=m. So the second kinetic energy is less than the first one, i.e. kinetic energy decreases.
    Yeah just followed what you did and it works. Thanks!!


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    (Original post by gcsestuff)
    Yeah just followed what you did and it works. Thanks!!


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    I would probably do this way in an exam to make sure.
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    Love how this forums is nocturnal


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    (Original post by gcsestuff)
    Love how this forums is nocturnal


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    I had to google what noctornul means lol

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    Name:  ImageUploadedByStudent Room1433633512.348967.jpg
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    Sorry for so many questions.

    Do I have the right idea about this?


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    (Original post by gcsestuff)
    Name:  ImageUploadedByStudent Room1433633512.348967.jpg
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    Sorry for so many questions.

    Do I have the right idea about this?


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    Can you post the full Q
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    Anyone know if there's solomon papers for physics?
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    (Original post by Paul Dirac)
    Anyone know if there's solomon papers for physics?
    Dont think so

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    (Original post by a.a.k)
    Dont think so

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    I thought so, quite sad

    olympiad ones are too hard
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    (Original post by Paul Dirac)
    I thought so, quite sad
    Well...its life

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    (Original post by Paul Dirac)
    Can you post the full Q
    Name:  ImageUploadedByStudent Room1433633814.389064.jpg
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    It's in the text book. And then this is an example questions

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    Part C

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    That's how I thought you did it but it's not


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    (Original post by Paul Dirac)
    I thought so, quite sad

    olympiad ones are too hard
    Have you done the old syllabus questions ??


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