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    (Original post by Boy_wonder_95)
    Do papers instead!
    Finished papers from 01 - 12. Got the jan 13 left, so planning to do that as a mock later on today. Ii need to revisit some M1 stuff though.
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    (Original post by StUdEnTIGCSE)
    Well they have asked to find the vertical component of the force acting on the rod at A.
    Now the rod is in equilibrium so upward forces=downward forces.

    One upward force is the component of F upwards. Now if we draw a horizontal at B (where F acts) you will get an angle F makes with the horizontal. Then draw the vertical and it will strike you why they use Fcos75 (or Fsin15)

    Hope you get it.
    oh yeah! gosh! that was so easy! thanks a lot!
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    Really quick question, I think it was the June 2012 paper and there was a question where you had to find the distance a trailer travells up a slope after the bar towing it gets broken. When working out the work done by resitive forces I put the force as friction and weight added up, it turns out you only had to use friction. why is this?
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    (Original post by Zaphod77)
    I'm doing well in the papers, so it seems to be going well for me! It's not failed me yet This is why I was saying about checking the M1 way though, it's a way of ensuring you put WD on the right side! But again, only do that if you have time at the end - it's better to get to the end of the paper than get 1 question completely right but not manage 2 easy questions worth loads of marks
    Okay, I'll keep this in mind for tomorrow and I normally have 30 minutes remaining to check through so I'll definitely do this as a back up, thanks for your help


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    (Original post by Callumhamby)
    Really quick question, I think it was the June 2012 paper and there was a question where you had to find the distance a trailer travells up a slope after the bar towing it gets broken. When working out the work done by resitive forces I put the force as friction and weight added up, it turns out you only had to use friction. why is this?
    Remember you only multiply by external forces, weight is not an external force!
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    ONLY READ THIS IF YOU HAVE DIFFICULTY WITH SIGNS IN CONSERVATION OF ENERGY OR WORK-ENERGY QUESTIONS

    (It's a different way of looking at those laws. I think it's better, but I don't want to confuse anyone who is happy with the method in the text books.)

    Spoiler:
    Show

    The textbooks don't do CofE/WEP very well. (The current Heinemann book isn't quite as bad, but the previous one was horrendous - it did almost every example a different way, and if you used any of the methods on the wrong example, you got the wrong answer!)

    It's actually quite easy if you forget about all this "change of KE" , "change of GPE" , "change of EPE" nonsense and just think about the total energy of the system.

    The final total energy (of all sorts - so KE and GPE, and EPE if you are doing M3) is equal to the initial total energy plus any gains minus any losses.

    In M2 and M3 questions, gains and losses only come from work done by external forces. If a force is helping the motion (i.e. is in the direction of motion or within 90 degrees of it), it is doing positive work on the system and causes a gain in energy. If it is opposing the motion (i.e. opposite to the direction of motion, to within 90 degrees ) it is doing negative work and causes a loss of energy. You calculate the work done using component force times distance. (Except possibly for a force of known power, e.g. the driving force provided by on engine, where you can use power times time.)

    However, you don't count work done by or against weights; we take account of gravity by putting in GPE instead.

    In an M2 or M3 question*, friction always opposes the motion and can only cause a loss of energy, and it will do so whenever the body slides.

    Cheers, Ken

    * It's possible for a moving body to gain energy because of friction with another moving body, but I'm pretty sure you won't get that in M2 or M3. Even then, a system including both bodies can still only lose energy because of the friction, and will do so whenever sliding occurs.
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    (Original post by MathsNerd1)
    Okay, I'll keep this in mind for tomorrow and I normally have 30 minutes remaining to check through so I'll definitely do this as a back up, thanks for your help


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    No problem Good luck! Hopefully it'll be an exam we both agree with
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    (Original post by Zaphod77)
    No problem Good luck! Hopefully it'll be an exam we both agree with
    Yeah and I'm hoping this reserve paper isn't as challenging as Jan 2010 or a few questions of June 2012, I'll be happy with any other style really

    Anyone got an predictions of questions that might come up?


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    (Original post by MathsNerd1)
    Yeah and I'm hoping this reserve paper isn't as challenging as Jan 2010 or a few questions of June 2012, I'll be happy with any other style really

    Anyone got an predictions of questions that might come up?


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    I was thinking the same, if the paper does not turn out to be anything like june12/jan10 i think i can cope. PLEASE EDEXCEL BE NICE.
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    I have a feeling that a COM question will include asking for the angle before the shape begins to topple as I haven't seen this come up once yet, any other suggestions?


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    (Original post by MathsNerd1)
    Yeah and I'm hoping this reserve paper isn't as challenging as Jan 2010 or a few questions of June 2012, I'll be happy with any other style really

    Anyone got an predictions of questions that might come up?


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    Everything's coming up.

    Projectiles, variable acceleration, finding COM, COM being hung, Work Energy and Power Principle, Easy collisions then slightly more involved collisions, and finding tension/thrust in moments and finding reaction and direction at A .

    In which order they come in, we don't know
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    (Original post by MLogan)
    Remember you only multiply by external forces, weight is not an external force!
    Actually, it is. But there is a special rule for weights. Instead of putting in work by or against weights, we put in a corresponding sort of potential energy. (It would give exactly the same answers if we left out GPE and put in the work done by or against weights.)

    The same choice is available for any "conservative force", not just weight. It you do this sort of thing at university you might come across electrical potential energy, magnetic potential energy, etc. If you put those into your equations, you leave out work done by or against electrostatic forces or magnetic forces.
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    (Original post by Boy_wonder_95)
    Everything's coming up.

    Projectiles, variable acceleration, finding COM, COM being hung, Work Energy and Power Principle, Easy collisions then slightly more involved collisions, and finding tension/thrust in moments and finding reaction and direction at A .

    In which order they come in, we don't know
    I don't see why the toppling thing wouldn't come up as its never been on any yet, as far as I know anyway. Plus there is a collisions question that asks to work out the total distance when a ball is dropped and it rebounds a lot of times. I don't think I've seen that yet either.

    I'd rather just have a normal straightforward paper where they already supply the diagrams to the questions, as I'm too lazy


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    (Original post by MAD Phil)
    Actually, it is. But there is a special rule for weights. Instead of putting in work by or against weights, we put in a corresponding sort of potential energy. (It would give exactly the same answers if we left out GPE and put in the work done by or against weights.)

    The same choice is available for any "conservative force", not just weight. It you do this sort of thing at university you might come across electrical potential energy, magnetic potential energy, etc. If you put those into your equations, you leave out work done by or against electrostatic forces or magnetic forces.
    I didn't get any of that :| But i guess i don't need to know that for this exam?
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    (Original post by MathsNerd1)
    I don't see why the toppling thing wouldn't come up as its never been on any yet, as far as I know anyway. Plus there is a collisions question that asks to work out the total distance when a ball is dropped and it rebounds a lot of times. I don't think I've seen that yet either.

    I'd rather just have a normal straightforward paper where they already supply the diagrams to the questions, as I'm too lazy


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    As in you'd have to use geometric series for total distance dropped? :O
    I'll go over toppling again now
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    (Original post by MLogan)
    I didn't get any of that :| But i guess i don't need to know that for this exam?
    Yeah, relax!
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    (Original post by MathsNerd1)
    I have a feeling that a COM question will include asking for the angle before the shape begins to topple as I haven't seen this come up once yet, any other suggestions?


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    hmmmm Thanks for mentioning that, need to go over it. I think maybe pulleys might come up <<Will dread it!!
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    (Original post by Boy_wonder_95)
    As in you'd have to use geometric series for total distance dropped? :O
    I'll go over toppling again now
    Yeah, it's as a example in the book but I'd prefer if it didn't appear tomorrow, just in case it does I'll be ready for it and so will you now


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    (Original post by MLogan)
    hmmmm Thanks for mentioning that, need to go over it. I think maybe pulleys might come up <<Will dread it!!
    In what way could they come up?


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    (Original post by MathsNerd1)
    In what way could they come up?


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    To work out P.E gain, K.E loss. Like the questions on pg 96 qs 46
 
 
 
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