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

1. (Original post by thedon96)
can someone please definitively explain the different degrees of damping? My teacher and textbooks all say different things about heavy, critical and overdamping
I haven't heard about over damping on this spec but critical damping is when it stops the system oscillating after being released from equilibrium and heavy damping is when there is no oscillation at all, it never reaches equilibrium. On a graph it would be a straight line that slopes downwards gradually but doesn't pass the midpoint
2. (Original post by Leonacatherine)
Anyone able to explain why the periodic frequency being in phase with velocity makes it resonate? Why can't it be in phase with displacement

Can anyone give the aqa definition of resonance ?
Not sure if this is what you're looking for but here's a try.

If you think about pushing someone on the swing you want to time your push so that you push them as soon as they momentarily stop when they reach your hand. At that point their velocity is on the verge of switching direction from "towards your hand" to "away from your hand", so when you push right when their velocity changes to "away from your hand" you are pushing in phase with the velocity. (If you plot the graph of displacement vs time for your hand and the swing they'll be the same with possibly different amplitude because your hand has the same period as the swing and is also always at the same direction as the swing so there is no "lag" between you and the swing and the two are consequently in phase.)
3. Hey who here does Section B first? Was wondering whether doing the long questions first would be better because i invest too much time into the multiple choice.
4. (Original post by NEWT0N)
Not sure if this is what you're looking for but here's a try.

If you think about pushing someone on the swing you want to time your push so that you push them as soon as they momentarily stop when they reach your hand. At that point their velocity is on the verge of switching direction from "towards your hand" to "away from your hand", so when you push right when their velocity changes to "away from your hand" you are pushing in phase with the velocity. (If you plot the graph of displacement vs time for your hand and the swing they'll be the same with possibly different amplitude because your hand has the same period as the swing and is also always at the same direction as the swing so there is no "lag" between you and the swing and the two are consequently in phase.)
Hey thanks! I get the swing metaphor but the brackets bit at the end confused me haha

I think I might be overthinking it haha but if you could wouldn't you push them at the top of their swing to make the amplitude even greater?
5. (Original post by thedon96)
can someone please definitively explain the different degrees of damping? My teacher and textbooks all say different things about heavy, critical and overdamping
I think we need to know about light, heavy, critical and overdamping. It's good to know about overdamping even if it's possibly not on the spec (think about polar orbits not being on the spec but you still get questions about them here and there).

Light: The amplitude of oscillation reduces slowly over time.
Heavy: Amplitude of oscillation reduces more quickly over time than for a lightly damped system
Critical: The system doesn't oscillate but returns to its equilibrium position in the shortest possible time (in less than one cycle, due to no oscillation)
Overdamping: The system doesn't oscillate but also doesn't return to its equilibrium position. This is why you would perhaps not want to overdamp a car suspension system (car won't return to equilibrium, which is bad). You can think of this as a pendulum moving through a thick gooey substance from maximum displacement

Those are the general ideas anyway.
6. (Original post by Specter)
Hey who here does Section B first? Was wondering whether doing the long questions first would be better because i invest too much time into the multiple choice.
I'd aim for either 60 long/45 Mc

or 55long/50 Mc

Definitely spend longer on long questions. If worse comes to worse you can always guess the Multiple choice if you run out of time. Not recommended tho just saying
7. (Original post by Leonacatherine)
Hey thanks! I get the swing metaphor but the brackets bit at the end confused me haha

I think I might be overthinking it haha but if you could wouldn't you push them at the top of their swing to make the amplitude even greater?
That's what I meant sorry for wording it confusingly. When I said "when they reach your hand" I meant "when they reach your hand, which you would want to place at the top of their oscillation" if that makes sense.
8. (Original post by Specter)
Hey who here does Section B first? Was wondering whether doing the long questions first would be better because i invest too much time into the multiple choice.
I'd aim for 60long/45Mc

or 55long/50Mc

Definitely spend longer on written section. If worse comes to worse you can guess the multiple choice. Only if you run out of time that is
9. This might be a silly question but why does the MC answer sheet have 50 response boxes
10. (Original post by NEWT0N)
This might be a silly question but why does the MC answer sheet have 50 response boxes
Because it's a general AQA one, not a PHYA4 specific one I think
I'd aim for 60long/45Mc

or 55long/50Mc

Definitely spend longer on written section. If worse comes to worse you can guess the multiple choice. Only if you run out of time that is
Yeah thats how I usually try to split my time. But as soon as I dont get the answer for a MC question I spend ages trying to work it out then realise I have less time for the long answer qs
12. what did everyone here get first time for the aqa june 2014 MC part?
13. (Original post by Kennethm)
what did everyone here get first time for the aqa june 2014 MC part?
I found that the hardest one, got 19/25 but some questions were a fluke. You?
14. (Original post by Kennethm)
what did everyone here get first time for the aqa june 2014 MC part?
14 in January lol

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15. But I redid I recently and got 20, I get between 18-21

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16. (Original post by Kennethm)
what did everyone here get first time for the aqa june 2014 MC part?
When I did it back in December 2014 I got 18. Improved since then thankfully

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17. June 2010 6 marker: "This ismost important for the high current winding (the secondary coil of astep-down transformer)."

18. woah all these replies - I did it a long time ago and did it for the first time today basically going into it without remembering what it was before and I must say this is certainly the most difficult MC I've done. The others pale in comparison and I nearly ace the others...

Im sure its been posted here before but could someone explain Q13 from that paper please? Its here : http://filestore.aqa.org.uk/subjects...1-QP-JUN14.PDF

Cannot wrap my head around it.
19. (Original post by NEWT0N)
June 2010 6 marker: "This ismost important for the high current winding (the secondary coil of astep-down transformer)."

theres more current in that winding and since energy lost is proportional to I^2 (current squared) it means more energy is lost because there is simply more current in it.
20. (Original post by Kennethm)
theres more current in that winding and since energy lost is proportional to I^2 (current squared) it means more energy is lost because there is simply more current in it.
Whoops brain fart, I was thinking V_p/V_s >= I_p/I_s which is clearly wrong.

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