[m1a1tank]

paul, for 11, im not sure how to do it calculation wise. but i just used logic. 24 hours is us now. So its not that. 160 hours is too long. 4.5x10^-2 would be too short. so by process of elimination i would go for 1.4 hours. But if you find out how to do it please let me know

[don'tTRIP.]

The keyword here is 'weight'.

Rearrange mg = mrw^2 and there's your answer

[swert]

can some please help me on this question im stuck on its from a legacy paper june 2007 question 9. I dont even know how to approach it :/

[FrightBright]

Hey guys I got less than 90% in my ISA. My teacher says I cannot get an A* because I need 90% + in all 3 units seperately..

Is he mistaken? I mean surely if I get 90% overall I get an A*.

Can someone confirm thanks.

[schizopear]

(Original post by FrightBright)
Hey guys I got less than 90% in my ISA. My teacher says I cannot get an A* because I need 90% + in all 3 units seperately..

Is he mistaken? I mean surely if I get 90% overall I get an A*.

Can someone confirm thanks.

you need a 90% average in the A2 modules and an A grade overall - not 90% in each module.

[number23]

for capacitors, why does the current fall to zero during charging and discharging?

also, what is the actual difference between charge, current and voltage? thnx

[number23]

also, can anyone explain ac generators and all that sine/cos stuff for emf and flux linkage? or have any good videos explaining it?

[FrightBright]

(Original post by swert)
can some please help me on this question im stuck on its from a legacy paper june 2007 question 9. I dont even know how to approach it :/

Do you know where I can find these legacy papers?

Cheers.

[swert]

(Original post by FrightBright)
Do you know where I can find these legacy papers?

Cheers.

http://www.freeexampapers.com/past_p...A%2FPhysics+A/
here knock yourself out oh if do mange to solve the question i was on about do let me know how to do it

[callmenighthawk]

(Original post by don'tTRIP.)
The keyword here is 'weight'.

Rearrange mg = mrw^2 and there's your answer

what? are you saying that you just get rid of the m in mrw^2? if mg is zero (the weight) then mrw^2 will equal zero..

To the guy asking about question 11, 21 and 22

I'm quite unsure about the quesiton 21, i don't think a flux linkage can decrease? as long as it is being turned flux linkage/emf will be induced, and the question doesn't state whether the emf is positive or not

someone help me out here !

edit : Q 11 was B, the guy who did it by common sense got it right
21 is B, the flux linkage increases. unless they gave you a value for the emf (which they wont in these questions) then it will increase
22 is D, 9 x 10-1 (0.9) - did by (150 x 10 x 60) area x b field

[don'tTRIP.]

(Original post by callmenighthawk)
what? are you saying that you just get rid of the m in mrw^2? if mg is zero (the weight) then mrw^2 will equal zero..

To the guy asking about question 11, 21 and 22

I'm quite unsure about the quesiton 21, i don't think a flux linkage can decrease? as long as it is being turned flux linkage/emf will be induced, and the question doesn't state whether the emf is positive or not

someone help me out here !

First you cancel out the m's so your left with g = rw^2

Then you have g = r(2pi/T)^2

Rearrange to get t on it's own; this is in seconds

Divide your answer by 3600 to get it into hours and bazinga

[callmenighthawk]

(Original post by don'tTRIP.)
First you cancel out the m's so your left with g = rw^2

Then you have g = r(2pi/T)^2

Rearrange to get t on it's own; this is in seconds

Divide your answer by 3600 to get it into hours and bazinga

thanks man.

anyone else finding the student room threads on exams to be very helpful? points out where you don't know certain methods/understanding of a subject rather than the physics itself, I feel like all of us can do the physics it's just the communication/looking at the question in the right way.

[callmenighthawk]

(Original post by swert)
http://www.freeexampapers.com/past_p...A%2FPhysics+A/
here knock yourself out oh if do mange to solve the question i was on about do let me know how to do it

that is very difficult, you're on about the capacitors one?

I tried this : V = Vo e^-t/rc

ln (1/2) / 0.035 = constant (RC)

then did ln (1/16) x constant(rc) to get a time of 54 seconds lolz

it's clearly C or D but I can't do the math..

ok i've rearranged wrong it should be RC = 0.035 / ln 1/2

still getting a answer not on the scheme


wow that is a tough question I think I worked it out

E = 1/2 QV when Q halves then V halves

when e = 1/16 it must equal 0.5 x charge x voltage

1/16 / 0.5 square rooted = 0.35 volts

to find RC you do 0.035 / ln (1/2) to get 0.05 or something

then you do ln (0.35 / 1) x 0.05 to get 51ms, apparently it isn't either C or D if what I did was right


surely they don't expect you to do all that for one mark?

[FrightBright]

(Original post by swert)
http://www.freeexampapers.com/past_p...A%2FPhysics+A/
here knock yourself out oh if do mange to solve the question i was on about do let me know how to do it

Thanks capacitance is constant.

Half life is 36ms.

Energy strored is 1/2(Q)(V^2)

So you want the voltage to drop to 1/4(V) because when you square it it's 1/16.

So just 36x2.

=72 ms.

-Oh callum the reason you make you feel weightless is because all your weight is being used for centriptal force. So mrw-^2=mg kinda like when your on top of a ride and you feel weightless.

+ your method for finding the E/16 just saw it.. Its longer if you do it that way.. when I did it i got 51 as time constant.

[callmenighthawk]

(Original post by FrightBright)
Thanks capacitance is constant.

Half life is 36ms.

Energy strored is 1/2(Q)(V^2)

So you want the voltage to drop to 1/4(V) because when you square it it's 1/16.

So just 36x2.

=72 ms.

-Oh callum the reason you make you feel weightless is because all your weight is being used for centriptal force. So mrw-^2=mg kinda like when your on top of a ride and you feel weightless.

+ your method for finding the E/16 just saw it.. Its longer if you do it that way.. when I did it i got 51 as time constant.

so which answer is right ?

[don'tTRIP.]

Should I take g as 9.8 or 9.81? The mark schemes are so inconsistent

[FrightBright]

(Original post by callmenighthawk)
so which answer is right ?

B-72 ms

51 is time constant which is RC
Thats not the same as t

[Next Level]

I don't get resonance, as in why the driving freq must equal the natural one, why it's 90 degrees out of phase, all this business of forced and free vibrations etc...any good videos on the topic?

[FrightBright]

(Original post by Next Level)
I don't get resonance, as in why the driving freq must equal the natural one, why it's 90 degrees out of phase, all this business of forced and free vibrations etc...any good videos on the topic?

Its in phase with velocity which is 90 degrees out of phase with displacement.

look at graphs pg 36 nelson thornes

Hope that helps.

[callmenighthawk]

(Original post by FrightBright)
B-72 ms

51 is time constant which is RC
Thats not the same as t

right, my teacher told me it's always best to use 1/2CV^2 when doing calculations.

yeah I thought I understood why it was 90 degree out of phase before because I said Force = mass x velocity, when it's mass x acceleration.

I know it's to oppose the natural restoring force but I don't know how to word it.