Dario

Member
Originally posted by McLake

So why isn't halway up the next slope ?
At Q, the gradient of the tangent to the rollercoaster is changing the fastest.

Also, I did Sydney Grammar School Trial HSC 2002 as practice and that exam had exactly the same question, so I knew the answer without thinking about it.

NATMAN

Member
HAIYAAAA.... got 2 wrong.... q2 and 4 silly silly silly silly meeeee!

superhubert

Member
another way to think of the rollercoaster one is the centripedal acceleration/force, and how it relates to velocity and g at any given instant.
centrapedal force is almost the same at p,q and r. (actually it's slightly less at p and r because the curve is parabolic). thus the descriminating factor must be either velocity or g. If you remember the slingshot effect you'll realise that the cart only gains angular momentum relative to the outside observer. to a passenger there is no momentum change. (as is the slingshot effect relative to the planet.) thus the descriminating factor must be the relationship of the centrapedal force and g. since g acts vertically downwards, the max g force will be when centrapedial force is acting in the same direction. this is at point Q.
right?

J

juzza

Guest
Originally posted by hk*star
haha damn i think i made up half of them
mc was the hardest part of the test
is copper magnetic??
yeah i only got 1 d as well for answer
Just incae no on answered this, no copper will not work as a magnet but it is a conductor - a dman good one too, so the eddy currents thing will work and the magnet will fall slower down the copper pipe

josie_is_slut

Member
CAN WE COMBINE EVERY FORMULA ON THE FORMULA SHEET TO MAKE ONE SUPER FORMULA, SO WE CAN ANSWER ANY QUESTION POSSIBLE

EXA BOY

Member
with the length contraction time dilation one i stuffed it up 2 start with, but then there was another one in the short answer bit which u actually had 2 work out and i found that the 22m long train was sumthin like 17.6m to the outside observer, then i decided to try simple numbers such as the velocity = 0.9, the c^2s cancel so its l=square root of 1-0.9 which means that 2 an outside observer the train contracts and then i did the same for time and found that the second to the obsever is also longer....

The roller coaster one the carriage thing is completely changing direction so i just knew it was max g-force hehehe no formulas no nuffin, just think about the demon, or even better still the dodgy little beast cause its simple 2 realise that as u climb the hill, gforce isnt as great as wen u come out of the trough bit

Minai

Alumni
I trust spice girl's words, so I got 13/15
I cant believe I got Q1 wrong....bloody hell

josie_is_slut

Member
how is question 2 A??
isnt it B?
coz in every question i done on time dilation the stationary observer always records a longer time than the person inside the spaceship or train or wateva, thats Tv > To and with length contraction it always happens that Lv < Lo.....jus have a look at the time dilation one, the stationary observers always have longer times

josie_is_slut

Member
oh shit, now im thinking i should of put A, my logic is right, so therefore the time is slower to the stationary observer and not faster Tv>To
so the answer is A.......shittt....i just contracdicted myself, the answer is definelty A

zemaj

Member
On question 2

Hmmm... everyone is saying it's a, or b. I'm pretty sure it's c - follow this:

Think of the twins paradox, on the spaceship when the person comes out after they have traveled, then they have aged less than the stationary observer. For this to have happened, time must have run slower if it had run faster, then they would be older.

Thus between a and b, it has to be a, but knowing the Lorenz transformations, it time runs slower (shorter) then length must have increased (lengthened).

Therefore it must be c!

Yeah, so my asnwers are:
1) b
2) c
3) b
4) b
5) d (Ha! Hadn't done any practice q's on these and spent about 10 mins and about 6 attempts to get it out - I remembered in the end though )
6) a
7) a
8) c
9) c
10) b (I'm pretty sure I put this - I've got a tick next to a on my sheet, but I'm pretty sure I changed it)
11) a
12) c
13) c
14) b
15) a

-zem

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kaseita

Member
??
according to the lorenz transformation equations for length, the spaceship would've contracted from the perspective of the observor

Length seen by observor = length seen by passenger x (sqrt(1-v^2/c^2)

The way Jacaranda writes it:

Lv = Lo x (sqrt(1-v^2/c^2)

where Lv is length of object seen by observor
Lo is length of object seen by passenger

where for any value of v, v not equal to c:
length seen by the observor < length seen by passenger

as 0 < sqrt(1-v^2/c^2) < 1

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zemaj

Member
No, Lo is the length on spaceship! Not on observer!

The spaceship is non-inertial!

And if time goes one way, length goes the other....

Anyways, bugger it. I should be doing chem, not this

-zem

superhubert

Member
nup, nup nup. think of it like this. it is harder to reach a speed of c, because as u speed up, time slows down. if it were the other way round there wouldn't be all those theories on tome travel. i.e if u can travel faster than c, the entrapy of time would reverse itself, and u could time travel. as for the length. the easiest way to think of it is with norm forces. if u push on something its gonna contract, not expand. remember your trading length and time for speed..
anyways, putting procrastination aside i'm going to do some major chem cramming...
and after tommorrow i can just add it to my 'pile of perpetual ignorage' just like all my other subjects....
ps zemaj is right, Lo is on the spaceship, but somehow he still got the wrong answer. the ppl who got this wrong prbably also got the relativity q wrong in the second sect....

kaseita

Member
Originally posted by zemaj
No, Lo is the length on spaceship! Not on observer!

The spaceship is non-inertial!

And if time goes one way, length goes the other....

Anyways, bugger it. I should be doing chem, not this

-zem
Lo is the length of the spaceship, according to the person inside the ship, as I presume your saying (from the q)
Lv is the length of the spaceship seen by an outside observor.

just because the subscript says 'o' doesn't mean its saying its the observor!

The question asks what effects on the spaceship are seen, by a stationary observor. Therefore you ARE looking at what the length of the spaceship is seen by the observor.

If you look at that transformation equation, your looking for Lv, not Lo!

To the person inside the spaceship, nothing has changed. Time is exactly the same, and the length of the spaceship is as it was before. Therefore I'm not talking about that person.

Plus the spaceship is inertial. Whilst it may have been in an non-inertial state when it accelerated to that speed, it now travels at a "very high speed" which is an inertial frame of reference.

and I have no idea what you mean by time going one way and length the other, because at high speeds, time is seen to dilate, and length contract. Dilate means go slower. If you look at the Jacaranda, they talk about time dilation and length contraction, not time dilation and length expansion.

-=«MÄLÅÇhïtÊ»=-

Gender: MALE!!!
laz, 10c is wrong coz like u said emf = /t
The greatest change in phi is when the coil experiences the greatest torque, ie. the initial position. So the graph has to start at its max.
Secondly, a one phase ac generate will only have a split ring commutator, so every 360 revolution, the current will change directions twice. In c, the current does that 4 times in 1 revolution.

THEonlyONe!

New Member

i get A for 12, i don't think it can be C as everyone else seems to have chosen...reasoning:

i think the logical answer is C as it has zero resistance, but how can it have more than one resistance for a given temperature...ie: the vertical part!

advise me to the error in my thinking and i will be greatful.

THEonlyONe!

New Member
actually that previous post was just stoooopid, ignore please...i take it all back.

bathtub

New Member

i put down my answer as (c) BUT i felt somewhat inclined to put down (d) , as how can anything ever have absolute 0 resistance???

super conductors are... SUPER conductors, but i cant fathom how they could ever have absolute 0 resistance. the answer has to be (d) if it is to be absolutely correct, however, as i mentioned earlier, for the purposes of the syllabus, i reasoned that they want us to have chosen (c)

does anyone care to correct me???

kaseita

Member
yes

hehe, using normal logic, yes, it would seem impossible that anything can ever have zero resistance. after all, resistance is caused from collisions with the lattice, or with each other (electron bumping into another electron).

however, with superconductors, they have this strange property that allows current to move without resistance. i.e. at some critical temperature, it doesn't collide with the lattice, or with each other. this has of course been proven using experiments, but what was strange was that they believe there would be minimal or no resistance at a 0 degrees kelvin, but instead it happens at a higher, critical temperature.

how is this possible?

the BCR theory (letters of the names of the people who came up with this) is that electron current in a superconductor is of a different form - i.e. it has an extra feature. Rather than moving singularly, in an unordered manner, it instead moves in pairs, called cooper pairs.

The first electron passing through the lattice, causes the lattice to distort. the positively charged lattice is attracted to the first electron, and so bends towards it. when it bends, it releases a phonon, a packet of sound energy. A second electron, that is behind the first, is accelerated towards the first, due to the distorted lattice (attracted to the positive charges, which have themselves, been attracted to the first electron). The energy for it to accelerate, is through the absorption of this phonon. So effectively the first electron has emitted a phonon, which is absorbed by the second, allowing the second electron to accelerate towards the first, and become a "pair"

Of course, this doesn't last long. Cooper pairs continually break and form, and its this pattern that allows the electrons to avoid colliding with the lattice, and with other electrons.