# General Thoughts: Physics (1 Viewer)

#### jordeps

##### New Member
Was 550m a diameter or a radius for the space station thingy?? :O
thanks

#### anomalousdecay

Was 550m a diameter or a radius for the space station thingy?? :O
thanks
It was diameter. I stuffed that up and thought it was radius.

#### MATHmaster

##### Member
Thanks. I really think that most students actually miss out on band 6's due to their extended response structure, lack of cause, effect, reason for every question and the curveball questions (like the photocell one).

Unfortunately you can't do much about curveball's except revise the whole syllabus. Impossible to do with 2 days after chem and still somehow do past papers. can't fit in that much in 15 hours of study. Only 4 past papers and 3 hours of revision.

I guess revising Physics since the trials might not be good enough for a 90, sigh.
The photocell wasn't a curveball. It was just insanely stupid to put it in the syllabus as something that cannot include a solar cell as an example.

#### minagee3

##### New Member
Yeah, I got about 0.163 I think.
With regard to the current, weren't they talking about the force not torque? I originally did the torque equation, but then when I was revising I look at it again and I used F=BILsin(theta). Because it was talking about the force that required the minimum current to lift the mass:/

#### panda15

##### Alligator Navigator
With regard to the current, weren't they talking about the force not torque? I originally did the torque equation, but then when I was revising I look at it again and I used F=BILsin(theta). Because it was talking about the force that required the minimum current to lift the mass:/
They were talking about force, but the force was being provided about an axis of rotation, not directly on the string holding the weight, so the force as a result of the torque would be greater than the force applied directly onto the string, hence why the minimum current is obtained from the torque formula.

#### hayabusaboston

##### Well-Known Member
I fucking wrote radius =225 from 550 lol. No wonder I was off with 46m/s.

But anyyway guys, physics is over, u cant change jack shit, cant believe this thread is still active for so many days haha

#### MATHmaster

##### Member
I fucking wrote radius =225 from 550 lol. No wonder I was off with 46m/s.

But anyyway guys, physics is over, u cant change jack shit, cant believe this thread is still active for so many days haha
Same hahahaha. gg

#### anomalousdecay

The photocell wasn't a curveball. It was just insanely stupid to put it in the syllabus as something that cannot include a solar cell as an example.
In the syllabus, its not very clear if you can or can't use it as an example.
That is a curveball, because not many students actually take the syllabus seriously.

Just gutted that option was too easy. Wanted an option curveball so I could get aligned to 25/25.

#### Hypem

##### Member
Yeah, I got about 0.163 I think.
I had no idea what to do and got this.

I'm a lot happier now..

Edit: Okay I had some idea. But I guessed for T = Fd that d was the radius lol. biggest wing ever

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#### minagee3

##### New Member
They were talking about force, but the force was being provided about an axis of rotation, not directly on the string holding the weight, so the force as a result of the torque would be greater than the force applied directly onto the string, hence why the minimum current is obtained from the torque formula.
yes I agree, though shouldn't they talk about torque then in particular. I know torque is a force, but its vague just to say force. Because both T=nbiacos(theta) and F=nbilsin(theta) are acting on the coil?

#### Hypem

##### Member

Looking back on it and people's results, looking at mid 80s raw. That'd go to 94 right?

#### anomalousdecay

Mid 80's raw would be anywhere between 90-95

#### Hypem

##### Member
Mid 80's raw would be anywhere between 90-95
Wasn't last year's B6 cut off 75?
I can't imagine 85 being the cut off this year

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#### panda15

##### Alligator Navigator
yes I agree, though shouldn't they talk about torque then in particular. I know torque is a force, but its vague just to say force. Because both T=nbiacos(theta) and F=nbilsin(theta) are acting on the coil?
But it didn't say force in the second part of the question. And while I agree that it is vague, it was more of a band 6 type of question, because it required a deep understanding of what torque is, a Newton Moment.

#### anomalousdecay

Wasn't last year's B6 cut off 75?
I can't imagine 85 being the cut off this year
Someone got 90 raw last year for a 96 mark.

So yeah, maybe mid 80's raw would be somewhere between 92 to 95.

#### anomalousdecay

But it didn't say force in the second part of the question. And while I agree that it is vague, it was more of a band 6 type of question, because it required a deep understanding of what torque is, a Newton Moment.
That requires a lot of deep thinking.
However, to an extension maths student, it is easy to recognise that you must use torque, not force, due to the stuff given in the question.
I realised straight away, chuckled thinking that "yes I got a hard question that not many others would get".

#### nifkeh

##### Member
ah i didnt talk about superconductors either, i had planned to, but tbh i didnt have enough space, i filled the whole page with semi conductors, its use, its impact, bla bla
and then i was like...meh should be good enough LMFAO
fuck ==
you could ask for more paper...

#### nifkeh

##### Member
Yes it's definitely related to back EMF. The key thing that they would have wanted here is to recognize the similarity of the DC motor and the DC generator (of course at the level expected here they are identical). So the one machine shifts seamlessly between being a motor and a generator, with the mode of operation depending only upon the relationship between the externally applied voltage and the back EMF.

When the externally applied voltage is greater than the internally generated voltage (aka the back EMF) then the direction of current *and power* is into the DC machine, so it operates as a motor. When the applied voltage is less than the back EMF however, then the direction of current and power is reversed, meaning that the machine is operating as a generator.

It would probably also be good to mention that when acting as a generator, that the direction of power is reversed and therefore that the direction of torque at the wheels must also be reverse. Hence the braking effort.
sigh totally didn't talk about emf at all..