Just did the exam. I thought it was a bit easier than expected so hoping for an A or a B.
How did everyone find it?
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Exam 07
Started by kingding, May 16 2007 03:13 PM
27 replies to this topic
#2
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Posted 16 May 2007 - 03:14 PM
Yeah same here, was easier than expected, couldn't do random uncertainties though! 
Just want a B and i'll be really happy
Any solutions for the paper?
Just want a B and i'll be really happy
Any solutions for the paper?
#3
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Posted 16 May 2007 - 03:18 PM
lol good good. Were there any random uncertainties? I thought there was only absolute uncertainties... Cant remember fully what i got for the questions. Wouldnt count on them being right anyway.. 
#4
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Posted 16 May 2007 - 03:28 PM
Thought that was much easier than I expected. Investigation was rubbish, but I got at least 70% in that exam, so hopefully I'll get the C I need.
There were no random uncertainties, and unless I'm mistake, the combined percentage was 5.2% ? Anyone able to confirm this?
There were no random uncertainties, and unless I'm mistake, the combined percentage was 5.2% ? Anyone able to confirm this?
#5
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Posted 16 May 2007 - 03:34 PM
Thought it was just absolute uncertainties - so i got 75 +/- 4 m/s i think. Not sure what the percentage uncertainty was. Nice one, 70% is a C or better for definite...
#6
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Posted 16 May 2007 - 03:44 PM
QUOTE(kingding @ May 16 2007, 04:34 PM) 
Thought it was just absolute uncertainties - so i got 75 +/- 4 m/s i think. Not sure what the percentage uncertainty was. Nice one, 70% is a C or better for definite...
Yeah, but have to include investigation which was pretty poor, though I worte it up with a view to getting as many marks as possible. No more than 12 out of 25 though, possibly only 7 or 8.
It told you the uncertainties and asked you to get the absolute uncertainty in the speed, so I worked out the percentage uncertainty for the two readings, combined them, and got 5.2%. I then took 5.2% of 75, which is 3.9.
#7
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Posted 16 May 2007 - 03:49 PM
Yeh got that then, couldnt remember the percentage, but i got that answer. My investigation was utter gash also, but i reckon iv done enough to maybe get 14 marks worth. Hoping that the written paper was good enough like.
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Posted 16 May 2007 - 03:50 PM
everyone thought it was quite good, For the inertia questions did you use 1/2 mr^2?
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#9
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Posted 16 May 2007 - 03:52 PM
I thought that paper was really easy compared to the past papers.
AM4R I used (1/2)mr^2 for the moment of inertia of a disc, I'm sure that's right
QUOTE(David_07 @ May 16 2007, 04:44 PM)
It told you the uncertainties and asked you to get the absolute uncertainty in the speed, so I worked out the percentage uncertainty for the two readings, combined them, and got 5.2%. I then took 5.2% of 75, which is 3.9.
Aye, that's what I got too. Then for the next part the absolute uncertainty wasn't increased as although the percentage uncertainty decresases but the speed of the wave increases and it actually works out as a bigger absolute error.AM4R I used (1/2)mr^2 for the moment of inertia of a disc, I'm sure that's right
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Posted 16 May 2007 - 03:53 PM
oh and did you all use the value for mean radius of moon orbit for most of the earthy type questions
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#11
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Posted 16 May 2007 - 04:07 PM
QUOTE(ad absurdum @ May 16 2007, 04:52 PM)
I thought that paper was really easy compared to the past papers.
AM4R I used (1/2)mr^2 for the moment of inertia of a disc, I'm sure that's right
QUOTE(David_07 @ May 16 2007, 04:44 PM)
It told you the uncertainties and asked you to get the absolute uncertainty in the speed, so I worked out the percentage uncertainty for the two readings, combined them, and got 5.2%. I then took 5.2% of 75, which is 3.9.
Aye, that's what I got too. Then for the next part the absolute uncertainty wasn't increased as although the percentage uncertainty decresases but the speed of the wave increases and it actually works out as a bigger absolute error.AM4R I used (1/2)mr^2 for the moment of inertia of a disc, I'm sure that's right
Argh! Forgot it was a disc, I just used the normal one
Should only lose one or 2 marks for that though? I also fluffed that uncertainty bit, I said the wavelength decreased when you increased the frequency, but of course that is not the case for a standing wave 
Not so confident now, maybe got just under 70%. Still, I am praying that I did enough for a C.
#12
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Posted 16 May 2007 - 04:22 PM
You'll lose one mark max for forgetting the (1/2). I might be wrong on the standing wave part, it was 50/50 between whether I went for that or the wavelength decreasing.
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Posted 16 May 2007 - 04:46 PM
QUOTE(ad absurdum @ May 16 2007, 05:22 PM)
You'll lose one mark max for forgetting the (1/2). I might be wrong on the standing wave part, it was 50/50 between whether I went for that or the wavelength decreasing.
Might be OK then. For the very last part (What would you do to reduce the absolute uncertainty) I said I would increase the mass (there was a mass on the end of the string in the diagram) which would increase the wavelength, while at the same time increasing the frequency. Maybe I shouldn't have bothered with the frequency part, but I'm sure increasing the mass is a valid method.
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Posted 16 May 2007 - 05:30 PM
I thought that exam was ok. I think if it was based on that paper alone I'd get an A. The fact my teacher didnt seem to know much about the investigations worries me on that front.
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Posted 16 May 2007 - 05:32 PM
What about the SHM question? What were your answers for that?
Oh yeh, and the gravitation one - you get a negative potential energy and therefore a negative total energy for the moon? Or is that totally wrong?
Oh yeh, and the gravitation one - you get a negative potential energy and therefore a negative total energy for the moon? Or is that totally wrong?
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Posted 16 May 2007 - 05:36 PM
QUOTE(kingding @ May 16 2007, 06:32 PM)
What about the SHM question? What were your answers for that?
I'm not too great at SHM. What did you put for the equation which described the piston's displacement? I got the max acceleration to be very high, something like 19000, and the max force was then 9000-odd. Max Ek I can't remember, but I used the equation Ek=0.5mw^2(A^2-y^2) with y = 0.
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Posted 16 May 2007 - 05:42 PM
For the SHM: maximum acceleration = 3.95 x 10^4
maximum force = 1.9 x 10^4
maximum Ek = 9.5 x10^2
For the Gravitation: Gravitational Force = 1.98 x 10^20
Tangential Speed = 1020.6 m/s
Potential of moon = -7.6 x 10^28
Total energy of moon = -3.8 x 10^28
Escape velocity = 2.2 x 10^4
maximum force = 1.9 x 10^4
maximum Ek = 9.5 x10^2
For the Gravitation: Gravitational Force = 1.98 x 10^20
Tangential Speed = 1020.6 m/s
Potential of moon = -7.6 x 10^28
Total energy of moon = -3.8 x 10^28
Escape velocity = 2.2 x 10^4
#18
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Posted 16 May 2007 - 06:02 PM
QUOTE(kingding @ May 16 2007, 06:42 PM)
For the SHM: maximum acceleration = 3.95 x 10^4
maximum force = 1.9 x 10^4
maximum Ek = 9.5 x10^2
For the Gravitation: Gravitational Force = 1.98 x 10^20
Tangential Speed = 1020.6 m/s
Potential of moon = -7.6 x 10^28
Total energy of moon = -3.8 x 10^28
Escape velocity = 2.2 x 10^4
maximum force = 1.9 x 10^4
maximum Ek = 9.5 x10^2
For the Gravitation: Gravitational Force = 1.98 x 10^20
Tangential Speed = 1020.6 m/s
Potential of moon = -7.6 x 10^28
Total energy of moon = -3.8 x 10^28
Escape velocity = 2.2 x 10^4
Got all the exact same or close enough answers for gravitation, I wasn't sure about the Energy of the moon though, I didn't think it could be a negative number as energy is scalar and can't be negative. (?).
How did you work out the max Acceleration? I used a=-w^2y , with y taken as -0.05m (the value which gives the greatest a).
#19
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Posted 16 May 2007 - 06:13 PM
What did you get for the acceleration? I think i did it that way. Yeah, i wasnt sure about the energy either, i think its wrong. What did everyone else get?
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Posted 16 May 2007 - 06:58 PM
QUOTE(kingding @ May 16 2007, 07:13 PM)
What did you get for the acceleration? I think i did it that way. Yeah, i wasnt sure about the energy either, i think its wrong. What did everyone else get?
For the acceleration I got 1.9x10^4 or something around that. As I said I used y = -0.05 (acceleration is greatest at this point), and I worked out w using w = 2(Pi)f as it mentioned the frequency was 100Hz. So w = 628 and a =-w^2 x -0.05 = 19740ms-2
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