

Exam 07
#1
Posted 16 May 2007 - 03:13 PM
How did everyone find it?
#2
Posted 16 May 2007 - 03:14 PM

Just want a B and i'll be really happy

Any solutions for the paper?

#3
Posted 16 May 2007 - 03:18 PM

#4
Posted 16 May 2007 - 03:28 PM
There were no random uncertainties, and unless I'm mistake, the combined percentage was 5.2% ? Anyone able to confirm this?
#5
Posted 16 May 2007 - 03:34 PM
#6
Posted 16 May 2007 - 03:44 PM
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
Posted 16 May 2007 - 03:49 PM
#8
Posted 16 May 2007 - 03:50 PM
#9
Posted 16 May 2007 - 03:52 PM
AM4R I used (1/2)mr^2 for the moment of inertia of a disc, I'm sure that's right

#10
Posted 16 May 2007 - 03:53 PM
#11
Posted 16 May 2007 - 04:07 PM
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


Not so confident now, maybe got just under 70%. Still, I am praying that I did enough for a C.
#12
Posted 16 May 2007 - 04:22 PM
#13
Posted 16 May 2007 - 04:46 PM
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.
#14
Posted 16 May 2007 - 05:30 PM
#15
Posted 16 May 2007 - 05:32 PM
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?
#16
Posted 16 May 2007 - 05:36 PM
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.
#17
Posted 16 May 2007 - 05:42 PM
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
Posted 16 May 2007 - 06:02 PM
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
Posted 16 May 2007 - 06:13 PM
#20
Posted 16 May 2007 - 06:58 PM
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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