(Original post by teachercol)
OCR Physics A G484 NW 24 Jan 2012
Usual disclaimers - these are purely my answers and are in no sense official.
They are just to give you an idea of how well you did.
They may contain errors and typos.
They are not intended to be a complete set of every possible answer.
Generally I give answers to 3sf - but 2sf is always Ok.
Q1 a) i) Linear momentum = mass x velocity (1)
ii) Momentum is a vector because - velocity is a vector and a scalar (mass) x a vector is always a vector
or because it has a direction or because you need to use vector addition when adding . (2)
b) i) 1 a=dv/dt = -7.5/0.28 = (-) 26.8 ms-2. (1)
( I wish they would ask for acceleration when a negative sign would be clear = I'm sure it will be ignored)
2 F=ma = 850 x 26.8 = (-) 2.28E4 N. (2)
ii) 1/2 mvmax^2 = 0.45E6 so v = 32.5 ms-1 (2)
c) m1v1 + 0 = (m1 + m2) v v = 3.11 ms-1 (2)
Nice and easy to start with.
Q2 a) i) Amplitude = 0.40m; Period = 5.0s (1)
ii) f = 1/T = 0.2Hz; w = 2 pi f = 1.26 rads-1 (2)
b) All crosses are on x=0 . (3) v=max here; acc=zero here; PE is min where KE is max ie here.
c) i) amax = (2 pi f)^2 A = 4.09E5 ms-2 (3) convert kHz and mm ...
ii) Cute question. Stretch and challenge.
1st Approack WD = force x distance. For one cycle distance = 4 x amplitude. Power = WD / T = (F x 4A)/T
2nd Approach Power = F x average speed = F x (4A/T) = same answer
P = 0.25 x4 x 1.8E-3 x 2.4E3 = 4.32W (3)
Q3 a) i) geostationAry (spelling) (1)
ii) so that stays above sma eplace on earths surface / doesnt move in sky (1)
iii) so can use fixed dishes for transmitting and receiving from satellite. (1) WTTE
iv) Use GMT^2 = 4 pi^2 r^3 T = 24x 60 x 60s
gives r = 4.2E7 m (3)
b) i) T^2 is prop to r^3 (1) - had a discussion with my students last week about whether this is on the syllabus!
ii) t1^2/t2^2 = r1^3/r2^3 so ratio is 27.3 ^2/3 = 9.1 (2)
Q4 a) Latent heat of fusion (spelling) (1)
b)i) Internal energy = the sum of the random distribution of KE and PE for all the particles in the system (2)
ii) Energy is supplied to break bonds / PE of particles increases. (2)
c) i) Pt = m c dT 12 x 60 x 60 = 15 x 1.2 x 99 x dT so dT = 2.4K (3)
ii) Heat lost to surroundings / heat is needed to heat greenhouse frame too /
heat to heat contents of greenhouse /
rate of energy transfer is likely to fall over the hour - all seem Ok to me. (3)
Looking forward to the mark scheme
Q5 a) i) Perfectly elastic = total KE stays constant (1)
ii) molecules bouce off walls perfectly eleastically.
molecules momentum changes ( = 2mv )
force on molecule is equal and opp to force on wall
force on moecule = change in momentum oer collision x no of collisions per unti time
pressure = force / area
lots of molecules colliding - add up ll forces. (4)
iii) when temp increases, velocity increases
larger change in momentum in each collision
more collisions per unti time
so bigger force so higher pressure. (2)
b) i) PV/T is constant either work out n and sub or use p1V1/T1 = p2V2/T2
Did you spot answer had to be in kPa? p2 = 36 kPa. (3)
ii) Balloon will expand. If gets too big, it may burst. Limit to how far envelope can stretch. (1)
Q6 a) m=Mm/NA = 2.02E-3/6.02E23 = 3.36E-27 or you could just add masses of two protons together (2)
b) E=3/2 k T = 2.28E-20 J (2)
c) E = 1/2 mv^2 so v=2.63E3 ms-1 (2)
d) E is the mean Ke per molecule - some have much more than the mean / Maxwell- Boltzman distribution sketch.
so the highest energy molecules can escape. (2)
Overall I thought the paper was pretty similar to June 2011. Yes there were one or two tricky bits but there were a
lot of easy marks too.
The grade boudaries were pretty low in June 44/40/36/33/30 . I'd expect about the same.
Most of my students reckon they dropped about 10 marks.
Of course the ones that messed it up proably wouldnt admit it to me!
Hope it went well for you. On to Frontiers of Physics.
Thats a monster paper.
Edited answer for 2 c i