Physics: Post your doubts here!

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Wolfgangs said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w10_qp_12.pdf

Q11. someone please explain it.
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The tension in the rope needs to balance out the weight of the student acting downwards, so since the total tension needs to be opposite and equal to the weight, Let tension in rope on left side be T1 and the tension on rope on right side T2 and :. Total tension = T1sino + T2sino
add them up... W= 2Tsino, rearrange to get T= W/ 2sino. Answer: B
 
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Ahmed H. Al-Neel said:
help with question 7,33,34 and 40 plz!
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_s07_qp_1.pdf
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7)
Both the other quantities are velocities. Thus X should represent the velocity due to the force F. Answer is thus C, as product of a and t would give us the velocity due to that acceleration for the specified time.
33)
V1= 5000/ (5000+5000) x 2 = 1V then V2= 3000/ (3000 + 2000) x 2 = 1.2 so V1 - V2 = -0.2 so C
34)
As Light intensity increases,Resistance decreases, R is proportional to voltage, Voltmeter reading decreases. So option A has only decrease so answer is A.
40)
We need to divide Proton numbers by their nucleon numbers, that is
for H speed would be 1 , for He it would be 1/2 ,for Li it would be 3/7 ,for Be it would be 4/9
Hence, Li has the lowest value of speed hence answer is Li
More detail :¬
u have to calculate the charge to mass ratio, whichever is the lowest will have the least acceleration.
q/m ratio of hydrogen = 1/1 = 1
q/m ratio of helium = 2/4 = 0.5
q/m ratio of lithium = 3/7 = 0.42
q/m ratio of beryllium = 4/9 = 0.44
 
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Ahmed H. Al-Neel said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_s11_qp_11.pdf
question 9,25,27,37
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9)
Initial m = Final m

mv + 0 = 2mx (note: after the bodies stick, their masses combine. Let their combined speed be 'x').
mv = 2mx
x = v/2 (m cancel out)

Since the initial momentum is mv, the final momentum must be the same, so its mv also.

K.E after is given by,

0.5 * (2m) * (v/2)^2
= mv^2/4

Hence A.

25)
Use the formula (2n - 1)*75 i.e, for n = 1 we have frequency of 75Hz
we are asked for n = 2,3
n = 2 --> 3*75 = 225
n = 3 --> 5*75 = 375
Hence answer is D

27)
find d which is 1/n -->1/500 * 10^ (-3) = 2 x 10^-6.
then d sin90=n x (600 x 10^-9) = 3 then he asked for the images so it is 3 orders for one side which is 45 degrees so for the 90 degrees it is 3 x 2= 6 + the normal ray = 7 so D

37)
he want the ratio of the V1/V2 so we need the distance of R1 from x divided by the distance of R2 from x...the distance from R1 from X is x and the distance of R2 from X is the total wire length - the distace of R1 from X so it is L-x so the answer is D x/L-x
 
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_Ahmad said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w05_qp_1.pdf

Q8,20,24,25
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8) I will solve it later.

20)
Simple yaar, take a look on book, that middle thing is work done..

24)
The amplitude increases 2 times so intensity increases by 2^2 = 4 times
The frequency decreases by 1/2 so intensity decreases by (1/2)^2 = 1/4 times
So net change in intensity= 4 * (1/4) = 1
So intensity is unchanged and is equal to I0
So the answer is B

25)
In air, the frequency would remain the same. Now do it yourself
Why frequency is same :
when waves travel, they carry energy. When passing from one media to another the energy content of the waves has to stay the same due to the law of conservation of energy.
we know that E=hf where E=Energy of wave, h=planck's constant, f=frequency of wave.
so h being a constant would not change, and f could not change either.
So, the only characteristic of the wave which can change is the velocity and wavelength, which adjust themselves so as to give the same frequence before and after passing through different media.

Hope you got it :)
 
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