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Physics: Post your doubts here!

Thought blocker

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_Ahmad said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w12_qp_13.pdf

Q15,21,26,33 PLEASE HELP!!!
Click to expand...
Just keep your mind concentrated while reading the questions and if all of your concepts are cleared you would definitely score well :) And believe me, As level is nothing and nothing is tough if your concepts are cleared. :) If you want some concepts to be cleared I have some thing for you:¬
i) Click --> studyguide and login from my account
Username : ElectroKid
Password : 9934356566
P.S. Dont misuse this account as my account is being operated by my more then 100 friends.
ii) Click ---> Khanacamdemy for video tutorials
iii) And if you still have doubts, don't hesitate to ask here ;)
Hope this helps you (y)

15)
viscous force - Force that act on a body moving through a fluid that are caused by the resistance of the fluid.
upthrust force - The upward force that a liquid exerts on a body floating or immersed in a liquid.
Gravitational force - Force exerted by one body on another, and is directly proportional to the product of their masses and inversely proportional to the square of the distance separating their center of masses.
So Here W is same in all option so you do not need to look at that
So now look at U that is mostly in liquid so it would be less.. so if you look at options in D it is very less.
So now look at V it would obviously be more, as hailstone is falling just below the cloud that is from the base and vertically towards the ground. Hence in option D viscous force is high
Hence answer is D

21)
There is more pressure near Y due to the height of the column. The water therefore is compressed and thus gains elastic potential energy.
As the rate of flow is constant, kinetic energy is out of the options.
We can say, K.E. doesn't increases because the question states that the rate of flow is constant. E.P.E. increases because pressure increases with depth.

26)
Distance b/w node to node is λ/2 = L and L is 1 m
Given there are 3 nodes hence 3λ/2 = 1 ---> λ = 2/3
Now V = f * λ f --> f = V / λ = 330 / 0.667 = 495Hz
Q26.png

33)
Resistance is based on surface area of a cross section of the wire
Area = π*R²
Area 1= π*0.5² =π*0.25
Area 2 = π*0.25² =π*0.0625
Area 1 is 4 times higher than Area 2 so has (1/4) of the resistance.
0.2 * 4 = 0.8
B 0.8Ω

All the best brother (y)

:) :)
 
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A

_Ahmad

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Thought blocker said:
Just keep your mind concentrated while reading the questions and if all of your concepts are cleared you would definitely score well :) And believe me, As level is nothing and nothing is tough if your concepts are cleared. :) If you want some concepts to be cleared I have some thing for you:¬
i) Click --> studyguide and login from my account
Username : ElectroKid
Password : 9934356566
P.S. Dont misuse this account as my account is being operated by my more then 100 friends.
ii) Click ---> Khanacamdemy for video tutorials
iii) And if you still have doubts, don't hesitate to ask here ;)
Hope this helps you (y)

15)
viscous force - Force that act on a body moving through a fluid that are caused by the resistance of the fluid.
upthrust force - The upward force that a liquid exerts on a body floating or immersed in a liquid.
Gravitational force - Force exerted by one body on another, and is directly proportional to the product of their masses and inversely proportional to the square of the distance separating their center of masses.
So Here W is same in all option so you do not need to look at that
So now look at U that is mostly in liquid so it would be less.. so if you look at options in D it is very less.
So now look at V it would obviously be more, as hailstone is falling just below the cloud that is from the base and vertically towards the ground. Hence in option D viscous force is high
Hence answer is D

21)
There is more pressure near Y due to the height of the column. The water therefore is compressed and thus gains elastic potential energy.
As the rate of flow is constant, kinetic energy is out of the options.
We can say, K.E. doesn't increases because the question states that the rate of flow is constant. E.P.E. increases because pressure increases with depth.

26)
Distance b/w node to node is λ/2 = L and L is 1 m
Given there are 3 nodes hence 3λ/2 = 1 ---> λ = 2/3
Now V = f * λ f --> f = V / λ = 330 / 0.667 = 495Hz
View attachment 43953

33)
Resistance is based on surface area of a cross section of the wire
Area = π*R²
Area 1= π*0.5² =π*0.25
Area 2 = π*0.25² =π*0.0625
Area 1 is 4 times higher than Area 2 so has (1/4) of the resistance.
0.2 * 4 = 0.8
B 0.8Ω

All the best brother (y)

:) :)
Click to expand...

THANKS ALOT!!!!!!!!!!!

sorry
but for the last question the answer is D:(
 
ZaqZainab

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_Ahmad said:
THANKS ALOT!!!!!!!!!!!

sorry
but for the last question the answer is D:(
Click to expand...
Thought blocker said:
Well MS might be wrong, I get B only :/
Click to expand...
No the MS is not wrong
you have taken the length for the stretched and unstretched wire equal which is wrong
as they have mentioned uniformly
=(pie(0.5)^2 )/(pie(0.25)^2)=4
so raio of length=4
so if i was 1 before it became 4
or if it was 2 it became 8
but if you calculate using the resistance the length was 0.2*pie*(0.5)^2= 1/20 pie
now it will be 1/20 pie * 4 = 1/5 pie
resistance=(1/5 pie)/(pie*(0.25)^2)
 
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Thought blocker

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ZaqZainab said:
No the MS is not wrong
you have taken the length for the stretched and unstretched wire equal which is wrong
as they have mentioned uniformly change of area= change of length
change of area =(pie(0.5)^2 )- (pie(0.25)^2)=4
so change of length=4
so if i was 1 before it became 4
or if it was 2 it became 8
but if you calculate using the resistance the length was 0.2*pie*(0.5)^2= 1/20 pie
now it will be 1/20 pie * 4 = 1/5 pie
resistance=(1/5 pie)/(pie*(0.25)^2)
Click to expand...
You know I am weak at this stuff. Go in basic level and then explain, and please have a gap after one line :)
 
ZaqZainab

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Thought blocker said:
You know I am weak at this stuff. Go in basic level and then explain, and please have a gap after one line :)
Click to expand...
okay so we have a wire and it is being stretched. By stretching a wire you only change its length or area you can't change the volume (to change volume you should either remove a part of it or add more copper to it)
and so Volume of stretched=volume of unstretched

A copper wire had a diameter of 1.0mm and it was stretched to 0.5mm diameter. The word stretched makes it quite clear that the length changes
Now the question is by how much did the length change?
We didn't add extra to it we didn't remove from it so that means we can use ratios

I guess you aren't getting this part
let me use an example. lets say you have got 2 bowls of chocolate each bowl contain 10 so you have a total of 20 it will always stay 20 unless you decide to eat one from it or maybe just add another chocolate to it but that's not happening You decide to pick one chocolate from bowl A and you drop it in bowl B
total number still 20 but there is 1 less in bowl A where did it go? to bowl B!

From our question Bowl A is Diameter and Bowl B is the length and total number is the total volume
You working shows that you did consider the change in Diameter but not length
Well basically if you do that your total will change to 19 and in the length case you are just removing a part of it which is not indicated in the question you are suppose to use the same 'long,stretched' wire .Have you played with play doh? if you have you might have noticed while making a snake or anything long the more you reduce the cross sectional area the longer the snake.

We have to find out the change in area in other words the area which was removed to take shape as a length
I have done a lot of talking lets start with the calculations
First thing first finding the change in area which you did already (y)
Area 1 (pie(0.5)^2 )
Area 2 (pie(0.25)^2 )
you have done it and found the ratio as 4/1
so goes for the length which you ignored
find the initial length using the formula R=resistivity*length/area
but as it is the same material i take resistiviy as 1
0.2=l/pie(1*0.5)^2
l=1/20 pie
this is the initial it should also change by 4/1 as the area did
so final l=(1/20 pie)*(4/1) =(1/5 pie)
now you can use the formula again to get the new R with Area=pie(o.25)^2 and l=(1/5 pie)

I tried my best to explain
I hope you got it by now
 
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Rimsha Noor1 said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w13_qp_11.pdf
Question 8. Why cant we just add up the speeds and divide them by 2? :|
Click to expand...

Short answer, because the time taken with each speed is different.

Let's do the math behind this - suppose the distance traveled with both speeds is denoted by "s".
Suppose the time taken at 600 kmph = t(1).

Then, since Speed = Distance/Time, we can write

600 = s/t(1)
So that
t(1) = s/600

Similarly, let's do the same for the other speed. The distance traveled is still "s", the speed = 400 kmph, and the time taken to traverse this distance = t(2). So:

400 = s/t(2)
So that
t(2) = s/400

The average speed is the total distance traveled dividing by the time taken (the average velocity is different - it is equal to total displacement divided by time taken).
= (Total Distance traveled)/(Time taken to cover that distance).

Since the plane travels a distance "s" in one direction and returns the same distance "s", the total distance traveled = s+s = 2s.

The time taken = t(1) + t(2) = s/400 + s/600 = 3s/1200 + 2s/1200 = 5s/1200 = s/240

Therefore, the average speed = 2s/(s/240) = 480 kmph = C.

Hope this helped!
Good Luck for all your exams!
 
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ZaqZainab said:
okay so we have a wire and it is being stretched. By stretching a wire you only change its length or area you can't change the volume (to change volume you should either remove a part of it or add more copper to it)
and so Volume of stretched=volume of unstretched

A copper wire had a diameter of 1.0mm and it was stretched to 0.5mm diameter. The word stretched makes it quite clear that the length changes
Now the question is by how much did the length change?
We didn't add extra to it we didn't remove from it so that means we can use ratios

I guess you aren't getting this part
let me use an example. lets say you have got 2 bowls of chocolate each bowl contain 10 so you have a total of 20 it will always stay 20 unless you decide to eat one from it or maybe just add another chocolate to it but that's not happening You decide to pick one chocolate from bowl A and you drop it in bowl B
total number still 20 but there is 1 less in bowl A where did it go? to bowl B!

From our question Bowl A is Diameter and Bowl B is the length and total number is the total volume
You working shows that you did consider the change in Diameter but not length
Well basically if you do that your total will change to 19 and in the length case you are just removing a part of it which is not indicated in the question you are suppose to use the same 'long,stretched' wire .Have you played with play doh? if you have you might have noticed while making a snake or anything long the more you reduce the cross sectional area the longer the snake.

We have to find out the change in area in other words the area which was removed to take shape as a length
I have done a lot of talking lets start with the calculations
First thing first finding the change in area which you did already (y)
Area 1 (pie(0.5)^2 )
Area 2 (pie(0.25)^2 )
you have done it and found the ratio as 4/1
so goes for the length which you ignored
find the initial length using the formula R=resistivity*length/area
but as it is the same material i take resistiviy as 1
0.2=l/pie(1*0.5)^2
l=1/20 pie
this is the initial it should also change by 4/1 as the area did
so final l=(1/20 pie)*(4/1) =(1/5 pie)
now you can use the formula again to get the new R with Area=pie(o.25)^2 and l=(1/5 pie)

I tried my best to explain
I hope you got it by now
Click to expand...
Oye thanks for typing this and giving a time, I got it dear..... Thanks a lot... :) (y) Zaqy ;) :p I would not stop using that :whistle: :p
 
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Hadi Murtaza

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papajohn said:
Zaq Zainab Thought blocker & Suchal Riaz
I`m tagging u guys because the questions is very challenging and i only expect the right answer from you guys.
Here the is the question. My confusion is that why we take highest gradient point as the the lowest resistance where the resistance is the reciprocal.
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w05_qp_1.pdf
Q 34
Click to expand...
m = k/R
R = k/m

Increasing gradient will decrease R, so at maximum gradient, R will b minimum
 
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