Physics: Post your doubts here!

[sagar65265]

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? :|

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!

 

[Wolfgangs]

Regarding this year paper 21, the first question was the define velocity, right?

 

[Thought blocker]

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
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

Oye thanks for typing this and giving a time, I got it dear..... Thanks a lot... Zaqy I would not stop using that

 

[papajohn]

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/...and AS Level/Physics (9702)/9702_w05_qp_1.pdf
Q 34

 

[Hadi Murtaza]

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

m = k/R
R = k/m

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

 

[RoOkaYya G]

m = k/R
R = k/m

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

genius!

 

[papajohn]

m = k/R
R = k/m

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

Well if u see the graph, at B it is most steepest which means higher gradient than any other?The correct ans is C which against ur explaination

 

[Hadi Murtaza]

Well if u see the graph, at B it is most steepest which means higher gradient than any other?The correct ans is C which against ur explaination

I just gave da answer 2 wat u said, i didnt read da question

 

[Thought blocker]

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

I din't got your tag...
My way of doing this question:¬
Method i)
Make accurate scale diagram... Then at each point (A,B,C,D) calculate resistance with formula R = 1 / m you'll find at C there is lower resistance...
Method ii)
As R = V / I ; R is the ratio of V:I so when drawing a line of best fit i.e is drawing a st. line from origin (zero , zero) check which point is closer to the line, I got exact at C

Hope you got it..
Method i) would take sometime but will give accurate result if your scale is correct
Method ii) is also valid... I would prefer this method rather than first...
P.S. After completing whole paper, if I had a time, I would check it with both of this method.
All the best PAPA

 

[sagar65265]

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

The fact is that the gradient is not what affects the resistance; at any point, the resistance of a material can be obtained by applying a potential difference V across it's terminals, and measuring the current I through it. After doing this, the value of V/I should give you the resistance of the material. If they asked you at what point the rate of change of resistance was highest, then B makes complete sense.

However, since differentiation is all about minute changes (whole of calculus, really, but only differentiation is worth a look in this case) and we are concerned with exact values, we do not need differentiation to obtain an answer.

What Thought blocker has said above is absolutely right, which I think looks a little like this:

Hope this helped!

Good Luck for all your exams!

 

[papajohn]

Thanks pals!

 

[Thought blocker]

Thanks pal!

Welcome papa ji

 

[Thought blocker]

The fact is that the gradient is not what affects the resistance; at any point, the resistance of a material can be obtained by applying a potential difference V across it's terminals, and measuring the current I through it. After doing this, the value of V/I should give you the resistance of the material. If they asked you at what point the rate of change of resistance was highest, then B makes complete sense.

However, since differentiation is all about minute changes (whole of calculus, really, but only differentiation is worth a look in this case) and we are concerned with exact values, we do not need differentiation to obtain an answer.

What Thought blocker has said above is absolutely right, which I think looks a little like this:View attachment 44027

Hope this helped!

Good Luck for all your exams!

Well I din't get that differentiation part, explain me that thing differentiation q karna tha ?
And yes I meant the same thing

 

[sagar65265]

Well I din't get that differentiation part, explain me that thing differentiation q karna tha ?
And yes I meant the same thing

Just saying kyunki gradient of curve ek baar lena tha na, so gradient at A, B and C would be found by differentiation/ drawing gradient, etc. No need for it, just saying.

 

[Hadi Murtaza]

The fact is that the gradient is not what affects the resistance; at any point, the resistance of a material can be obtained by applying a potential difference V across it's terminals, and measuring the current I through it. After doing this, the value of V/I should give you the resistance of the material. If they asked you at what point the rate of change of resistance was highest, then B makes complete sense.

However, since differentiation is all about minute changes (whole of calculus, really, but only differentiation is worth a look in this case) and we are concerned with exact values, we do not need differentiation to obtain an answer.

What Thought blocker has said above is absolutely right, which I think looks a little like this:View attachment 44027

Hope this helped!

Good Luck for all your exams!

I disagree wid dis

 

[Thought blocker]

Just saying kyunki gradient of curve ek baar lena tha na, so gradient at A, B and C would be found by differentiation/ drawing gradient, etc. No need for it, just saying.

Oki oki

I disagree wid dis

LOL, Genius, batao q ?

 

[Hadi Murtaza]

Oki oki

LOL, Genius, batao q ?

Have u guys heard of tangent on curves ??

 

[Thought blocker]

Have u guys heard of tangent on curves ??

Going to bath.. I just did my treatment of whole body... Ake shamjha ta hu maine kya kiya... Ruko app...
Be back soon dear <3

 

[Hadi Murtaza]

Going to bath.. I just did my treatment of whole body... Ake shamjha ta hu maine kya kiya... Ruko app...
Be back soon dear <3

alrighty

 

[Thought blocker]

alrighty

So, lemme explain you...
R = V / I yes ?
It means R is in the ratio of V : I yes ?
And we are given I - V graph yes ?
So we can draw a line of best fit, that is from origin, and the line of best fit here is resistance.
And we are asked to find the least resistance in the whole graph..
We are given points A,B,C,D..
Now you attach all point from origin to A,B,C,D
you'll see The steepest line at C point... (Steepest the gradient lower will be the Resistance, hope you got the answer, if no lemme elaborate)
And as you know, R = 1/m yes ? ; so, High the m, less is the R : Here we have the steepest line at C then answer is C
Well, if we were asked for the highest resistance, answer would be A, as it has the lowest steepness from other three.
Hope you got it