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physics qs help (unit 4) especially the vector qs (q 2)!

http://www.edexcel.com/migrationdocuments/QP%20GCE%20Curriculum%202000/January%202012%20-%20QP/6PH04_01_que_20120307.pdf

Can somebody help me out on how to solve qs 2 & 16b
I really really need details with the answer please and thanks
(edited 11 years ago)
Reply 1
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Effort
Original post by >>MMM<<
http://www.edexcel.com/migrationdocuments/QP GCE Curriculum 2000/January 2012 - QP/6PH04_01_que_20120307.pdf

Can somebody help me out on how to solve qs 2 & 16b
I really really need details with the answer please and thanks


Can you show how to derive question 1 exactly even though it makes sense.

For 16b)
Q=CV so as plates move closer together, capacitance increases so do the potential difference across the capacitor decreases. This causes the P.D across the resistor to increase. When plates become further apart it reverses, so voltage across resistor decreases.

Thats how I answered it but not sure if I fully understand
Original post by Effort
Can you show how to derive question 1 exactly even though it makes sense.

For 16b)
Q=CV so as plates move closer together, capacitance increases so do the potential difference across the capacitor decreases. This causes the P.D across the resistor to increase. When plates become further apart it reverses, so voltage across resistor decreases.

Thats how I answered it but not sure if I fully understand


X: K.e=p^2/2m
Y: 2*K.e=p^/2m by dividing
1/2=p(x)^2/p(y)^2 simplify
p(x) =radical 2

can u help me solve question 2 please!
Reply 3
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Effort
Original post by >>MMM<<
X: K.e=p^2/2m
Y: 2*K.e=p^/2m by dividing
1/2=p(x)^2/p(y)^2 simplify
p(x) =radical 2

can u help me solve question 2 please!


Hey I'm sorry I don't understand your answer to question 1. Can you try and explain it again?:confused:

For Question 2: the reason it is C is because of conservation of momentum. The combined effect of 1 and 2 has to be the same as the combined effect of 3 and 4.:smile:
Original post by Effort
Hey I'm sorry I don't understand your answer to question 1. Can you try and explain it again?:confused:

For Question 2: the reason it is C is because of conservation of momentum. The combined effect of 1 and 2 has to be the same as the combined effect of 3 and 4.:smile:


why can it be B? and what dont you understand in my simplification?
Reply 5
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Effort
Because you have to use the vector directions ad magnitudes given in the picture.

I dont understad what you said for after Y

Is Y meant to be p squared/2m?
(edited 11 years ago)
Original post by Effort
Because you have to use the vector directions ad magnitudes given in the picture.

I dont understad what you said for after Y

Is Y meant to be p squared/2m?


The qs said that the particle had double the k.e so what i mean by Y is case 2 i.e. the equation for the new case

and i didnt understand what u mean by Because you have to use the vector directions ad magnitudes given in the picture. :confused:
Reply 7
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Effort
Im confused about your working after y. In the question it shows the velocity vectors. In b, the vectors correspond to a different direction to the one in the picture.

Ps we are hopeless why can't a more abled person answer our queations haha
Original post by Effort
Im confused about your working after y. In the question it shows the velocity vectors. In b, the vectors correspond to a different direction to the one in the picture.

Ps we are hopeless why can't a more abled person answer our queations haha


do you mean that vector 2 have to be after vector 1 and vector 3 have to be before vector 4
in the second case it would have double the ke so ke*2
then the new momentum p(y)^2/2m (2 m since mass hasnt changed) is equal to 2ke

i hope u understand now
Reply 9
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Effort
In B, 3 and 4 have a different combined effect to 1&2 therefore momentum isn't conserved whereas in C the combined effects are the same.

Okay so k.e.=p^2/2m

So ke= 2p^2/2m =p^2/m?

im confused how do you get root 2 p? Sorry if I sound thick but can't you just go through all the algebraic steps?
Original post by Effort
In B, 3 and 4 have a different combined effect to 1&2 therefore momentum isn't conserved whereas in C the combined effects are the same.

Okay so k.e.=p^2/2m

So ke= 2p^2/2m =p^2/m?

im confused how do you get root 2 p? Sorry if I sound thick but can't you just go through all the algebraic steps?


look you have the eqn ke=p^2/2m if u double this ke u'll get 2ke=p(new)^2/2m divide both eqns ke will cancel and mass will cancel

giving you 1/2 = p(old)^2 / p(new)^2 solve for p(new) and u'll get the answer
Reply 11
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Effort
Original post by >>MMM<<
look you have the eqn ke=p^2/2m if u double this ke u'll get 2ke=p(new)^2/2m divide both eqns ke will cancel and mass will cancel

giving you 1/2 = p(old)^2 / p(new)^2 solve for p(new) and u'll get the answer


Ahh finally your making sense i get it now thanks:biggrin:. Do you get question 2 and 16 now?
The reason why C is the correct answer for question 2 is conservation of momentum.
Sum of momentum before must be equal to sum of momentum after.
So the sum of vectors 1 plus 2 must equal the sum of vectors 3 plus 4
You add vectors by placing them end to end.
In C, placing 1 and 2 end to end finishes up at the same place as putting 3 and 4 end to end.
So momentum before (1 plus 2) = momentum after (3 plus 4)
Original post by Stonebridge
The reason why C is the correct answer for question 2 is conservation of momentum.
Sum of momentum before must be equal to sum of momentum after.
So the sum of vectors 1 plus 2 must equal the sum of vectors 3 plus 4
You add vectors by placing them end to end.
In C, placing 1 and 2 end to end finishes up at the same place as putting 3 and 4 end to end.
So momentum before (1 plus 2) = momentum after (3 plus 4)


Is it necessary to be head to tail like in C (end of one and the beginning of two) or can it be like B tail to tail (beginning of 1 and 2) in this case why cant it be A? and thanks
(edited 11 years ago)
Original post by Effort
Ahh finally your making sense i get it now thanks:biggrin:. Do you get question 2 and 16 now?


can you answer my quote to stonebridge and in qs 16 the markscheme says the current reverses do you have any idea why?:s-smilie:
Reply 15
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Effort
Original post by >>MMM<<
can you answer my quote to stonebridge and in qs 16 the markscheme says the current reverses do you have any idea why?:s-smilie:


because the vectors represent momenta which have to end in the same place due to conservation of momentum. For question 16, the vibrating plates cause the capacitance to change. C=Q/V. Changing C changes the voltage of the capacitor. And as Vt=Vc+Vr The voltage of the resistor must change. Changing voltage means, that the current changes too.
Original post by >>MMM<<
Is it necessary to be head to tail like in C (end of one and the beginning of two) or can it be like B tail to tail (beginning of 1 and 2) in this case why cant it be A? and thanks


To add vectors in this way they must be placed head to tail.
C is the only one where
- they are head to tail
-1 and 2 are added, three and 4 are added

It can't be A because that adds the wrong vectors.
(edited 11 years ago)
Original post by Effort
because the vectors represent momenta which have to end in the same place due to conservation of momentum. For question 16, the vibrating plates cause the capacitance to change. C=Q/V. Changing C changes the voltage of the capacitor. And as Vt=Vc+Vr The voltage of the resistor must change. Changing voltage means, that the current changes too.


if the voltage decrease doesnt that mean that the current will just decrease and not change direction . In question 2 im going to assume that it has to be head to tail right?
Reply 18
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Effort
Original post by >>MMM<<
if the voltage decrease doesnt that mean that the current will just decrease and not change direction . In question 2 im going to assume that it has to be head to tail right?


Yeah the vectors are tip to tat or whatever you call it:P

That's what I'm confused about aswell, I think the MS could be slightly poorly worded and reverse just means it fluctuates up and down.

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