Physics: Post your doubts here!

[Alice123]

for 18 : E= power output / input ... out put is already given ...for input you multiply fuel comsumption by energy content ..convert to SI units. to check wether you did it correct multiply with units and you'll get ______j /s that is the unit of power . (sorry i think this is not what you asked for , but anyhow might be useful for sm1 else)


question 24: since both wires of same material so E young modulus is same
equate equations for both
F is same
so , FL/ pi r^2 x1 = F 3L/ 4 pi r^2 x2 ........................you will get x2= 3/4 x1

question 34 :

thanks...

 

[queen of the legend]

thanks...

for s-2012/12 question 17 :
take pe for each block :first does not move
2nd : m 2h g
3rd: m 3h g
4th : m 4h g

add them you get 3m 9h 3g
take LCM ... 3 (m 2h g ) ..= 6 mgh ... ......it doesnt make sense for me still but this is how i did it

 

[queen of the legend]

ok when at Y, the body has traveled 40 m, in 12 seconds so the speed at Y is v = d / t = 40 / 12 = 3.3 ms^-1
this is going to be the initial speed for the length YZ
the speed at Z = d / t = 40 / 6 = 6.6 ms^-1
this is the final speed for the length YZ
now use v^2 = u^2 + 2as for YZ
(6.6)^2 = (3.3)^2 + 2 a (40)
a = 0.36 ms^-2

you get 0.4 ms not 0.36 ?!

 

[queen of the legend]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w11_qp_12.pdf
question 10 need help urgently~!
From my knowledge, momentum is constant if there's no external force acting on a system, so p = mv for the truck, since p is constant, when m increases v decreases, but why does the velocity stays the same when the sand is released from the truck? (Mass of the truck decreases?)

first of all newtons first law applies here .....only when a force acts on the object it causes a change in its velocity
so when sand is added first v decreases due to increase in mass ......when sand falls out the velocity stays the same since there is no force to increase its velocity later ...it tend to continue its motion along the same path with same speed

 

[Goku]

http://www.xtremepapers.com/papers/...and AS Level/Physics (9702)/9702_s03_qp_1.pdf

^ Can someone please help me with questions 7,10,14,17,30,40,36
Thaankyou!

 

[Khalifa]

Can somebody post explainations to http://www.xtremepapers.com/papers/...and AS Level/Physics (9702)/9702_w08_qp_1.pdf

Questions 1, 27: the answers are C for both. I got the answer but messed up the power in both
Question 24C and 36B

 

[EdmundH]

first of all newtons first law applies here .....only when a force acts on the object it causes a change in its velocity
so when sand is added first v decreases due to increase in mass ......when sand falls out the velocity stays the same since there is no force to increase its velocity later ...it tend to continue its motion along the same path with same speed

I've got it from my friend, i think his explanation sounds more reasonable :
when mass is added, velocity decreases due to the increase in mass (p = mv), where p is a constant,
when the sand is realeased, looking a the truck itself there's loss in mass, but if we look at the sand and the truck as a system,
p = (m1 + m2)v (m1 = mass of sand, m2 = mass of truck)
so when the sand is released, the sand continues to move with velocity v, so thus the truck, as we can see from
p = m1v + m2v (expanding the equation)
So momentum is still conserved throughout the system!
Hope this helps!

 

[Sriyog Sharma]

in this question originally the pressure = rho g h ....when hieght incresed by h again p= rho g 2h

Why does the height increase again ? Wont it be the same height. I agree the height increases by "h" in the right tube. but after that i dont understand.

 

[EdmundH]

Why does the height increase again ? Wont it be the same height. I agree the height increases by "h" in the right tube. but after that i dont understand.

U got trolled by the one who set this question lol, (me too actually xD)
u see, when the right tube raise by a height h, the left tube too will raise through a height h, (imagine the water flowing from the left tube to the right tube)
so the change in height is acutally 2h

 

[autumnsakura]

Why does the height increase again ? Wont it be the same height. I agree the height increases by "h" in the right tube. but after that i dont understand.

the liquid has to move. If liquid increase in one tube, it has to come from somewhere which is the another tube. So as liquid increase, liquid decrease kinda thing....Another example, if you have 5 apples each in two basket and you had to move 2 apples from one basket to another, one basket has 7 and another will only have 3 left. 7-3=4 .....net difference is 4

 

[autumnsakura]

http://www.xtremepapers.com/papers/...nd AS Level/Physics (9702)/9702_w09_qp_12.pdf
Q12,13,14 please!!

http://www.xtremepapers.com/papers/...nd AS Level/Physics (9702)/9702_w11_qp_12.pdf
Question 23 please!!

 

[EdmundH]

http://www.xtremepapers.com/papers/...nd AS Level/Physics (9702)/9702_w09_qp_12.pdf
Q12,13,14 please!!

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w11_qp_12.pdf
Question 23 please!!

Oct09
Q12. since the btm belt is slacked, so tension is only at the upper belt, torque at Q = 3.0 N m,
so 3.0 = T x (100 x 10^-3)/2
T = 60N
Torque at P = T x (150 x 10^-3)/2 = 4.5
Q13. assuming it is projected with velocity v, so the initial KE, E= (mv^2)/2
at the highest point, the vertical component of the veolcity = 0, so its left with v cos 45,
so final KE = 1/2 x m x (v cos 45)^2 = 0.5E
Q14. By conservation of momentum, 2 x 2 = 1 x v, v =4
so the 1kg trolley moves 4ms-1 to the right,
calculating the total KE of the both trolleys, which is 1/2 x 2 x 2^2 + 1/2 x 1 x 4^2 = 12J
assuming no energy is lost, the energy stored in the spring is converted to KE gained by the trolleys, so its 12J

Oct11/12
Q23. the area under graph is the elastic PE, so we can c that the answer is obviously (i hope its obvious for u too) B. not much that i could explain here

 

[Sriyog Sharma]

the liquid has to move. If liquid increase in one tube, it has to come from somewhere which is the another tube. So as liquid increase, liquid decrease kinda thing....Another example, if you have 5 apples each in two basket and you had to move 2 apples from one basket to another, one basket has 7 and another will only have 3 left. 7-3=4 .....net difference is 4

Then left tube will have height of x-h. and the right tube will have x + h right???? and now how come the difference is 2h ?

 

[autumnsakura]

Oct09
Q12. since the btm belt is slacked, so tension is only at the upper belt, torque at Q = 3.0 N m,
so 3.0 = T x (100 x 10^-3)/2
T = 60N
Torque at P = T x (150 x 10^-3)/2 = 4.5
Q13. assuming it is projected with velocity v, so the initial KE, E= (mv^2)/2
at the highest point, the vertical component of the veolcity = 0, so its left with v cos 45,
so final KE = 1/2 x m x (v cos 45)^2 = 0.5E
Q14. By conservation of momentum, 2 x 2 = 1 x v, v =4
so the 1kg trolley moves 4ms-1 to the right,
calculating the total KE of the both trolleys, which is 1/2 x 2 x 2^2 + 1/2 x 1 x 4^2 = 12J
assuming no energy is lost, the energy stored in the spring is converted to KE gained by the trolleys, so its 12J

Oct11/12
Q23. the area under graph is the elastic PE, so we can c that the answer is obviously (i hope its obvious for u too) B. not much that i could explain here

Thank you so much! You are a saviour for dumb people like me !!

 

[Sriyog Sharma]

U got trolled by the one who set this question lol, (me too actually xD)
u see, when the right tube raise by a height h, the left tube too will raise through a height h, (imagine the water flowing from the left tube to the right tube)
so the change in height is acutally 2h

Why would the left tube will also raise by h ? wont it decrease by h ?

 

[Sriyog Sharma]

Then left tube will have height of x-h. and the right tube will have x + h right???? and now how come the difference is 2h ?

Can you please workout all the steps how you got the answer.

 

[autumnsakura]

Then left tube will have height of x-h. and the right tube will have x + h right???? and now how come the difference is 2h ?

yes. SO taking the difference, (x+h)- (x-h) = 2h

 

[hussamh10]

http://www.xtremepapers.com/papers/...nd AS Level/Physics (9702)/9702_s11_qp_13.pdf
Q10

 

[Sriyog Sharma]

yes. SO taking the difference, (x+h)- (x-h) = 2h

Holy shit.I didnt understand this for days . Thank you so much ,

 

[Sriyog Sharma]

Holy shit.I didnt understand this for days . Thank you so much ,

Do you know one question about the ladder leaning against a wall which is about to slide. If you know can you please explain me.I m searching the pastpapers for that question but cann't find it.