Physics: Post your doubts here!

[Ashique]

Walikum,
I haven't studied the whole course yet, and I can just coordinate with you on Q.14 and 16 for now.
Q.14
You take the "perpendicular" distance from the line of force to the pivot.
The 10 N force produces an anticlockwise moment. The 5N force also produces the same anticlockwise moment and the 20N force produces a clockwise moment about the pivot , P (see diagram).
You can take anticlockwise, or clockwise direction as positive and the other as negative. In this case, let us consider the anticlockwise moment as the positive direction for this question;
Torque = F1.d1 + F2.d2 - F3.d3
Torque = 10(2) + 5(2)-20(3)
Torque=-30N , i.e Torque=30N in the clockwise direction. Ans A

I think multiplying 20 times 3, would be wrong. Yes. you got the answer right, but the approach is wrong.
To calculate the resultant moment, we multiply the force and the perpendicular distance; in this case 2m for both 5N and 20N.
Now think about you how the 10N will have a resultant on the entire system as a whole. This is not just a simple bean and a pivot. One end of the system is bent at 90 degrees, and all the forces are acting on this end of the system. Now, for forces to have a resultant moment to this end, it has to act perpendicularly. And from the diagram we can see that the 10N is parallel not perpendicular. So the only two forces to have a resultant are the 5N and the 20N. The 10N is a trick, and is there to confuse you.
Now, f1d1-f2d2=0
20(2)-5(2)
40-10= 30 Nm. Hence, answer is A.

Asalam-o-alaikum

Can you please help with these physics paper 1 questions?
Questions 13, 14, 16, 25, and 33. Thank you very very much.
http://www.xtremepapers.com/CIE/International A And AS Level/9702 - Physics/9702_s05_qp_1.pdf

Please help me with these?

I've tried explain question 13 above.
Question 16-
The derivate of an energy/time graph will give us the power. We can see the graph is steepest from 10J to 40J. So the gradient=
(40-10)/(3-2)= 30W

Question 25-
I is directly proportional to A^2 (A square)
So 2I is proportional to x^2 (Since we need to find the corresponding amplitude, let's assume it to be x).
Now a ratio calculation-
2IA^2= Ix^2
2A^2=x^2 (I gets cancelled with each other)
so square root of 2A^2=x
The square root cancels indice of A ("the raised to the power 2"). which leaves us with "square root" of 2 * A.
I know it's complicated, but I hope you get it.


Question 33
We use the formula
R=("row"L)/A
L is directly proportional to R. and A is inversely proportional to R. It is mentioned in the question that the length is doubled. So since L is proportional to R, R will also be doubled. But they also mentioned that the volume remains constant. Now for a wire, whose length has been double, and for it's volume to remain constant, there has to be a reduction in the crossectional area of the wire; and in our case, since the length has been doubled, the coressectional area has to be halved, in order for the volume to remain constant. Now how will this increase in length, but the decrease in area, affect the resistance over all? The doubling of the length doubles the resistance, since it's directly proportional. And halving the area, will again double the resistance, since the area is inversly proportional. So in total, the resistance would be increased by a factor of 4. Hence the answer is D, 4R.

And as for question 14, I really can't do it, and need help myself.

 

[unique840]

how did v get o.92Ao??

we have to find time for total error of 10%. it already has an error of 2%. means we have to find the time having an error of further 8%. the value given is Ao. 2% error is included in it. further error of 8% means Ao - 8% = 0.92Ao

 

[Oliveme]

14 D as the resultant together with other forces should originate from the same point. it should be springing outwards n upward direction. we can't show dat on a ppr so D looks appropriate in this case i guess.

16: the gradient will show us the power so the graph looks steepest from 2 till 3 seconds. the rise during this time is 30 joules and the time is 1 second so P= W.d / Time = 30W

Thank you

 

[unique840]

can u plz tell me how does the 10 N force has an anticlockwise moment?

if u apply the 10 N force on the lever, it will move upwards as it is pivoted at point P. therefore it is anticlockwise.

 

[unique840]

can smby explain me the derivation of PV=NkT????

PV = nRT
n is the number of moles. R is the molar gas constant
number of moles * avogadro constant(avg) = number of particles(N)
n = N/avogadro constant
k = R/avogadro constant
R= avogadro constant * k
PV = (N/avg) * (avg*k) * T
avg cancels out therefore
PV = NkT

 

[Oliveme]

33) resistance = (resistivity*length)/area
volume is constant. volume = area*length
length is doubled but volume is constant so area must be half.
area = A/2
length = 2l
resistance = (resistivity * 2l) / (A/2)
= (4*resistivity*l) / A
=4R

Thank you very much. God bless you.

 

[Oliveme]

25) intensity = A^2
I = A^2
2I = (A')^2 A' is the new amplitude
replace I with A^2
2 (A^2) = (A')^2
taking square root on both sides
A' = A * square root of 2
ans is B

Thank you so very much.

I've got more questions please? Physics paper 1 may 2011 questions 14, 15. thank you.

 

[unique840]

Thank you very much. God bless you.

ur welcum

 

[unique840]

june o9

question 8 prt b please!
jxt let me know that what is being asked in the question. Am unable to pick up the meanings!

p2 or p4?

 

[leadingguy]

p2 or p4?

p4

 

[Oliveme]

I think multiplying 20 times 3, would be wrong. Yes. you got the answer right, but the approach is wrong.
To calculate the resultant moment, we multiply the force and the perpendicular distance; in this case 2m for both 5N and 20N.
Now think about you how the 10N will have a resultant on the entire system as a whole. This is not just a simple bean and a pivot. One end of the system is bent at 90 degrees, and all the forces are acting on this end of the system. Now, for forces to have a resultant moment to this end, it has to act perpendicularly. And from the diagram we can see that the 10N is parallel not perpendicular. So the only two forces to have a resultant are the 5N and the 20N. The 10N is a trick, and is there to confuse you.
Now, f1d1-f2d2=0
20(2)-5(2)
40-10= 30 Nm. Hence, answer is A.



I've tried explain question 13 above.
Question 16-
The derivate of an energy/time graph will give us the power. We can see the graph is steepest from 10J to 40J. So the gradient=
(40-10)/(3-2)= 30W

Question 25-
I is directly proportional to A^2 (A square)
So 2I is proportional to x^2 (Since we need to find the corresponding amplitude, let's assume it to be x).
Now a ratio calculation-
2IA^2= Ix^2
2A^2=x^2 (I gets cancelled with each other)
so square root of 2A^2=x
The square root cancels indice of A ("the raised to the power 2"). which leaves us with "square root" of 2 * A.
I know it's complicated, but I hope you get it.


Question 33
We use the formula
R=("row"L)/A
L is directly proportional to R. and A is inversely proportional to R. It is mentioned in the question that the length is doubled. So since L is proportional to R, R will also be doubled. But they also mentioned that the volume remains constant. Now for a wire, whose length has been double, and for it's volume to remain constant, there has to be a reduction in the crossectional area of the wire; and in our case, since the length has been doubled, the coressectional area has to be halved, in order for the volume to remain constant. Now how will this increase in length, but the decrease in area, affect the resistance over all? The doubling of the length doubles the resistance, since it's directly proportional. And halving the area, will again double the resistance, since the area is inversly proportional. So in total, the resistance would be increased by a factor of 4. Hence the answer is D, 4R.

And as for question 14, I really can't do it, and need help myself.

Thank you.

 

[unique840]

june o9

question 8 prt b please!
jxt let me know that what is being asked in the question. Am unable to pick up the meanings!

we hav to xplain relation in which h is the constant of proportionality for example
energy is directly proportional to frequency.
when we equate energy and frequency we apply a constant h which is to remove the sign of proportionality therefore it is used as the constant of proportionality

 

[leadingguy]

we hav to xplain relation in which h is the constant of proportionality for example
energy is directly proportional to frequency.
when we equate energy and frequency we apply a constant h which is to remove the sign of proportionality therefore it is used as the constant of proportionality

thanx

 

[rz123]

kindly have a look.
test yourself question from chadda's book. page 542

A candidate obtains the following data between two variables t and r.
r/cm | t/s
6.2 | 4.6
12.0 | 6.0

the first value of r has an uncertainty of 0.2 cm, which is greater then the percentage uncertainty in t. Do the results show that t^2 is proportional to r ?

 

[unique840]

thanx

ur welcum

 

[Mustehssun Iqbal]

I think multiplying 20 times 3, would be wrong. Yes. you got the answer right, but the approach is wrong.
To calculate the resultant moment, we multiply the force and the perpendicular distance; in this case 2m for both 5N and 20N.
Now think about you how the 10N will have a resultant on the entire system as a whole. This is not just a simple bean and a pivot. One end of the system is bent at 90 degrees, and all the forces are acting on this end of the system. Now, for forces to have a resultant moment to this end, it has to act perpendicularly. And from the diagram we can see that the 10N is parallel not perpendicular. So the only two forces to have a resultant are the 5N and the 20N. The 10N is a trick, and is there to confuse you.
Now, f1d1-f2d2=0
20(2)-5(2)
40-10= 30 Nm. Hence, answer is A.



I've tried explain question 13 above.
Question 16-
The derivate of an energy/time graph will give us the power. We can see the graph is steepest from 10J to 40J. So the gradient=
(40-10)/(3-2)= 30W

Question 25-
I is directly proportional to A^2 (A square)
So 2I is proportional to x^2 (Since we need to find the corresponding amplitude, let's assume it to be x).
Now a ratio calculation-
2IA^2= Ix^2
2A^2=x^2 (I gets cancelled with each other)
so square root of 2A^2=x
The square root cancels indice of A ("the raised to the power 2"). which leaves us with "square root" of 2 * A.
I know it's complicated, but I hope you get it.


Question 33
We use the formula
R=("row"L)/A
L is directly proportional to R. and A is inversely proportional to R. It is mentioned in the question that the length is doubled. So since L is proportional to R, R will also be doubled. But they also mentioned that the volume remains constant. Now for a wire, whose length has been double, and for it's volume to remain constant, there has to be a reduction in the crossectional area of the wire; and in our case, since the length has been doubled, the coressectional area has to be halved, in order for the volume to remain constant. Now how will this increase in length, but the decrease in area, affect the resistance over all? The doubling of the length doubles the resistance, since it's directly proportional. And halving the area, will again double the resistance, since the area is inversly proportional. So in total, the resistance would be increased by a factor of 4. Hence the answer is D, 4R.

And as for question 14, I really can't do it, and need help myself.

But there's still a perpendicular distance from the 10 N force to the pivot. And that distance is upwards, rather than towards left. see the uploaded diagram. The dotted line there show the perpendicular distances. There is still moment of force produced by the 10 N force and the other two forces as well.

 

[omg]

we have to find time for total error of 10%. it already has an error of 2%. means we have to find the time having an error of further 8%. the value given is Ao. 2% error is included in it. further error of 8% means Ao - 8% = 0.92Ao

thanks a lot dude

 

[DragonCub]

can smby explain me the derivation of PV=NkT????

The original form of the equation for ideal gas is p V = n R T where
n = the mole number of gas
R = ideal gas constant
T = temperature in absolute scale

In derivation,
N = the number of gas molecules = n × NA (Avogadro's constant)
k = Boltzmann's constant = R ÷ NA

So you can see p V = N k T = (n NA) (R / NA) T = n R T
Hope this is helpful to you

 

[omg]

The original form of the equation for ideal gas is p V = n R T where
n = the mole number of gas
R = ideal gas constant
T = temperature in absolute scale

In derivation,
N = the number of gas molecules = n × NA (Avogadro's constant)
k = Boltzmann's constant = R ÷ NA

So you can see p V = N k T = (n NA) (R / NA) T = n R T
Hope this is helpful to you

yeahhh thankss

 

[unique840]

thanks a lot dude

ur welcum