Physics: Post your doubts here!

[Rizwan Javed]

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Anyone please help?

They are asking for a hypothetical relationship, a "possible relationship" as they say. You know that Power depends on v and F, and F depends on v. So power depends on v twice as much as F depends on v. Thus, in a potential relationshiip, the dependence of Power on v must be one exponent higher than the dependence of F on v. A is impossible 'cause this shows the reverse. B and C are impossible cause here both are dependent on v to the same extent. This leaves only D as plausible answer where Power shows dependence on v to the cube while F shows dependence on v to the square. I'm not so good at maths hopefully someone else can explain if I didn't make sense.

well if I were to do this, I would have done it like this:
P = Fv
Since the object is moving, it definitely possesses some Ek. So substitute it into the above equ.

Ek/t = Fv

Take 't' on the other side, you will get:

Ek = F*d 0.5*m*v^2 = F*d

take 'd' on other side and divide. you will get:

0.5m/d * v^2 = F

since speed is constant, resistive force = driving force, so (Fr for resistive force)

Fr = 0.5m/d * v^2

Mass of the object, and distance moved, let them to be constant. so Fr depends on V^2 like this: Fr = k v^2

=> Fr ∝ v^2

Now to find the relationship with power, we know: P = Fv

F= kv^2 <--- put this into above ^ equ

P = kv^2 * v
P = k v^3
P ∝ v^3

Hence the answer is D.

 

[Metallic9896]

31 and 32 please

Q31:

V = IR (Voltage = Current / Resistance)

R = pL/A (Resistance = Resistivity x Length / Area)

Thus, V = IpL/A (Voltage = Current x Resistivity x Length / Area)

I = VA/pL (Current = Voltage x Area / Resistivity x Length)

V is the same, p is the same, L is the same. A is directly proportional to I (also proven by I = nAvq), so we can say I = kA, where k is a constant.

A itself = (pi x d squared)/4, so A is proportional to sqaure of d or we can say A = kd, where k is a constant.

Wire S has double the diameter of Wire T, so based on A = kd^2, if we double d, we multiply A by 4. So Wire S has 4 times the area as Wire T.

I = kA, and if A is 4 times in Wire S, then I in Wire S is also 4 times that of Wire T.

Ratio of current in Wire S to Wire T then must 4, D.

Q32:

Power does not depend on the sign of the current, only on the magnitude. Mean power = (Power when current = 2) + (Power when current = 1) / 2

Get power from I^2 x R

Mean power = (2 x 2 x 100) + (1 x 1 x 100) / 2 = (400) + (100) / 2 = 500/2 = 250 W.

 

[Metallic9896]

well if I were to do this, I would have done it like this:
P = Fv
Since the object is moving, it definitely possesses some Ek. So substitute it into the above equ.

Ek/t = Fv

Take 't' on the other side, you will get:

Ek = F*d 0.5*m*v^2 = F*d

take 'd' on other side and divide. you will get:

0.5m/d * v^2 = F

since speed is constant, resistive force = driving force, so (Fr for resistive force)

Fr = 0.5m/d * v^2

Mass of the object, and distance moved, let them to be constant. so Fr depends on V^2 like this: Fr = k v^2

=> Fr ∝ v^2

Now to find the relationship with power, we know: P = Fv

F= kv^2 <--- put this into above ^ equ

P = kv^2 * v
P = k v^3
P ∝ v^3

Hence the answer is D.

So beautiful. :')

 

[Holmes]

Speed = distance / time

When distance is X,
For outward flight,
Time = X / 600

For Return Flight,
Time = X/400

Total Time = (X / 600) + (X / 400)
Average Velocity = 2X / Total Time = 480

Great shot!

 

[Holmes]

http://papers.gceguide.com/A Levels/Physics (9702)/9702_s16_qp_13.pdf Q4

Percentage uncertainity in:
L = 0.25% L^3 so L= 0.25 * 3 = 0.75%
M= 1.667%
t = 1.67% t^3 so t = 1.67 * 3 = 5.0 %
w = 2.174%

so t is the answer
ANS: C

 

[Holmes]

 

[GarryTheGhost]

help plz

 

[Your 69Mom]

View attachment 62531

Help Please!

Why do i need to take average of the density? Can somebody explain? URGENT!

 

[Your 69Mom]

View attachment 62533

I think,
at time of launch, there is no change of height, so there is no change in gravitational force. The resultant force change, but the gravitational force remains constant. So B.

 

[Rizwan Javed]

View attachment 62531

Help Please!

The question states that density decreases LINEARLY with altitude. So you can consider it in terms of a graph like this (taking sea level to be altitude of zero!)

Finding the area under the graph will give you the value for 'ph' which can be used in the pressure equation (P = pgh)

So find the area = 0.5 * (1.22+0.74) * 5000 = 4900

now find the pressure change that occurs = pgh = 4900 * 9.81 = 48069
Note that this is the PRESSURE CHANGE, not the pressure at 5000m

so you can calculate the pressure at 5000m altitude as:

change = pressure(sea level) - pressure(5000m)
pressure(5000) = pressure(sea level) - change
pressure(5000) = 10^5 - 48069 = 51931 ~ 52000 Pa

 

[techgeek]

 

[amina1300]

There was one question from the waves section about over lap of interference does anyone recall solving something like this..?

 

[Shimmery woods]

que no 15 please im not getting the answrr C

 

[Thelastmoment]

Good luck for tomorrow

 

[Rizwan Javed]

View attachment 62536

The question states that the water does not rebound off the wall, that means the velocity of the water becomes zero, when it strikes the wall.
The momentum of the water at the mouth of pipe is : mv
The speed is zero at wall, so change in momentum is : mv
Force = rate of change of momentum
Force = mv/t

we are going to consider the force exerted on wall per second, so t=1s

F = mv --(i)

now we need to remove mass 'm' from the above equation.

density = mass/volume
p = m/V => m = p*V

volume of water = A * v

m = p * A * v <--- substitute into (i)

F = p * A * v * v
F = pAv^2

Hence the answer is D.

 

[shahzaib ihsan]

plz tell me how to solve these type of problems e.g.

a copper wire is to be replaced by an aluminium allow wire of the same length and resistance. copper has half the resistivity of the allow. what is the ratio
diamerter of alloy wire/diameter of copper wire?
i mean every time there is a question like this i can't understand how to solve them

and like this:

fringes of separation x are observed on a screen 1.00m from a double slit that is illuminated by a yellow light of wavelength 600 nm.
at which distance from the slits would fringes of the same separation x be observed when using blue light of wavelength 400nm?

 

[Thelastmoment]

plz tell me how to solve these type of problems e.g.

a copper wire is to be replaced by an aluminium allow wire of the same length and resistance. copper has half the resistivity of the allow. what is the ratio
diamerter of alloy wire/diameter of copper wire?
i mean every time there is a question like this i can't understand how to solve them

and like this:

fringes of separation x are observed on a screen 1.00m from a double slit that is illuminated by a yellow light of wavelength 600 nm.
at which distance from the slits would fringes of the same separation x be observed when using blue light of wavelength 400nm?

for the first apply R=pl/A seperately for both
then get ratio

For the second use lambda= ax/D
you change lambda see how it affects by applying seperately again

 

[GarryTheGhost]

 

[Thelastmoment]

que no 15 please im not getting the answrr C

First P=Fv
=300kN x 40 ms-1
=1.2 x 10^7 W / 1M = 12MW

 

[Thelastmoment]

View attachment 62538

is the answer B?