Physics: Post your doubts here!

[hellodjfos;s'ff]

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Shouldn't D be correct here as the air resistance is negligible and using the equation s=ut + 0.5at² isn't it possible?







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15: It is A simply because the ball's energy is being converted from kinetic to gravitational, since there is no air resistance, none of the ball's energy is being lost to the air as heat. So, the ball's energy is conserved. B is incorrect because the option says that the momentum of the ball is conserved. This is incorrect because the momentum of the ball and AIR/EARTH is being conserved. The momentum being lost by the ball is being gained by the air on earth(the ball is pushing the air causing it to flow with a velocity meaning it is getting momentum). C is incorrect because at max height, the velocity is zero so the kinetic energy must be zero. D is incorrect because the potential energy(they mean the gravitational energy) is increasing but not at a constant rate since the ball is slowing down(due to gravity). So the rate at which the ball is gaining height is decreasing until it reaches a max, so the ball's potential energy is increasing but not at a constant rate.

8: There is acceleration so there must be a resultant force. This rules out A which states that the upward and downward forces are equal. The force exerted by the man on the floor is an upward force, contact force. Since the acceleration is downwards, the resultant force must also be acting downwards. This means that the downward force is greater than the upward force(weight>contact force). The option that states this idea is D, so that's the answer.

 

[hellodjfos;s'ff]

how to solve this???

When finding the gravitational energy of regular objects, you take their height from the centre of mass. Initially, the centre of mass of the block is 0.3m above the edge of the step. So the initial gravitational energy is 500(0.8+0.3)=550J. After the block falls, its centre of mass is 0.2m above the surface. Now the gravitational energy is 500(0.2)=100J. The change in gravitational energy is 550-100=450J (C)

 

[Kanekii]

For 9, since the force of gravity acting on her is constant, her acceleration and deceleration would be constant too. Consequently, the gradient of the graphs must be constant too, this rules out B and C. To decide between A and D, find the diagram with a completely vertical line which would be D. There would be a completely vertical line because when the girl is on the trampoline, the spring of the trampoline suddenly causes her velocity to change direction(go opposite) but remain of the same magnitude. This means that her velocity changes instantly(no time taken). So the answer is D.

A*** already explained 11 and 24.

For 26, you find the period of the wave using T=1/f . The period will turn out to be 4 ms. Now in a stationary wave, the wave goes up(max displacement), zero displacment, down(minimum displacement). So using this concept, for this wave, the wave will go to zero displacement in 1st ms, then up in the 2nd ms, then zero displacement in the 3rd ms, then down to the original position in the 4th ms, and then back to zero displacement in the 5th ms. That means after 5ms, the wave will have zero displacement giving the answer B.

Thanks alot but whats the ratio of up zero and down and how to find it?

 

[hellodjfos;s'ff]

Thanks alot but whats the ratio of up zero and down and how to find it?

It's not really a ratio. You got to understand how stationary waves move and work. Check this gif here to understand: http://www.sengpielaudio.com/StandingWaves.htm . There are 5 stages to a cycle/period of a stationary wave. First is the point/state in which it starts usually when it is up, this is at 0 degrees. Then at 90 degrees, it goes down to 0 zero displacement. Then at 180 degrees, it goes down. At 270 degrees back to zero displacement. At 360 degrees its back to its original position(at the start). In the question you asked, it was asking about it in terms of the period not degrees. So 4ms for the whole cycle. At 0 ms, the wave is down. At 1 ms, it is at zero displacement. At 2 ms, it is up. At 3 ms, it is at zero displacement. At 4 ms, it is back down(original position). At 5 ms, it is back at zero displacement. So the final answer is 0(B).

 

[aisya farooq]

When finding the gravitational energy of regular objects, you take their height from the centre of mass. Initially, the centre of mass of the block is 0.3m above the edge of the step. So the initial gravitational energy is 500(0.8+0.3)=550J. After the block falls, its centre of mass is 0.2m above the surface. Now the gravitational energy is 500(0.2)=100J. The change in gravitational energy is 550-100=450J (C)

thankyouu

 

[Kanekii]

Help.

 

[Ayesha_m01]

can someone explain me this

 

[MShaheerUddin]

Its B 4.2 N

 

[MShaheerUddin]

Torque of Couple = Force x Distance between two forces which is perpendicular to the line of action of force
Here we will take vertical component of force (8Sin60) so that distance is perpendicular to it.
8Sin60 x 0.6 = 4.2 N

 

[angryrider_56]

Help?

 

[Ebrahim12]

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Q26. Taking velocity to the right as positive gives:

Total P = (2m)(u) - (m)(u) = mu

A) (m)(5u/3) - (2m)(u/3) = mu
B) (m)(2u/3) - (2m)(u/6) = mu/3
C) (m)(2u/3) + (2m)(u/6) = mu
D) (3m)(u/3) = mu

Only B doesn't follow the principle of conversation of momentum.

 

[Ebrahim12]

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Q21. According to principle of conversation of momentum :
Total momentum before emission (P1) = Total momentum after (P2)

P1 = o (stationary)

The nucleus emits one proton, so the daughter nucleus will have a nucleon number of A- 1.
It's mass will be (A-1)m, where m is the mass of one nucleon.

Momentum of nucleus = (A-1)mu
Momentum of proton = -mv
(Opposite signs because of the opposite directions of the velocities).

P2 = (A-1)mu - mv

P1 = P2
0 = (A-1)mu - mv
mv = (A-1)mu
v = (A-1)u (B)

 

[angryrider_56]

Q21. According to principle of conversation of momentum :
Total momentum before emission (P1) = Total momentum after (P2)

P1 = o (stationary)

The nucleus emits one proton, so the daughter nucleus will have a nucleon number of A- 1.
It's mass will be (A-1)m, where m is the mass of one nucleon.

Momentum of nucleus = (A-1)mu
Momentum of proton = -mv
(Opposite signs because of the opposite directions of the velocities).

P2 = (A-1)mu - mv

P1 = P2
0 = (A-1)mu - mv
mv = (A-1)mu
v = (A-1)u (B)

You sir, deserve a f***ing medal....Thanks alot dude

 

[Ebrahim12]

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Q22. The gradient of displacement-time graph is velocity, so when the gradient is zero the velocity and thus KE are also zero.
The gradient is zero at time = 0, 0.5T, T, 1.5T and 2T
So the KE must be zero at those time coordinates too, only graph D shows this.

Q14. F is the force provided by each beam.
They act towards the hinge like this:


The system is in equilibrium, the vertical components = the weight and the horizontal components cancel out.
Vertical component of one beam = Fsin(50)
Total upward forces = 2Fsin(50)

2Fsin(50) = 4 x 10^4

solving for F gives F = 2.6 x 10^4 (A)

Q10.

ma = force in direction of motion - opposing force

for block M:
Ma = Mg- T
for block m:
ma = T - mg

Add equation 2 to 1:
Ma + ma = Mg- mg
Solving for a gives B as the answer.

Q37.

Use KCL (I flowing into = I flowing out of a junction) to find the currents in the resistors.

Use KVL (EMFs = PDs across a closed loop) to find R.

EMF = PD
12 = (4)(0.5) + (1)(R) + (1.5)(R)
10 = 2.5R
4 = R (B).

Q6. The sandbag was in motion in the balloon, meaning that it's initial velocity when dropped is 3m/s upwards.
It will have a parabolic motion while under acceleration of gravity.
The displacement of the bag during the 5 seconds will be the distance between its starting position and the ground, which is the height of the balloon when it was released.

S = ut + 0.5at^2
S = (-3)(5) + 0.5(9.81)(5)^2
S = 107.625 (B)

Note that the initial velocity and acceleration are in opposite directions.

 

[Ebrahim12]

You sir, deserve a f***ing medal....Thanks alot dude

No problem

 

[Mush999]

Help pls!!

 

[Mush999]

What do they mean by this?

 

[hellodjfos;s'ff]

Help pls!!

I answered this on page 898. You check the answer there.

 

[hellodjfos;s'ff]

What do they mean by this?

You apply v=f(lamda) to find the frequency. You know the wavelength of the visible spectrum ranges from 400nm to 700nm, and that the velocity of electromagnetic waves is 3x10^8.
So 3x10^8=400x10^-9(f)
The f turns out to be 7.5x10^14 Hz. Hence, the order of frequency is 10^14 Hz(C).

 

[hellodjfos;s'ff]

What do they mean by this?

Oh yeah i forgot to answer your main question lol.
They are asking us to find the order(standard form) of frequency of the visible spectrum.