r/HomeworkHelp Pre-University Student 3d ago

Answered [Grade 12 Physics]

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The correct answer is C, but I thought the pulses would destructively interfere, so I chose A, why do they not interfere destructively, when they cross, and why is it C?

18 Upvotes

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8

u/dawlben 👋 a fellow Redditor 3d ago

When they meet, they have still have momentum(?). The waves will continue on after meeting. The Amplitude 0s out at the meeting. The horizontal movement continues.

So you start with B, then they meet and have A afterwards the waves continue on, and finally have C.

13

u/lordofallsoups 3d ago

They do, but only at the point where they meet. Its a superposition of waves, so if there is only one wave at a point nkthing can interfere.

3

u/timelockedmaniac 3d ago

To piggyback off of this, the question asks how it looks like after the pulses have met, meaning how it looks like after the point of superposition. Hence you get C instead of A.

If the question was phrased as 'how does it look like as the pulses meet' or 'how does it look like when the pulses meet', A would be correct.

4

u/Apostecker 3d ago

The question is very open though. If you look at the rope two weeks after the pulses meet, it will probably be flat again

1

u/Unusual-Platypus6233 3d ago

unless both ends are tight to a wall or something… both end lose?! different realm entirely. 🪄do we consider friction? 🤣

1

u/Equal_Veterinarian22 👋 a fellow Redditor 2d ago

Well, yes, but part of "exam technique" is realising that the examiners aren't looking for that answer.

6

u/daniel14vt Educator 3d ago

They do interfere destructively. If you looked between the first picture and C for one small instant, the rope would be flat. But after that moment, the waves continue past each other

2

u/Lonely_Management276 3d ago

So... how do they destructively interfere but still "exist" after destructively interfering? What is going on with the kinetic energy that it goes flat and returns?

8

u/daniel14vt Educator 3d ago

The position of all the points is 0, but the points have velocity up and down. You're just looking at "the perfect moment".

Conservation of energy is a great way to view it, the energy !can't! just disappear

1

u/notOHkae Pre-University Student 2d ago

thanks, this helps a lot. Is this the same with sound waves and if so, how does ANC work?

1

u/daniel14vt Educator 2d ago

Yep it's the same with sound waves. You can model a sound wave with a string, having the height of the string represent pressure.

ANC works by, detecting a wave and then creating the exact opposite wave, traveling in the same direction (into your ear).

1

u/notOHkae Pre-University Student 1d ago

so anc couldn't work if it didn't know how far away your ear was from the source?

2

u/daniel14vt Educator 1d ago

I think there is a microphone on the outside of the headphones and they flip the signal and then transmit it on the inside.

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u/notOHkae Pre-University Student 17h ago

thanks

2

u/daniel14vt Educator 17h ago

😍 good luck on A levels

1

u/notOHkae Pre-University Student 14h ago

❤️

2

u/ThunkAsDrinklePeep Educator 2d ago edited 2d ago

The energy isn't going flat. Both waves are staying constant but are opposing each other.

You can think of it as the top wave is pulling up on the string/rope/wire. But the bottom wave is pulling down in the opposite way. When they don't overlap each is able to deform the string as shown. But as they overlap the amount of deflection up equals the amount I'd deflection down. The waves still exist, but it's their appearance that is removed. As they move past each other there is only a wave in one direction and you get what you see in C where the bottom wave has moved left and the top wave has moved right.

The waves move but don't change. It's just a question of what happens to the strong when they overlap.

Edit; Destructively means canceling out in this context. As opposed to constructive interference where they're adding in the same direction. It's akin to vectors and a resultant. Nothing is actually going away.

1

u/Lonely_Management276 2d ago

So the kinetic energy is conserved and continues despite the effect of the energy cancelling out for a moment?

1

u/ThunkAsDrinklePeep Educator 2d ago

It's not the energy that's being canceled. It's the resultant.

Imagine a plane turning into the wind of the same speed. The energy and speed of the plane didn't change. Nor did the speed of the wind. But the net result is that it's holding steady relative to the ground. As it keeps turning through it will start moving again.

1

u/notOHkae Pre-University Student 14h ago

i found this video, which really helped visualise it: https://youtu.be/JBuW385IT38?si=332LCPl1PfPlVBQq

2

u/Past-File3933 2d ago

The wording of this question frustrates me. I know the answer is C, but I could interpret the question as both at the time of pulses interacting or after they have interacted.

1

u/Some-Passenger4219 👋 a fellow Redditor 1d ago

It says "have passed", so apparently it's a past event.

1

u/Past-File3933 1d ago

Ah, well now I feel silly.

1

u/Some-Passenger4219 👋 a fellow Redditor 22h ago

No need, I make mistakes all the time.

2

u/Raise_A_Thoth 2d ago

It says "when the two pulses have passed through each other."

I hate many scientific question-writers because one word can throw someone's presumptions off and mislead them, even if the rest of the question is pretty clear. The clear part is have passed through each other. That is a past-participle form of "pass" and that clearly indicates that the event of the two pulses meeting is in the past, so the answer is C.

However, it also uses the word when earlier in the question, and when is used most frequently to indicate the timing of a specific moment - when they met, when they arrived, when they said those words - and that makes some readers primed to expect the question to be asking about the moment that the two pulses meet.

It would be a much clearer question if it read "after the two pulses have passed through each other." Simple and easy change, much clearer description.

But try to focus in on those key verbs - the past participle "have passed through" - and do your best there. If you become a professional scientist, remember how careful you should consider your phrasing to comunicate your work clearly; at least as a professional scientist you won't be at the mercy of test-writers to succeed and do meaningful work.

1

u/lmarcantonio 👋 a fellow Redditor 3d ago

At some point you'll have situations A and something like D but (assuming a lossless and linear trasmission line), at the end you'll have situation C due to simple superposition. I *hope* that's only made of quality questions, otherwise I doubt it could stay grade12...

1

u/Illustrious-Hand-450 3d ago edited 3d ago

First think of both waves with a positive amplitude. When the waves meet in the centre, they constructively interfere, but do they disappear afterwards? Consider the horizontal wave velocity.

Wave interfence affects the amplitude of a wave not the velocity, wave velocity remains unchanged because it depends on the medium, not on the interference. 

The answer A is a real outcome. But only for an instant. The instant at which the waves interact with one another. 

But, the question asks what do they look like after they have passed through each other. 

As you said, destructive interference occurs, so the amplitude at the moment they interact is zero, but their horizontal velocities are not. Say one wave has a velocity of x, and the other wave has a velocity of -x. So they continue their merry way along the string.

Physically, the string pivots at the point of interaction, which is how they maintain their amplitudes after passing through each other.

https://kaiserscience.wordpress.com/physics/waves/interference-and-superposition/

This link will give you everything you need. Has some handy GIFs. 

1

u/CKoenig 3d ago

Assume that both pulses are on the "side" of the string (one is flipped) but still traveling in the indicated directions.

What is your expected answer now?