Gr. 11 Physics question [Archive] - Team17 Forum

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[UFP]Ghost
14 Sep 2007, 22:02
Here goes: "After a transverse ware pulse had traveled 2.5 m through a medium, it has a speed of 0.80 m/s. How would this speed have differed if:
a) the pulse had been twice the size?
b) the pulse had had twice the energy?
c) the pulse had traveled twice the distance?"
I think the answer to all three is not at all from how I understand it?
If I'm wrong could you explain why?
Thanks in advance,
J K

Made in
15 Sep 2007, 01:59
a) Speed = size x freq.
Speed ' = 2 size x freq.
Speed ' = 2 Speed
The speed would be twice as fast.
b) Not at all. The amplitude of the wave has no relations with it's speed.
c) Speed = distance / time, if the distance is 2x, the speed is 2x.
I think this is it. If you don't think so, tell me.
Physics! :D

[UFP]Ghost
15 Sep 2007, 02:51
a) Speed = size x freq.
Speed ' = 2 size x freq.
Speed ' = 2 Speed
The speed would be twice as fast.
b) Not at all. The amplitude of the wave has no relations with it's speed.
c) Speed = distance / time, if the distance is 2x, the speed is 2x.
I think this is it. If you don't think so, tell me.
Physics! :D
I got 3, after i posted this while thinking about it at dinner. it's #1 I'm confused about. The 'pulse size' like the size of the medium or? Could you expand on that.
edit: yes but in 3, if distance increases doesn't frequency decrease?

Made in
15 Sep 2007, 03:27
In 3, I think the question makes a referenco to the distance of the medium, not the wave size, so that wouldn't change the frequency.
In 1, the wave size would be the double, and the frequency the same. As we know, speed = wave size/length x freq, if the size is 2x, speed is 2x.
I would recommend you to ask your teacher to have an clearer explanation of that. :P

[UFP]Ghost
15 Sep 2007, 03:58
In 3, I think the question makes a referenco to the distance of the medium, not the wave size, so that wouldn't change the frequency.
In 1, the wave size would be the double, and the frequency the same. As we know, speed = wave size/length x freq, if the size is 2x, speed is 2x.
I would recommend you to ask your teacher to have an clearer explanation of that. :P
Now i just don't get 1.
If the wave size is larger then it would take longer for one cycle?
edit: btw thanks for all the help so far.
edit 2: according to my teachers handout: frequency and wavelength are inversely proportional - when one increases, the other decreases.

Made in
15 Sep 2007, 04:00
Ghost;608002']Now i just don't get 1.
If the wave size is larger then it would take longer for one cycle?
edit: btw thanks for all the help so far.
No, because the frequency (cycles per second) would be the same.

[UFP]Ghost
15 Sep 2007, 04:02
No, because the frequency (cycles per second) would be the same.
but i don't understand why that is. If I increase the distance of one cycle and then it takes longer to do it.

Made in
15 Sep 2007, 04:06
Not if you keep the same frequency. If you do one cycle per second at first, for example, and you keep the same frequency, you would still do one cycle per second, even if it's a longer cycle. As a consequence, the speed would increase.

[UFP]Ghost
15 Sep 2007, 04:10
so i assume it's the same frequency or is it somewhere in the question, or is it implied?
so the speed is the same because the size of the wavelength doubled but the frequency is the same so it covers more per second?

Made in
15 Sep 2007, 04:17
See if you can understand if I put like this:

Made in
15 Sep 2007, 04:21
The speed is the double, because the wave length doubled but the frequency is still the same. It does the same number of cycles per second, but longer cycles. Would reach longer distances in same time.

[UFP]Ghost
15 Sep 2007, 04:21
I understand that. I'm just not perfect on the non frequency changing. Are all these things (a,b,c) if i keep everything else but change this one thing then: this will happen.

Made in
15 Sep 2007, 04:27
I'll see if I can think how can I explain the frequency thing in a clearer way later.
Got to go now, see ya.
PS: Also, check out this site: http://www.glenbrook.k12.il.us/gbssci/Phys/Class/waves/u10l1c.html , it has some information about waves and stuff.

AndrewTaylor
15 Sep 2007, 08:51
The speed is the double, because the wave length doubled but the frequency is still the same. It does the same number of cycles per second, but longer cycles. Would reach longer distances in same time.
Who says the wavelength doubled?
"Pulse size" is a very ambiguous term and could refer to the wavelength, amplitude, pulse length, pulse area or any one of several combinations of those. I wouldn't assume wavelenth had changed, particularly because wavelength is dependant on frequency, so it can't change without the frequency changing first.
This is a very poorly phrased question, but it really matters not. Light is a transverse wave, so if it's possible to generalise (which it probably is, and if it isn't then this question is impossible) then speed is constant in any scenario, because of relativity.

[UFP]Ghost
15 Sep 2007, 16:06
so was my original answer of none of these change the speed, correct, AT?

AndrewTaylor
15 Sep 2007, 16:31
Yeah, I reckon so.

[UFP]Ghost
15 Sep 2007, 16:40
Thanks AT. I'll come back with an answer on monday...if she shows up this class.

Alien King
15 Sep 2007, 18:13
Get some clarificaiton on that last one too. It's terribly written.

[UFP]Ghost
15 Sep 2007, 18:57
Get some clarificaiton on that last one too. It's terribly written.
Thats why I've been having so much trouble.

SomePerson
15 Sep 2007, 20:15
Yeah, I can't possibly imagine "size" to refer to anything but amplitude. Wavelength is wavelength, and in my mind is entirely different.

farazparsa
15 Sep 2007, 21:39
Now I can't sleep until I find the answer to this. :eek:
It never said the wavelength doubled.

Paul.Power
15 Sep 2007, 21:44
Ambiguous question. Talk to your teacher.

Alien King
15 Sep 2007, 21:55
Now I can't sleep until I find the answer to this. :eek:
It never said the wavelength doubled.
Well my initial thoughts say that the speed doesn't change for any of them.
The wavelength doesn't double either, or if it does, then the frequency would be halved.

Plasma
15 Sep 2007, 22:02
Twice the distance shouldn't make any difference, unless there's some hidden catch to the question.
But I can't figure out how "twice the size" and "twice the energy" come into it.
The wavelength doesn't double either, or if it does, then the frequency would be halved.
I was under the impression that only the speed and the feature that's doubled would change, and everything else would have the same value. At least, that's how I think these kind of questions normally work.

Alien King
15 Sep 2007, 22:08
Twice the energy basically means twice the amplitude.
Increased size could only mean amplitude as far as I can tell. If it means twice the wavelength, then it's worded very badly.
A wave's speed is constant when travalling through the same medium. For example, the speed of light is constant and as far as I know, the same applies to any wave. Just grab a piece of string or something and make those wave patterns. The speed is always the same, no matter what you do to it.
Oh, and Ghost mentioned this:
according to my teachers handout: frequency and wavelength are inversely proportional - when one increases, the other decreases.

Plasma
15 Sep 2007, 22:11
Twice the energy basically means twice the amplitude.
Increased size could only mean amplitude as far as I can tell. If it means twice the wavelength, then it's worded very badly.
I thought that too, but both questions asking the very same thing couldn't be right.
Oh, and Ghost mentioned this:
Again: very badly worded question!

thomasp
15 Sep 2007, 22:13
Surely if it's a transverse wave and the medium remains constant, then the speed of the wave will remain the same?
Think of a lightwave - that's a transverse wave, isn't it? Also, if you drop a stone into a pond, the ripples that you get are transverse waves. I'll use that as an example
Let's assume the "pulse size" is the cross sectional area of the wave transmitter - the size of your stone
Therefore, if the size of the pulse is doubled (ie: area doubled), as in q1, the speed will be the same, it's just occupying a larger area. You've just thrown a boulder into the pond, as opposed to a pebble. The ripples will still go out from the centre at the same speed. And you'll get soaked :p
If you throw the pebble into the pond harder, as in q2 (as opposed to just using gravity's force), you'll get bigger initial waves (greater amplitude), but they'll still move out at the same speed. You'll probably also get wet :p
Now, question 3. This is a strange one. The waves/ripples will travel at a constant velocity through the pond, if you assume the water is in a vacuum or neglect air resistance - but their amplitude will decrease as the distance from the stone increases. Throwing the stone in from a greater height will not affect the waves, as that's just the same as throwing it in faster/slower.
If I were you, I'd ask for clarification.

[UFP]Ghost
15 Sep 2007, 23:51
Now I can't sleep until I find the answer to this. :eek:
It never said the wavelength doubled.
monday....monday....damn it's been going through my mind since monday....to many days off school.....DAMN!

[UFP]Ghost
16 Sep 2007, 21:00
I thinks me got it!
a) amplitude itself increasing.
b) is talking about how much energy is moving through it.
c) Wavelength increasing and frequency would decrease as a result making the speed the same.
Therefore none change as AT said.

SomePerson
16 Sep 2007, 21:16
I think C is asking if the wave slows down as it gets further away. Note the "had traveled 2.5m" and then if the pulse "had traveled twice as far". Basically it's asking what speed the wave will be travelling after going twice as far.
But I still agree that none change.

farazparsa
16 Sep 2007, 23:03
I couldn't understand what C was asking. Talk to your teacher.

[UFP]Ghost
18 Sep 2007, 00:38
a) was bigger amplitude.
b) was more energy causing bigger amplitude
c) was like i send a wave down a slinky and it comes back to me and not just from one end to the other.
so I was right about what a/b meant and i knew all 3 didn't change the speed.
Thanks for all your help :)

farazparsa
18 Sep 2007, 00:48
http://www.archinofsky.com/stuff/may/20x10_winner_is_you_web.jpg

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