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# How far does the wave travel? watch

1. I'm confused, what is the difference between the distance a wave travels and the wavelength. My textbook tells me speed =wavelength/time period.
The question is the frequency of a soundwave is 50Hz, represented as a transverse wave find:
a) How many waves are emitted in 20 seconds. It's 50*20 =1000 correct?
b) The wavelength has changed to 20cm. how far does the wave travel in 20 seconds, I really don't understand this part.
c)Speed of the wave, doesn't provide context, I assumed it is the speed of the wave with a wavelength of 20 cm but both to be on the safe side.
2. Wavelength is the distance between one peak of the wave and the next. Wavelength doesn't tell you anything about how far the wave has travelled or will travel.
Are you sure you're not misquoting the question?
You can calculate a wave speed if you know what the wavelength and frequency are at the same time.
But if the question is saying the wavelength changes without giving you the new frequency you don't have enough to work out the speed.
3. What other information is needed to calculate how far a wave has travelled in a given amount of time?
4. What medium are the waves travelling in and at what temperature? Sound waves travel at different speeds in different mediums and temperatures.

A) The amount of waves depends on how much is emitted from the source. I don't exactly understand a question, a better question would be what you have in b, how far has it travelled in 20 seconds.
B) If the wavelength has changed to 20cm, that means the frequency has changed.
C) This depends entirely on the medium the wave is travelling in and at what temperature. This refers back to my first sentence.
5. a) f=50Hz, T=20, find the number of waves emitted
b) wavelength =20cm (I found this contradictory with part a so I assumed it entered a new medium, is this not the case?), find how far it has travelled in 20s
c)Find the speed
6. (Original post by Primus2x)
a) f=50Hz, T=20, find the number of waves emitted
b) wavelength =20cm (I found this contradictory with part a so I assumed it entered a new medium, is this not the case?), find how far it has travelled in 20s
c)Find the speed
TBH this would probably work much better if you gave us the exact text of the question instead of inviting us to interpret your interpretation of it.
7. (Original post by Joinedup)
TBH this would probably work much better if you gave us the exact text of the question instead of inviting us to interpret your interpretation of it.
Very well
10 A source of waves has a frequency of vibration of 50Hz.
a) How many complete waves are emitted in 20s?
b)The wavelength is 20cm, how far has the wave travelled in 20s?
c)Calculate the speed of the wave.
8. (Original post by Primus2x)
I'm confused, what is the difference between the distance a wave travels and the wavelength. My textbook tells me speed =wavelength/time period.
The question is the frequency of a soundwave is 50Hz, represented as a transverse wave find:
a) How many waves are emitted in 20 seconds. It's 50*20 =1000 correct?
b) The wavelength has changed to 20cm. how far does the wave travel in 20 seconds, I really don't understand this part.
c)Speed of the wave, doesn't provide context, I assumed it is the speed of the wave with a wavelength of 20 cm but both to be on the safe side.
Pretend a wave is your finger that you will move in a straight line. the wavelength can represent the length of your finger/wave. Then the distance the wave travels is how much you move your finger .

a) the Time period of a wave,T, is 1/frequency. so do you think you were correct?

(Original post by Primus2x)
Very well
10 A source of waves has a frequency of vibration of 50Hz.
a) How many complete waves are emitted in 20s?
b)The wavelength is 20cm, how far has the wave travelled in 20s?
c)Calculate the speed of the wave.
b) You didn't know the wavelength before so now they are telling you. Clue is your own post:
(Original post by Primus2x)
What other information is needed to calculate how far a wave has travelled in a given amount of time?
what else do you think is needed to work out distance traveled during a specific time which they have given you?

c) since they have only given you one wavelength, yes I would assume you need to use that one only.

Think about it and get back to me(quote me)
I hope I could help!
9. (Original post by RVNmax)
Pretend a wave is your finger that you will move in a straight line. the wavelength can represent the length of your finger/wave. Then the distance the wave travels is how much you move your finger .
a) the Time period of a wave,T, is 1/frequency. so do you think you were correct?
(1/50)/20=1/1000, a thousandth?
(Original post by RVNmax)
b) You didn't know the wavelength before so now they are telling you. Clue is your own post: what else do you think is needed to work out distance traveled during a specific time which they have given you?
The wavelength is 20cm or 2*10-1m to find the distance travelled do I multiply the wavelength by the number of waves there are? So it's my answer to part a) * the wavelength
(Original post by RVNmax)
c) since they have only given you one wavelength, yes I would assume you need to use that one only.
Think about it and get back to me(quote me)
I hope I could help!
Speed is distance over time, so part b)/20
So this all depends on part a) being correct.
10. f=50hz so in one second you get 50 complete wave cycles, peak and trough. The waves move away from the source at an unknown speed.
The distance between peaks (wavelength) is 0.2 meters.
The distance covered by one seconds worth of waves is the wavelength multiplied by the number of waves emitted in a second. I.e. At the instant you finish emitting the 50th peak, the first peak is 50*0.2 meters away.
It took one second to emit those 50 complete wave cycles.
Distance per second is a speed, it's the speed of the wave.

This is also in the formula :
lambda=v/f which you should try memorise imo.

Once you've got the speed you can answer the questions.

The question didn't say anything about changing the wave media so I still don't understand where you think the contradiction arose.
11. (Original post by Joinedup)
f=50hz so in one second you get 50 complete wave cycles, peak and trough. The waves move away from the source at an unknown speed.
The distance between peaks (wavelength) is 0.2 meters.
The distance covered by one seconds worth of waves is the wavelength multiplied by the number of waves emitted in a second. I.e. At the instant you finish emitting the 50th peak, the first peak is 50*0.2 meters away.
It took one second to emit those 50 complete wave cycles.
Distance per second is a speed, it's the speed of the wave.

This is also in the formula :
lambda=v/f which you should try memorise imo.

Once you've got the speed you can answer the questions.

The question didn't say anything about changing the wave media so I still don't understand where you think the contradiction arose.
So I work backwards from what I thought? Find the speed first, then the distance then the number of waves?
12. (Original post by Primus2x)
So I work backwards from what I thought? Find the speed first, then the distance then the number of waves?
I would find the answer to c (Speed in m/s) and use it to answer b (distance in 20s).
Part a. you can just answer straight away without knowing the wavelength.

Fwiw old exam questions used to ask you for the answers in the 'wrong order' i.e. Intermediate result last, quite often. Otoh modern exam questions tend to ask you for the answers in the order in which you'd calculate them. Just something to bear in mind if you're practicing on old exam papers.
13. (Original post by Primus2x)
(1/50)/20=1/1000, a thousandth?
The wavelength is 20cm or 2*10-1m to find the distance travelled do I multiply the wavelength by the number of waves there are? So it's my answer to part a) * the wavelength

Speed is distance over time, so part b)/20
So this all depends on part a) being correct.
a) why did you do that calculation? Do you know what time period means? it means the time for one full wave of given wavelength to pass a given point. so when you worked out 1/50 that's the time taken for one wavelength to pass in unit seconds(as the unit for frequency was in Hz), so how many wavelengths will pass in 20 seconds?

b)
(Original post by Primus2x)
So I work backwards from what I thought? Find the speed first, then the distance then the number of waves?
yes. like I said earlier, you've got the time and you need to work out distance, so which other quantity do you need? It is the speed, which you can work out.

c) I didn't want to give you the formula because I wanted you to look back at your notes/remember it/work it out looking at what you were given.

I think you won't have any problems on that now. If you do struggle, just keep at it for a while before you ask again. If you need confirming of final answers then quote me.
14. Right, I've worked backwards;
c)V=lamda*f=0.2*50=10ms^-1
b)V=d/T=10=d/20, 20*10=d=200m
a)200/50=4 complete waves, still not sure on this part, frequency is not just the reciprocal of time period, its how many waves emitted in 1 unit time, so distance over frequency? I'd have thought that wavelength is involved. 200/0.2 wavelengths=1000 waves.
15. How far a wave has travelled and wavelength are independent. Wavelength is the distance from one peak of the wave to the other. However how far a wave has travelled can be worked out using a few equations, e.g wavelength= speed/frequency
16. (Original post by Primus2x)
Right, I've worked backwards;
c)V=lamda*f=0.2*50=10ms^-1
b)V=d/T=10=d/20, 20*10=d=200m
a)200/50=4 complete waves, still not sure on this part, frequency is not just the reciprocal of time period, its how many waves emitted in 1 unit time, so distance over frequency? I'd have thought that wavelength is involved. 200/0.2 wavelengths=1000 waves.
yeah you had part A right in the first place, 50hz is 50 cycles per second. In 20 seconds it'll make 50*20 waves. That's all there is to part A.
17. (Original post by Primus2x)
Right, I've worked backwards;
c)V=lamda*f=0.2*50=10ms^-1
b)V=d/T=10=d/20, 20*10=d=200m
a)200/50=4 complete waves, still not sure on this part, frequency is not just the reciprocal of time period, its how many waves emitted in 1 unit time, so distance over frequency? I'd have thought that wavelength is involved. 200/0.2 wavelengths=1000 waves.
c and b are correct.

You don't need c or b for part a). It is very simple -
A source of waves has a frequency of vibration of 50Hz.
a) How many complete waves are emitted in 20s?

If you know how long one wave takes to pass a certain point, a.k.a. the Time Period, how many of those wave will pass in 20s. I would do 20s/(1/50Hz), yes that is the answer you originally got, which I never said was wrong.
Though the way you did it right at the end was also correct as total distance in 20s divided by length of each wave just says same thing.
18. Thanks, wont hear back from it until after Easter.

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