How Does Amplitude, Energy, and Travel Distance Affect Wave Speed?

[BoggyP]

Homework Statement

"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?"

Homework Equations

V = (Lambda) x (Frequency)

The Attempt at a Solution

a) I think pulse size = amplitude, and doesn't affect the speed?
b) Have no clue what twice the energy means.
c) Well I think nothing again because when you increase distance, you decrease frequency?

 

[Dick]

The energy of a wave (as you might guess) is a function of the amplitude. So it's really a change in size. So the answer to b) is the same as a). Why would you think increasing distance would decrease frequency?

 

[BoggyP]

i understand that now, thanks :)

For a) (only one i don't get) if the pulse size increases, is that where the frequency decreases. because frequency and wavelength are inversely proportional.

 

[Dick]

Now I have to ask why you would think increasing pulse size would affect either of those. To me, pulse size means the amplitude of the pulse, not it's length. Or am I interpreting the word 'size' wrong?

 

[BoggyP]

Now I have to ask why you would think increasing pulse size would affect either of those. To me, pulse size means the amplitude of the pulse, not it's length. Or am I interpreting the word 'size' wrong?

I think pulse size = wavelength and energy = amplitude. From what i understand now at least. (not positive)

 

[Dick]

I think pulse size = wavelength and energy = amplitude. From what i understand now at least. (not positive)

That's a little confusing. I would assume pulse size=amplitude and answer on that basis. You've said correctly that velocity is determined by wavelength and frequency. Do you think any of the changes in the questions would affect either of those?

 

[BoggyP]

going by what you think, only c) and it would double. From the way i understand it a) & c) would. also another important formula V=(lambda) / (Period)

 

[Dick]

going by what you think, only c) and it would double. From the way i understand it a) & c) would. also another important formula V=(lambda) / (Period)

Uh, frequency=1/period. They are really the same formula. I'm really having a hard time understanding what you are saying. "only c) and it would double", what would double? "From the way i understand it a) & c) would." Would what? Could you use complete sentences?

 

[BoggyP]

Uh, frequency=1/period. They are really the same formula. I'm really having a hard time understanding what you are saying. "only c) and it would double", what would double? "From the way i understand it a) & c) would." Would what? Could you use complete sentences?

It asked how would it affect the speed? If I go by your belief that a) & b) are amplitude then in c) the speed doubles. If I go by my belief that a) is wavelength then both a) and c) have twice the speed while b) is amplitude and doesn't affect speed.

 

[Dick]

Thanks, that helps. No wonder it was so confusing. We were talking about different things. If you believe that 'size' and 'distance' both refer to 'wavelength' and the frequency remains constant, then, yes, a) and c) both double the speed. I don't think that was the intention of the question - but then I don't read minds. BTW, by how I read the questions I would say none of them affect the speed. In c) distance for me just means how far the wave has travelled. Can't affect speed. Maybe time to have a chat with the author of the question.

 

[BoggyP]

Thanks, that helps. No wonder it was so confusing. We were talking about different things. If you believe that 'size' and 'distance' both refer to 'wavelength' and the frequency remains constant, then, yes, a) and c) both double the speed. I don't think that was the intention of the question - but then I don't read minds. BTW, by how I read the questions I would say none of them affect the speed. In c) distance for me just means how far the wave has travelled. Can't affect speed. Maybe time to have a chat with the author of the question.

Should i call up McGraw-Hill Ryerson?

When i ahve physics next i'll come back here and post the answer and i'll come back if I have any further break thoughts. Thanks for your help so far. (btw, originally i agreed with your final answer :P)

 

[Dick]

Nah, leave McGraw-Hill alone. If you look at any question hard enough you can find something to bicker about. To go back to the only thing we've discussed that has substance beyond word definitions the energy in a wave of fixed frequency and wavelength is proportional to the amplitude squared. Word to the wise.