# homework help: physics help pleaseeeee. - Help.com

christine445

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## physics help pleaseeeee.

Periodic waves spread out over the surface of a lake where two women, Jenny and Melissa, are fishing in separate boats 47 meters apart. Each woman’s boat bobs up and down 30/min. At a time when Jenny’s boat is at a crest, Melissa’s boat is at its lowest point, and there are 2 additional crests between them. Calculate the wavelength of these water waves. And what is the speed of these water waves?

ohh…. turn # of vibrations per minute into vibrations per seconds (called frequency).

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so how many waves in that 47 meters?

So I know this is probably a bit after you needed it since it have been a few days but maybe this will help anyway.

First read the question and only take the information you need from it and what you need to find
Distance between the two points , rate of which the boats crest [i.e. the frequency of the waves] and how many waves are in the space.
Need Speed and wavelength
Next isolate that information so you can see it clearly
Distance between boats = 47m
frequency = 30 waves/ 1 minute = 30/60s = 0.5waves/sec = 0.5hz
Waves in space = 2 crests

Third step make a diagram
J |–~~-47m-~~–|M
n_____n______n_____n
|-.~.~-|
…~..~.|-~-.-.-|
….~..~..~.~….|-~-
= distance between crests = Waves = 2.5 waves in 47m
*We know it’s 0.5 of a wavelength because M is said to be at the lowest point, in otherwords at the half point if we are measuring from the crests
Now change that into something we can use
47 m / 2.5 waves
=18.8m wavelength
Good, one unknown found

All we need to do now is to put it in the formula
s = f * wavelength
s = .5hz * 18.8m
s = 9.4m/s

That should be correct answer but it’s really the technique that’s important. Being able to make a diagram that will help you figure out the problem is the biggest challenge but once you’ve got it you should be ok.
Good luck!

p.s. I appologize if my diagram is a little wonky it’s difficult to get these things to work without being able to draw ; )

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