# Wave problem

[SOLVED] Wave problem

## Homework Statement

a small stone fell into alake causing 40 waves after 4 seconeds, if the radius of the outter circle is 1.6 meters, find the length of the occurred wave and its velocity

## The Attempt at a Solution

well i got two different solutions so far cause i couldn't determine what exactly was meant by (r=1.6m & length of the occurred wave):

1- f = 10 Hz

lambda/4 = 1.6
lambda = 6.4 m

v = lambda.f
= 6.4 × 10
= 64 m/s

========================

2- v = x/t

x = 2pi.r
= 10 m

v = 10/4
= 2.5 m/s (oO)

Doc Al
Mentor
Neither approach is good. Instead, picture what's going on. The waves travel outward from the center. The outer radius tells you how far the first wave traveled in the given time. Use that to find the speed of the wave.

Also, you have 40 waves (and thus 40 wavelengths) stretched out over that radius. How big is each wavelength if 40 of them add to the outer radius?

aha! so:

v = 1.6/4
= 0.4 m/s

and...

lambda = 0.4/10
= 25 m

i hope i got at least closer ^^

Doc Al
Mentor
aha! so:

v = 1.6/4
= 0.4 m/s
Good!

and...

lambda = 0.4/10
= 25 m
Not good. If 40 wavelengths span 1.6 m, how can each wavelength be 25 m?

I don't think i get it is it that i reversed the final result 25->0.04 (this happens all the time and its really annoying) or i simply skipped a step that i cant figure?

Doc Al
Mentor
is it that i reversed the final result 25->0.04 (this happens all the time and its really annoying) or i simply skipped a step that i cant figure?
That must have been what you did. You wrote .4/10 (which is correct), but you reversed the answer.

What I would have done is this:
40 Wavelengths = 1.6 m
1 Wavelength = (1.6 m)/40 = 0.04 m

I knew it :rofl:

the problem was that i haven't known that the wavelengths make out the radius as this is my first encouter with circular waves

anyways thanx alot Doc Al cya 