# Wave Speed

## Homework Statement

A group of swimmers is resting in the sun on an off shore raft. They estimate that 2.2 m seperates a trough from an adjacent crest of surface waves on a lake. They count 18 crests that pass by the raft in 31s. How fast are the waves moving?

## Homework Equations

F= 1/t v=wavelength x frequency

## The Attempt at a Solution

2.2(2)=4.4= wavelength

4.4 x 31? or 4.4 x 18? or something else?

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Dick
Homework Helper
I think pugfug was being alarmist about the link. Welcome back. Sorry about the panic. Anyway we now want frequency, the number of wavelengths per second. How many wavelengths in 31 secs?

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You need to calculate how many crests pass per second, then multiply by wavelength. This gives distance/time which is a velocity.

Astronuc
Staff Emeritus
2.2(2)=4.4= wavelength
Correct. The wavelength is twice the peak-to-trough distance.

Now speed is length/time, so find the total length of the waves passing the raft in t=31s.

Ok so would I do 4.4 x 31? Or do I have to incorporate the 18 in there somewhere?

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Sorry guys I know Im like kinda slow at this, I just suck really bad at physics and math in general really

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Moonbear
Staff Emeritus