# Periodic Waves

1. Jan 13, 2008

### sheri1987

1. The problem statement, all variables and given/known data

A jetskier is moving at 9.8 m/s in the direction in which the waves on a lake are moving. Each time he passes over a crest, he feels a bump. The bumping frequency is 1.1 Hz, and the crests are separated by 5.6 m. What is the wave speed?

2. Relevant equations

velocity = frequency * wavelength

3. The attempt at a solution

I thought to multiply the 1.1 Hz which is the frequency by the 5.6 which is the wavelength to get a velocity...I got 6.16 m/s then I was not sure what to do next...should I add it to the jetskier's velocity 9.8 m/s because they are going in the same direction? What should I do?

2. Jan 13, 2008

### Kurdt

Staff Emeritus
This is the doppler effect. For an observer moving away from the source of the waves (i.e. moving the the same direction) then the observed frequency is:

$$f'=\left( 1-\frac{v_o}{v}\right) f$$

3. Jan 13, 2008

### sheri1987

only the bumping frequency is given...how do I solve for the other frequency to plug into the equation

4. Jan 14, 2008

### Kurdt

Staff Emeritus
Remember that frequency is the wave speed divided by the wavelength.