Speed of a Wave: Solve for V in 4.6m and 7.4s

[pmd28]

Homework Statement

A person fishing from a pier observes that five wave crests pass by in 7.4 s and estimates the distance between two successive crests to be 4.6 m. The timing starts with the first crest and ends with the fifth. What is the speed of the wave?

Homework Equations

f=1/T
v=λf

The Attempt at a Solution

I solved for the period first, so in 7.4 sec there was 5 waves. T=1.48
f=1/T=.6757
v=λf=4.6*(.6757)=3.11 m/s

I know my error has to do wit that sentence, "The timing starts with the first crest and ends with the fifth," but I'm not sure what I'm doing wrong.

 

[TSny]

Draw a sketch of a wave and number the crests 1 through 5. How many wavelengths are contained between 1 and 5?

 

[pmd28]

There are 4 wavelengths.

 

[TSny]

Good. So what should the period T actually be?