Water waves problem

1. Sep 6, 2009

songoku

1. The problem statement, all variables and given/known data
The figure shows water waves traveling between two points P and Q, 20.0 cm apart. A dust particle on the water surface makes 5 complete oscillations in 2.0 s.

What is :
i. The amplitude;
ii. The wavelength;
iii. The frequency;
iv. The speed of the waves ?

2. Relevant equations
$$v=\lambda f$$

3. The attempt at a solution
i. amplitude = 4 cm

ii. I don't understand the question. The question states that "A dust particle on the water surface makes 5 complete oscillations" and I think there should be 5 waves, but in the picture there are only 2.5 waves.
The answer on the book is 8 cm. I guess it's from $$\lambda=\frac{20}{2.5}$$

But the answer for frequency is 2.5 Hz. I guess it's from $$f=\frac{5\;\text{waves}}{2\;s}$$

So I confused whether there are 2.5 waves or 5 waves.

Thanks

2. Sep 6, 2009

Chewy0087

Re: wave

the information "5 oscillations per 2 seconds" is not linked to the picture

therefore the 8cm is correct, there are only 2.5 waves

you wrote down the formula you need;

v= lambda f

therefore using the information as you quite rightly said the frequencey is 2.5 Hz, frequency is after all number of oscillations per second

now you have the wavelength & frequency, the speed should be easy to work out

Last edited: Sep 6, 2009
3. Sep 6, 2009

songoku

Re: wave

Hi Chewy0087

Ah I get it now. The information "5 oscillations per 2 seconds" is not about the picture but it's still related to the question because we can't determine the frequency just based on the picture.

Thanks