A progressive wave in a stratched string has a speed of 20m/s and a frequency of 100Hz. What is the phase difference between two points 25mm apart?

A) zero
B) pi/4 rad
c) pi/2 rad
d) pi rad

the ans is B but I'm not sure how to obtain it

Work out the wavelength, work out how many waves you can fit in your 25mm, you'll get a remainder of a wavelength and you need to work out how many radians it correponds to.

OK so lambda =c/f =3*10^8 /100 =3*10^6

am I going I the right direction, I don't think so should I have used lambda =ws/d

So... am I on the right track , anyone

Hello?

This is a wave in a string, not electromagnetic radiation, the speed isn't c.

100cycles per second also 20m per second so ::::
20m:100cycles:: 25E-3:?
100X25E-3/20 = 1/8cycle=Pi/4 rad