Solve Wave Form Problem: Two Buoys 5m Apart, Wavelength 15m

[luysion]

Homework Statement

Two buoys are floating in the sea, separated by 5 m. A wave with a wavelength of 15 m travels from left to right along the line between the buoys. The left-most buoy moves vertically up and down as described by the equation y=Acos(ωt). Which equation best describes the motion of the buoy on the right?

a) y = A cos (wt-pie/2)
b)y = A cos (wt + 90deg)
c) y = A cos (wt - 2pie/3)
d) y = A cos (wt- pie/4)
e) y = A cos (wt + pie/4)

Homework Equations

The Attempt at a Solution

I got my answer as C which is correct but I just guessed by finding the wavenumber and timsing it by 5.. i don't understand what x is in the cosine function for wave form exactly and i don't know how to solve this properly! cheers

 

[rl.bhat]

The general equation of a propagating wave is given by,
y = A*cos(2πt/Τ - 2πx/λ).
Here T is the period, λ is the wavelength and x is the distance where y is measured.