I'm given the following problem:
For a harmonic travelling wave travelling in the positive direction, the initial transverse speed is 3 m/s and the initial $y$ displacement or transverse displacement is 0.1m. If the frequency of oscillation is 10 Hz, find amplitude and $\phi$ constant.
It's not a difficult problem. However, I do not understand initial speed and initial y displacement conditions.
Namely, I don't understand the following:
The general wave formula is:
y(x,t) = Asin(kx - wt + phi) and if initial "y" displacement is 0.10, then
y(0, 0) = 0.1 and then 0.1 = Asin(phi) and
u(0, 0) = 3 and 3 = -Acos(phi).
How do we know that x in these formulas will be zero?
And could someone explain me, why when we derive y(x,y) to get u(x,t), we must keep x as a constant ?