To describe sinusoidal waves traveling along a rope
in the positive x direction we use the formula
(If the ``-'' sign is changed to ``+'',
it will travel in the negative x direction.)
The quantity A is the amplitude of the wave, and tells us the size of
the displacement of the rope.
The quantity 
is just the angular frequency of the motion of the rope
at any fixed value of x.
Just as is 
divided by the period of the wave motion in time,
the quantity k is divided by the period of the wave in space:
where 
is called the wavelength of the wave, and is the distance
from the crest of one wave to the crest of the next.
The quantity k is called the wavenumber.