For fundamental frequency of a string to occur, the length of the string has to be half the wavelength. That is,
1/2y = L, where L = length of the string, y = wavelength.
Therefore,
y = 2L = 2*0.75 =1.5 m
Additionally,
y = v/f Where v = wave speed, and f = ferquncy
Then,
v = y*f = 1.5*220 = 330 m/s
To calculate the force between two negative charges, we use the formula which is given by the Coulomb`s Law as

Here,
and
are the charges on the pith balls, r is the separation between the charges and k is constant and its value is
.
Given
and
.
Substituting these values in above formula we get,

Thus, the repulsive force between two pith balls is
.
It’d fall 29.4m or 96.46ft
Explanation:
Uhh since gravity is 9.8m/s then in three seconds it’d drop 29.4m or 96.46ft
That is assuming there isn’t a lot of wind resistance, but if you take that into account, then it’d probably be somewhere around 25m since the water bottle is going to be heavier than the wind resistance, and since we don’t know the weight of the water bottle it can’t really be calculated.
Hope this helps!