If A is your cell array, you can access the cell suing A{1}, A{2}, ... A{i},..etc.
for i = 1:length(A)
A{i}
end
Also have a look on cell2mat where you can convert the cell into a matrix and access the elements. Also have a look on vertcat. You can get the indices of elements/ x velocity as zero using:
idx = a==0 ;
a(idx)