second order differential equation with variable coefficient

Hi,

As wrote in a previous question I have to solve the equation: d^2x/dt^2= (q/m)(E/v^2)

I have wrote the following code:

 q=1; m=1; v=1; E=1;

 rhs=@(z,x)[x(2);(q/m)*(E/v^2)];

 [za, xa] = ode45(rhs, [0 100], [0 0]);

and it works fine.

The problem is that E for me is a function of z that I can write as:

   Ez=zeros(1,100);

     for i=1:1:100

         if (i<=10 || i>=50)

             Ez(i)=0;

         else

             Ez(i)=1;

         end
     end

I have found an example here: http://www.mathworks.it/help/techdoc/ref/ode23.html where is shown how to solve ode with time-dependent terms, but is is about a first order problem and I'm not able to write something similar for my second order problem... could you help me?