The Euler's method was used in your case to do the integration.
It only good for small time step and very far away case.
You need to use Runge-Kutta 4th order method to do the integration.
i.e. calculate v(t) from a(t).

You need to know numerical method or copy RK4 code from others to solve it correctly.
[url=]EJS[/url] is a tool for non-professional user who want to create simulation without much knowledge about numerical simulation.