# Integrating 2nd order ODE using midpoint rule

1. Apr 28, 2012

### Niles

Hi

I am trying to integrate Newtons equations for my system
$$a = \frac{F}{m} = \frac{d^2x}{dt^2}$$
This is only for the first coordinate of the particle. I wish to do it for y and z as well, but let us just work with x for now to make it simple.

The force in the x-direction depends on the velocity in the x-direction, vx, and the y- and z-coordinate. In other words
$$F=F(v_x, y, z)$$
Now, I wish to solve this equation, and I have currently implemented an Euler method. This is how I iterate
$$v_{n+1} = v_n + dt\cdot a(v_{x,n},y_n,z_n) \\ x_{n+1} = x_{n} + dt\cdot v_{n}$$
I now want to improve the error, and use a 2nd order Runge-Kutta method, i.e. the midpoint rule as briefly summarized here: http://www.efunda.com/math/num_ode/num_ode.cfm

I am not quite sure how to do this. In the link they say that now I should generally write
$$y_{n+1} = y_{n} + dt\cdot f(x_n + dt/2, y_n + k_1/2)$$
where
$$k_1 = dt\cdot f(x_n, y_n).$$
This is where my confusion arises: What does $f(x_n + dt/2, y_n + k_1/2)$ correspond to for me?

I would really appreciate a hint or two with this.

Best,
Niles.

2. Apr 29, 2012

### djelovin

Here you have it explained:
Computational physics

page 292, "13.4 More on finite difference methods, Runge-Kutta methods"

3. May 4, 2012

Thanks!