In 1687 Sir Isaac Newton formulated the principles governing the the motion of two particles under the influence of their mutual gravitational attraction in his famous Principia. However, Newton was unable to solve the problem for three particles. Indeed, in general, systems of three or more particles can only be solved numerically. Write a program to simulate the motion of N particles, mutually affected by gravitational forces, and animate the results. Such methods are widely used in cosmology, semiconductors, and fluid dynamics to study complex physical systems. Scientists also apply the same techniques to other pairwise interactions including Coulombic, Biot-Savart, and van der Waals.
The physics. We review the equations governing the motion of the particles according to Newton's laws of motion and gravitation. Don't worry if your physics is a bit rusty; all of the necessary formulas are included below. We'll assume for now that the position (rx, ry) and velocity (vx, vy) of each particle is known. In order to model the dynamics of the system, we must know the net force exerted on each particle.
The numerics. We use the leapfrog finite difference approximation scheme to numerically integrate the above equations: this is the basis for most astrophysical simulations of gravitational systems. In the leapfrog scheme, we discretize time, and update the time variable t in increments of the time quantum Δt. We maintain the position and velocity of each particle, but they are half a time step out of phase (which explains the name leapfrog). The steps below illustrate how to evolve the positions and velocities of the particles.
Creating an animation. Draw each particle at its current position using standard draw, and repeat this process at each time step. By displaying this sequence of snapshots (or frames) in rapid succession, you will create the illusion of movement. After each time step (i) draw the background image starfield.jpg, (ii) redraw all the bodies in their new positions, and (iii) control the animation speed using StdDraw.show().
Input format. The input file is a text file that contains the information for a particular universe. The first value is an integer N which stores the number of particles. The second value is a real number R which represents the radius of the universe: assume all particles will have coordinates that remain between -R and R. Finally, there are N rows, and each row contains 6 elements. The first two are the x and y coordinates of the initial position; the second two are the x and y coordinates of the initial velocity; the third is the mass; the last is a String that is the name of an image file used to display the particle. As an example, the input file planets.txt contains data for our solar system (in SI units).
There are additional data files in the nbody directory of the COS 126 ftp site. You can either download everything in the nbody directory or download nbody.zip which contains all the files that you will need. This directory also contains the corresponding GIF images.5 2.50e11 1.496e11 0.000e00 0.000e00 2.980e04 5.974e24 earth.gif 2.279e11 0.000e00 0.000e00 2.410e04 6.419e23 mars.gif 5.790e10 0.000e00 0.000e00 4.790e04 3.302e23 mercury.gif 0.000e00 0.000e00 0.000e00 0.000e00 1.989e30 sun.gif 1.082e11 0.000e00 0.000e00 3.500e04 4.869e24 venus.gif
Your program. Write a program NBody.java that reads in the universe from standard input using StdIn, simulates its dynamics using the leapfrog scheme described above, and animates it using our StdDraw. Maintain several arrays to store the data. To make the computer simulation, write an infinite loop that repeatedly updates the position and velocity of the particles. When plotting, consider using StdDraw.setXscale(-R, +R) and StdDraw.setYscale(-R, +R) to scale the physics coordinates to the screen coordinates.
Optional finishing touch. For a finishing touch, play the theme to 2001: A Space Odyssey using StdAudio and the file 2001.mid. It's a one-liner using the method StdAudio.play().
Compiling and executing your program. Follow these instructions [ Windows · Mac · Linux ] to configure the command-line on your system. To compile your program from the command-line, type:
% javac NBody.java
in your Terminal application. To run your program from the command-line on the input file planets.txt, redirect the file to standard input:
% java NBody < planets.txt
It produces an animation like the following movie.
Extra credit. Submit a universe in our input format along with the necessary image files. If its behavior is sufficiently interesting, we'll award extra credit.
Challenge for the bored.
There are limitless opportunities for additional excitement and discovery here.
Try adding other features, such as supporting elastic or inelastic collisions.
Or, make the simulation three-dimensional by doing calculations for x,
y, and z coordinates, then using the z
coordinate to vary the sizes of the planets.
Add a rocket ship that launches from one planet and has to land on another.
Allow the rocket ship to exert force with consumption of fuel.