I built a 2D gravity physics simulator in Java using JavaFX. The idea was simple: simulate bodies falling under gravity, bouncing off walls, and eventually colliding with each other.

How It Works

Each object in the simulation is represented by a Body class that tracks its position, velocity, size, mass, and color.

public Body(double x, double y, double vx, double vy, double diameter, double density, Color color) {
    this.x = x;
    this.y = y;
    this.vx = vx;
    this.vy = vy;
    this.diameter = diameter;
    this.mass = diameter * diameter * density;
    this.color = color;
}

Mass is calculated from the diameter squared times density, so bigger bodies are heavier.

Physics Update Loop

Every frame, gravity is added to the vertical velocity and the position is updated:

public void update(double grav) {
    vy += grav;
    x += vx;
    y += vy;
}

Wall Collisions

When a body hits a wall it bounces back with some energy loss — 80% velocity on the axis it hit, and a little friction on the ground:

public void handleWalls(double width, double height, double wall_length) {
    if (y > height - diameter / 2.0 - wall_length) {
        y = height - diameter / 2.0 - wall_length;
        vy = vy * -.8;
        vx *= .9;
    }
    if (x > width - diameter / 2.0 - wall_length) {
        x = width - diameter / 2.0 - wall_length;
        vx *= -.8;
    } else if (x < diameter / 2.0 + wall_length) {
        x = diameter / 2.0 + wall_length;
        vx *= -.8;
    }
}

What’s Next

Body-to-body collision detection is partially stubbed out — that’s the next thing I want to implement. Once that’s done I want to experiment with different gravity values and see how the simulation behaves.

Check out the full code on GitHub.