This is probably not the BEST example.. but it might give you a good idea.

http://users.design.ucla.edu/~mflux/interactive/ballbounce/index.html

Hmm.. the source is ...

http://classes.design.ucla.edu/Fall03/157A/cursos/00/index_visor.php?id=8&ejercicio_id=5&persona_id=20

It's overly complicated. I'll write a simplified version some time for people who need to grasp this.

Code:

particle [] p = new particle [10];
void setup(){
  for(int i = 0; i < p.length; i++){
    p[i] = new particle(random(400),random(400),random(2,12));
  }
  size (400,400);
  ellipseMode(CENTER);
  smooth();
}
void draw(){
  background (255);
  for(int i = 0; i<p.length; i++){
    p[i].gmove();
    p[i].draw();
  }
  line(mouseX,0,mouseX,height);
}
class particle{
  float x,y,thisMass,xVelocity;
  particle (float x, float y, float thisMass){
    this.x = x;
    this.y = y;
    this.thisMass = thisMass;
    xVelocity = 0;
  }
  void gmove(){
    float mouseMass = 5;
    float distX = mouseX - x;
    //introduce the y and use pythagoras for the distance with two axises
    /*
    float distY = mouseY - y;
     float dist = sqrt(sq(distX) + sq(distY));
     */
    float dist = abs(distX); //remove this line for two axises
    //kill acceleration to create an orbit if too near mouse
    if (dist < 15){
	dist = 15;
    }
    //gravity equations - x axis only so you can pick it apart
    float grav = (thisMass * mouseMass) / sq(dist) * 5;
    float xGrav = grav * (distX/dist);
    float xAccel = xGrav / thisMass;
    xVelocity += xAccel;
    x += xVelocity;
    /*
    float yGrav = grav * (distY/dist);
     float yAccel = yGrav / thisMass;
     yVelocity += yAccel;
     y += yVelocity;
     */
  }
  void draw(){
    ellipse(x,y,thisMass,thisMass);
  }
}