This code is adapted from the Gravity Particles applet by Keith Peters.  It ran fine for a while, but when I changed "force = 100/distSQ" to 100*distSQ and tried to run it, the text window at the bottom of the editor went crazy and processing crashed.  Now it seems to do it no matter what for this one program.  I can run other programs I've written, but this one crashes it.  The text window displays a whole lot of messages, and I can't see most of them because the environment freezes, but many of them look like
"at java.lang.Thread.run(Unknown Source)" or
"at processing.core.PApplet.run(PApplet.java:951)" or
"at processing.core.PGraphics.requestDisplay(PGraphics.java:362)"
Those three messages have occurred a lot with only the number at the end changing.  Here's the code:

int numParticles = 100;
float[] positionX = new float[numParticles];
float[] positionY = new float[numParticles];
float[] velocityX = new float[numParticles];
float[] velocityY = new float[numParticles];
int w = 500;
int h = 500;
void setup() {
 size(w, h);
 smooth();
 strokeWeight(2);
 stroke(0);
 for (int i=0; i<numParticles; i++){
   positionX[i] = random(w);
   positionY[i] = random(h);
   velocityX[i] = 0;
   velocityY[i] = 0;
 }
}

void draw(){

 background(255);
 
 for (int i=0; i<numParticles; i++){
   float dx = mouseX - positionX[i];
   float dy = mouseY - positionY[i];
   float distSQ = dx*dx + dy*dy;
   float dist = sqrt(distSQ);
   float force = 100/distSQ;
   
   velocityX[i] += force * dx/dist;
   velocityY[i] += force * dy/dist;
   velocityX[i] = min(velocityX[i], 20);
   velocityX[i] = max(velocityX[i], -20);
   velocityY[i] = min(velocityY[i], 20);
   velocityY[i] = max(velocityY[i], -20);
   
   positionX[i] += velocityX[i];
   positionY[i] += velocityY[i];
   
   if(positionX[i] > w){
     positionX[i] = w;
     velocityX[i] *= -1;}
   if(positionX[i] < 0){
     positionX[i] = 0;
     velocityX[i] *= -1;}
   if(positionY[i] > h){
     positionY[i] = h;
     velocityY[i] *= -1;}
   if(positionY[i] < 0){
     positionY[i] = 0;
     velocityY[i] *= -1;}
   
   point(positionX[i], positionY[i]);
 }
}

The operation I changed is in line 30.  I had another question about this program before the glitch, though.  Is there a way to make the program more efficient?  I have a more than capable system, but this program lags sometimes. I can't remember how many particles I put into it but I remember thinking that it should have been able to handle more when it was completely bogged down.  So I'm wondering if there are simpler ways to make these calculations, or maybe a different way to structure the program.  Any ideas there would be helpful.  Thanks!