public double timeToHit Particle Particle this if return INFINITY doub

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public double timeToHit(Particle b) {
Particle a = this;
if (a == b) return INFINITY;
double dx = b.rx - a.rx;
double dy = b.ry - a.ry;
double dvx = b.vx - a.vx;
double dvy = b.vy - a.vy;
double dvdr = dx*dvx + dy*dvy;
if (dvdr > 0) return INFINITY;
double dvdv = dvx*dvx + dvy*dvy;
double drdr = dx*dx + dy*dy;
double sigma = a.radius + b.radius;
double d = (dvdr*dvdr) - dvdv * (drdr - sigma*sigma);
// if (drdr < sigma*sigma) StdOut.println("overlapping particles");
if (d < 0) return INFINITY;
return -(dvdr + Math.sqrt(d)) / dvdv;
}
public void bounceOff(Particle that) {
double dx = that.rx - this.rx;
double dy = that.ry - this.ry;
double dvx = that.vx - this.vx;
double dvy = that.vy - this.vy;
double dvdr = dx*dvx + dy*dvy; // dv dot dr
double dist = this.radius + that.radius; // distance between particle centers at collison
// normal force F, and in x and y directions
double F = 2 * this.mass * that.mass * dvdr / ((this.mass + that.mass) * dist);
double fx = F * dx / dist;
double fy = F * dy / dist;
// update velocities according to normal force
this.vx += fx / this.mass;
this.vy += fy / this.mass;
that.vx -= fx / that.mass;
that.vy -= fy / that.mass;
// update collision counts
this.count++;
that.count++;
}