public void step(double t)
{
s.clearForces();
s.applyForces();
double halftt = 0.5 * t * t;
double tt = t * t;
for (int i = 0; i < s.numberOfParticles(); i++)
{
Particle p = s.getParticle(i);
if (p.isFree())
{
XYZ a = p.getForce() / p.getMass();
XYZ xmm = p.getOldPosition();
XYZ xm = p.getPosition();
XYZ x = xm.Add(xm - xmm) + a * tt;
XYZ vm = p.getVelocity();
p.setPosition(x);
p.setVelocity((x - xmm) / (2 * t));
}
}
}
