From ....../samples/CHOP/CHOP_Spring.C [gree...]:

        d1 = block->myDn1;
	d2 = block->myDn2;

	// Loop over each sample in the track.
	for(j=0; jgetTrackLength(); j++)
	{
	    t = myClip->getTime(myClip->getStart() + j);
	    oldp = f = clip->evaluateSingleTime(track, t);

	    // Run the spring equation
	    if(!force_method)
		f *= spring_constant;
	    else
		oldp /=spring_constant;

	    vel = (d1-d2) / inc;

	    acc = (f - vel*damping_constant - d1*spring_constant)/mass;
	    vel += acc * inc;
	    d = d1 + vel * inc; 
		
	    delta = SYSabs(oldp - d);
	    if (delta > 0.001)
		mySteady = 0;

	    new_track->getData()[j] = d;

	    d2 = d1;
	    d1 = d;
	}

	// update the displacements in the realtime data block for the next cook
	// to use.
	block->myDn1 = d1;
	block->myDn2 = d2;