restLength = 20;
damp = .95;
leader = thisComp.layer("leader");

fDur = thisComp.frameDuration;
currFrame = Math.round(time / fDur);

p2 = position.valueAtTime(0);
v2 = 0;
for (f = 0; f <= currFrame; f++){
    t  = f*fDur;
    p1 = leader.transform.position.valueAtTime(t);
    delta = p2 - p1;
    nDelta = normalize(delta);
    a = 2 * nDelta * (length(delta) - restLength) * fDur;
    v2 = (v2 - a) * damp;
    p2 += v2;
}
p2