After Effects: expressions for Particular - particles from thrust

gistfile1.txt

// apply this to the PoI of a spot light to point it in the direction of its acceleration vector
nextVec = transform.position.valueAtTime(time+0.08)-transform.position.valueAtTime(time);
prevVec = transform.position.valueAtTime(time)-transform.position.valueAtTime(time-0.08);
theVec = prevVec-nextVec;
theVec = theVec * 10;
theVec = theVec + transform.position;
theVec

// apply this to the Particles/sec parameter
prevVel = length(thisComp.layer("TEAL").transform.position.valueAtTime(time+0.04) - thisComp.layer("TEAL").transform.position.valueAtTime(time));
nextVel = length(thisComp.layer("TEAL").transform.position.valueAtTime(time+0.08) - thisComp.layer("TEAL").transform.position.valueAtTime(time+0.04));
Math.abs(nextVel-prevVel)*750

// and this to the particle velocity param
prevVel = length(thisComp.layer("TEAL").transform.position.valueAtTime(time+0.08) - thisComp.layer("TEAL").transform.position.valueAtTime(time));
nextVel = length(thisComp.layer("TEAL").transform.position.valueAtTime(time+0.08) - thisComp.layer("TEAL").transform.position.valueAtTime(time+0.08));
Math.abs(nextVel-prevVel)*50 // change to a negative value to emit against thrust