Velocity b w.r.t a =
V2vector -v1vector
Position of b w.r.t a =
Vector rb -vector r a
Particle b will collide with particle a if the velocity of b w.r.t a is directed towards a
Then vector v2-vector v1÷modulus of vector v2-vector v1=vector r2- vector r1÷ modulus of vector v2-vector r1
![](https://s3mn.mnimgs.com/img/shared/content_ck_images/ck_5ba5e25164d25.jpeg)