Two particles a and b, move with constant velocities →v1 and →v2. at the initial moment their position vectors are →r1 and →r2 respectively. the condition for particles a and b for their collision is :

(1)vector r1 - vector r2 = vector v1 - vector v2

(2)vector r1 - vector r2/magnitude of (vector r1 - vector r2 ) = vector v1 - vector v2/ magnitude of (vector v1 - vector v2)

(3)vector r1 . vector v1 = vector r2 . vector v2

(4)vector r1 x vector v1 = vector r2 x vector v2