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
- r1 v1 = r2 v2
- r1 - r2 = v1 - v2
- r1 - r2| r1 - r2| = v2 - v1| v2 - v1|
- r1 v1 = r2 v2
Answer
(3) r1 - r2| r1 - r2| = v2 - v1| v2 - v1|
Solution
For collision, both particles must be at the same point at some time t > 0:
r1 + v1t = r2 + v2t
r1 - r2 = ( v2 - v1)t
∵ t is a positive scalar, r1 - r2 and v2 - v1 must point in the same direction.
∴ Their unit vectors are equal: r1 - r2| r1 - r2| = v2 - v1| v2 - v1|
Answer: (3)