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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. r₁ · v₁ = r₂ · v₂
2. r₁ × v₁ = r₂ × v₂
3. r₁ − r₂ = v₁ − v₂
4. (r₁ − r₂) = (v₂ − v₁) / |v₂ − v₁|

Correct answer: (r₁ − r₂) = (v₂ − v₁) / |v₂ − v₁|

Solution

For two particles to collide, their relative position vector (r₁ - r₂) must align with their relative velocity vector (v₂ - v₁). This is expressed as (r₁ − r₂) = (v₂ − v₁) / |v₂ − v₁|, ensuring they meet at the same point.

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