A ship A is moving Westwards with a speed of 10 km h-1 and a ship B 100 km South of A, is moving Northwards with a speed of 10 km h-1. The time after which the distance between them becomes shortest, is
- 5 2 h
- 10 2 h
- 0 h
- 5 h
Answer
(4) 5 h
Solution
Take A at rest: velocity of B relative to A, vBA = vB - vA = 10 km/h North + 10 km/h East
⇒ | vBA| = 10 2 km h-1, at 45 to the line BA (North).
The distance is shortest when B reaches the foot of the perpendicular from A on its relative path.
Distance moved along the relative path = 100 45 = 50 2 km
∴ t = 50 210 2 = 5 h
Answer: (4)