Rain has velocity v ⃗ \vec{v} v = 6î - 8ĵ m/s. A man runs horizontally eastward with speed u and carries a flat board inclined at 45° to the horizontal. For the rain to strike the board normally, the man's speed must be:

Question

G1 Physics · Accepted Answer

Relative rain velocity is v⃗_RM = (6 - u)î - 8ĵ. A board at 45° has a normal direction whose horizontal and vertical component magnitudes are equal. For the incoming rain to be normal to the board, take 6 - u = -8. Hence u = 14 m/s east. Therefore option B is correct.