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Question

To a man walking at the rate of 3 km/h the rain appears to fall vertically. When he increases his speed to 6 km/h, it appears to meet him at an angle of 45 with vertical. Find the speed of rain.

A
3 km/h
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B
32 km/h
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C
23 km/h
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D
32 km/h
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Solution

The correct option is B 32 km/h
Le the velocity of rain w.r.t ground, Vr=x^i+y^j

Let
Case (i): When the man is walking at the rate of 3 km/h.
Relative velocity of rain w.r.t man,
Vrm=VrVm=(x^i+y^j)+3^i=(3x)^iy^j
Since rain appears to fall vertically,
x component =0
x=3
Case (ii): When the man is walking at the rate of 6 km/h
Vrm=(6x)^iy^j
Now rain appears to fall at an angle of 45 to the vertical.
tan45=VyVx=y6x=yx6=y3
(as x=3)
y=3
Vr=3^i3^j
|Vr|=32+(3)2=32 km/h

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