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Question

A man moving with a velocity of 5 m/s on a horizontal road observes that raindrops fall at an angle of 45 with the vertical. When he moves with a velocity of 16 m/s along an inclined plane, which is inclined at 30 with the horizontal, he observes raindrops falling vertically downward as shown in the figure. Find the actual velocity Vr of the raindrops.



A
Vr=83^i(835)^j
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B
Vr=43^i(435)^j
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C
Vr=23^i(235)^j
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D
Vr=3^i(35)^j
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Solution

The correct option is A Vr=83^i(835)^j
Case I: Man moving on horizontal road

Velocity of man, Vm=5^i

Let the velocity of rain be, Vr=Vx^i+Vy^j

Since, the man see the raindrops falling at 45 with vertical, so

VyVx5=tan45

Vy=(Vx5)....(1)

Case II: Man moving on inclined plane

Velocity of man,

Vm=16cos30^i16sin30^j

Vm=1632^i162^j

Vm=83^i8^j m/s

Now, the velocity of rain with respect to man,

Vrm=(Vx83)^i+(Vy+8)^j

Given that, the man see the raindrops falling in the vertical direction so the resultant is in vertical direction. So,

Vx83=0

Vx=83 m/s

From equation (1),

Vy=(835) m/s

So, the velocity of rain,

Vr=Vx^i+Vy^j=83^i(835)^j m/s

Hence, option (a) is correct answer.

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