A boy is standing on a truck moving with a constant velocity of 15m/s on a horizontal road. The boy throws a ball in such a way that it returns to the truck after the truck has moved 60 m. The speed of projection as seen from the road is

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20 m/s

25 m/s

15 m/s

10 m/s

answer is B.

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Detailed Solution

Velocity of truck = horizontal velocity of projectile ;

$u_{x} = ucos θ = 15 m / s$

distance moved by truck = range of the projectile = 60 m
$∴ 60 = \frac{2 u_{x} u_{y}}{g}$
$u_{y} = 20 m / s$
$∴$ Velocity of projection as observed from road
$u = \sqrt{u_{x}^{2} + u_{y}^{2}} = 25 m / s$

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