First slide

Projection Under uniform Acceleration

Question

Two bullets are fired horizontally and simultaneously towards each other from roof tops of two buildings 100 m apart and of same height of 200m with the same velocity of 25 m/s. When and where will the two bullets collides. (g =10 m/s²)

Moderate

A

after 2s at a height 180 m
B

after 2s at a height of 20 m
C

after 4s at a height of 120 m
D

they will not collide

Solution

Let bullets collide at time t
${x_1} + {x_2} = 100\,m$
$25t + 25t = 100;\;t = 2\,s$
$y = \frac{1}{2}g{t^2} = \frac{1}{2} \times 10 \times {2^2} = 20\,m$
$h = 200 - 20 = 180\,m$
Hence bullets will collide after 2s at height 180 m above the ground.

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