A rubber ball escapes from the horizontal roof with a velocity `v = 5 m s^-1`. The roof is situated at a height, `h = 20 m`. If the length of each car is equal to `x_0 = 4 m`, with which car will the ball hit ?
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Explanation

The initial velocity of the ball is in horizontal direction.
The initial velocity in vertical direction is zero.
For vertical motion, `u_x = 5 m s^-1, u_y = 0`
Displacement in vertical direction, `y = -h = -20 m`
`-h = (1)/(2) - "gt"^2 rArr (2 xx 20)/(10) = t^2 rArr t = 2 s`
Horizontal displacement : Distance covered in `2 s`,
`x = d = u_x xx t = 5 xx 2 = 10 m`
Since the length of one car is `4 m`, it will hit the third car for which `8 m lt d lt12 m`.