A ball is projected horizontally. After 3 second from projection its velocity becomes 1.25 times of the velocity of projection. Its velocity of projection is:-

a) 10 m/s

b) 20 m/s

c) 30 m/s

d) 40 m/s

Answer: d) 40 m/s

Solution:

v_y = u_y + a_yt

v_y = 0 + 9.8 x 3

= 29. 4 m/s

v’ = √v_x² + v_y²

= √v_x² + (29.4)²

(1.25 v_x)² = v_x² + (29.4)²

1.562v_x² = v_x2 + (29.4)²

0.562v_x² = (29.4)²

v_x = (29.4)²/0.562

v_x ≈ 40 m/s