A person sitting on the top of a tall building is dropping balls at regular intervals of one second. Find the positions of the 3rd. 4th and 5th ball when the 6th ball is being dropped.

Sol. For every ball, u = 0, a = g = 9.8 m/s2 ∴ 4th ball move for 2 sec, 5th ball 1 sec and 3rd ball 3 sec when 6th ball is being dropped. For 3rd ball t = 3 sec S3 = ut + ½ at2 0 + ½ (9.8) 32 = 4.9 m below the top. For 4th ball, t = 2 sec S2 = 0 + 1/2 gt2 = 1/2 (9.8)22 = 19.6 m below the top (u = 0) For 5th ball, t = 1 sec S3 = ut + 1/2 at2 = 0 + 1/2 (9.8)t2 = 4.98 m below the top.