Question

A ball thrown down from a balcony lands in 0.8 s
at a speed of 13 m/s. Find (a) the initial velocity;
(b) the height from which it was thrown; (c) the
time to land if it were thrown up from the balcny
with the same initial speed.​

Answer 1

(a) Let the ball is thrown down from a balcony with velocity u m/s.

time taken to land the ball , t = 0.8s

final speed of ball, v = 13m/s .

using formula, v = u + at

here, v = 13, a = 10 m/s² and t = 0.8s

so, 13m/s = u + 10m/s² × 0.8s

os, u = 5m/s

so, initial velocity of ball is 5m/s

(b) using formula, s = ut + 1/2 at²

here, s is height from which ball was thrown, u = 5m/s, t = 0.8s and a = 10m/s²

so, s = 5 × 0.8 + 1/2 × 10 × (0.8)²

= 4 + 3.2 = 7.2 m

(c) using formula, t = {u + √(u² + 2gs)}/g

= {5 + √(25 + 2 × 10 × 7.2)}/10

= {5 + √(25 + 144)}/10

= {5 + 13}/10

= 18/10 = 1.8sec.

[ Note : Don't be confuse if mentioned answer is little difference from your actual answer. it happens because of value of g it can be chosed either 9.8m/s² or 10m/s². but for easier calculation, I prefer 10m/s² . you can prefer 9.8m/s². ]