A boy throws a ball vertically upwards with a velocity of 9.8m/s from the roof of a building 20m high. How long will the ball take to reach the ground? What will be it's velocity when it strikes the ground? [g=9.8m/s]
Answers
answer : 3.25 sec and 22.1 m/s
explanation : A boy throws a ball vertically upwards with a velocity of 9.8 m/s from the roof of a building 20m high.
so, initial velocity of ball, u = 9.8 m/s²
initial height of ball , s = 20m
applying formula, s = ut + 1/2at²
or, -20 = 9.8t + 1/2 (-9.8)t²
or, -20 = 9.8t - 4.9t²
or, 4.9t² - 9.8t - 20 = 0
or, t = [9.8 ± √{9.8² + 4(20)(4.9)}]/9.8
= (9.8 ± 22.1)/9.8
= (9.8 + 22.1)/9.8 , t ≠ (9.8 - 22.1)/9.8 , because it is negative term.
= (9.8 + 22.1)/9.8
= 31.9/9.8 sec ≈ 3.25 sec
so, time taken to ball to reach the ground is 3.25 sec
velocity of ball before striking the ground is v
using formula, v² = u² + 2as
v² = u² + 2(-g)(-s)
v² = u² + 2gs
v² = (9.8)² + 2(9.8)(20)
v ≈ 22.1 m/s
hence, velocity of ball before striking the ground is 22.1 m/s
Answer:
this is my solved problem
