Question

A boy is standing on the edge of the roof at height 78.4m. He throws a ball vertically upward with velocity of 19.6 m/s . After how much time the ball will reach the ground?. Take a= 9.8 m/s square downward.

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Answer 1

Answer:

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Answer 2

The total time = time taken to for the ball to reach the roof + time taken for the ball to reach the ground from the roof with velocity -19.6 m/s.

=> time to reach the roof = 2(19.6)/9.8 = 4

Now, to find the time taken to reach the ground:

initial velocity=19.6

Height=78.4 m

final velocity = ?

time=?

=>  v^2 = u^2 + 2gh

Substitute the values:

=> v = sqrt(19.6^2 + 2(78.4)(9.8)

v = 43.8

To find time: v = u + at

=> 43.8 - 19.6 = 9.8t

=> t=2.47

threfore the total time= 6.47