Question

A boy throws a ball at time t = 0 with upward initial velocity = 20 m/s, from the corner of the roof of a

builiding, whose height above the ground is 25m. If the velocity of the ball is simply reversed after collision

from the ground (magnitude remaining same), and the 3rd collision of the ball with the ground occurs at

t = t1

sec, What is the value of t1

?​

Answer 1

ANSWER

Upward velocity v

0

=20m/s

Horizontal acceleration,a

x

=−4m/s

2

we know, Time of flight, T=

g

2v

y

=

g

2v

0

sinθ

=4sinθ

Also,range R=v

0x

T+

2

ax

T

2

0=20cosθT−2T

2

2T

2

=20cosθT

T=10cosθ

10cosθ=4sinθ

tanθ=

4

10

=2.5

θ=tan

−1

(2.5)