a boy drops a stone from the top of a building 100m high. another boy throws an iron ball, with certain velocity, 2 second later the stone was dropped. calulate the velocity with which iron ball was thrown , so that both the ball and the stone strike the ground at the same time.

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Solution

Hi,
For the stone that is dropped,
Initial velocity, u = 0
Distance covered, h = -100 m
Acceleration due to gravity, g = -10 ms^-2
So, the stone reach the ground in 4.5 s after it is dropped.

Now, at t = 0, the iron ball is thrown.
At, t = 2 s, the stone is dropped.
So, the stone reach the ground at t = 2 + 4.5 = 6.5 s
Which means the iron ball was in air for 6.5 s.

For the iron ball,
Initial velocity is say, v
Final velocity will be –v
Acceleration due to gravity, g = -10 ms^-2
Time of flight, t = 6.5 s
So, the iron ball has to be thrown at a speed 32.5 m/s to make both the projectiles reach the ground at the same time.

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