I have a question of velocity-time graph in which a girl throws ball from the top of cliff.The ball has mass of 250 g. How to calculate the magnitude of change in momentum of the ball between the time it leaves the girls hand to time t=4.0s.

Answer 1

v = -9.8t
p = mv
Now just plug in your numbers.

Answer 2

That is if she dropped the ball, not threw it :)