The ball travels up to a maximum height, then returns to the ground.
We need to find the rock's momentum as it strikes the ground. Let v be the final speed of the rock. Its final speed is as same as initial speed i.e. 8 m/s but in negative direction. So
p = mv
p = 1 kg × (-8 m/s)
= -8 kg-m/s
So, the rock's momentum as it strikes the ground is (-8 kg-m/s).
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