Help!!Momentum problems? 1. A girl jumps from a tree onto the ground. As she falls she speeds up.Explain in details how the law of conservation of momentum applies in this case? Can I consider the girl and the earth(the ground) as a system?? Why or why not?? 2. mass1*initial velocity1+mass2*initial velocity2=mass1*final velocty1+mass2*final velocity2 3.I think that this the girl and the earth should be in one system. But I don't exactly know when is the initial status and final status. I am confused if i should consider the moment when the girl hits the ground to be final status or after the impact. Before the impact, the girl's v is downwards and the earth's v is upwards based on Newton's 3rd Law. So what is the direction of this system's momentum????How do I know????