Final answer:
Upon striking the ground, the person's momentum goes to zero as they stop, with the momentum transferred to the Earth. Bending the knees upon landing extends the stopping distance, reducing the force experienced.
Step-by-step explanation:
When a person jumps from a tree to the ground, the momentum of the person upon striking the ground changes drastically. Momentum is a product of the mass of an object and its velocity. In this case, the person carries momentum as they fall, due to their velocity and mass. Upon impact with the ground, this momentum is quickly reduced to zero because the person comes to a stop. According to the law of conservation of momentum, the momentum lost by the person is transferred to the Earth.
However, because the Earth's mass is so immense compared to that of the person, the change in the Earth's velocity is negligible and unobservable. If the person lands with stiff knees, a large force is experienced over a shorter stopping distance, while bending the knees extends the stopping distance, resulting in a smaller force felt by the person.