The hammock can support up to four people because of the tension in the ropes connected to the two trees. The tension equals the force of the person's (my sister) weight. The tension isn't able to keep the hammock completely straight, so there is always the dip in the strings. The system itself is fairly stable until either the ropes wear down and no longer can take the weight and force of the people using the hammock, or the trees connected to the hammock also can't support it. When someone uses the hammock like a giant swing, their motion becomes a part of a vertical circle. At the bottom of the swing, the force is the greatest.