phlipdascrip wrote:If both climbers weigh about the same, the leader should not have created a stronger force than the follower, so no slack would be pulled back towards the leader upon impact.
This is the all-too-common confusion between force and energy. Suppose two identical climbers have the same speed. Then they have the same kinetic energy. The forces on the climbers, however, are
1. Gravity. This force is the same for both because they have the same mass and the gravitational field is effectively the same at both positions. Since gravity is the same, we can ignore it in this discussion.
2. Elastic force due to the rope. This force is governed, in first approximation, by Hooke's Law, which applied to a rope of uniform construction says that the shorter the rope the higher the force for the same elongation.
Initially, when the rope has just come taut and the tensions are low, static friction prevents the rope from moving across the biner and the climber with less rope is decelerated more violently. However, the tension in the rope on his/her side increases until the difference in tension between the two sides cannot be compensated by friction. Then the rope moves. It's Newton's second law of motion, nothing more, nothing less.
phlipdascrip wrote:But I'm also thinking that in the moment the follower pulls the leader off the rock, the follower's fall is decelerated a bit as he accelerates the leader. This would give the leader a small "head start" over the follower, and falling faster than him the leader 1) might pass the last piece of pro before the rope comes tight, and/or 2) have a stronger fall force to actually pull some rope back towards him upon impact.
What happens is that elastic waves travel along the rope.