This is conceptually the same as the lesson behind the horse-carriage, Newton''s Third Law paradox where a horse with a little knowledge in physics is basically telling the rider on his carriage that he cannot possibly pull the rider and carriage, since the carriage would pull back on the horse with an equal and opposite force thereby cancelling any motion(smart horse). The fault in the horse''s reasoning has to do with the interaction of forces INTERNAL to the system versus those EXTERNAL. When a bungee jumper is falling under the influence of gravity with a rope attatched to him, why should you as the physics problem solver, care about the force the bungee jumper exerts on the rope? If solving for the acceleration of the jumper, the only relevant forces on the jumper are that of the rope and gravity. The jumper is the system. A free body diagram would look something similar to this, with J representing the jumper.

^
| Ft- force of tension from the rope, can be approximated
| from Hooke''s Law as F=-kx , the x representing the
J change in displacement from the unstretched position
| of the rope, k just a spring constant
|
| Fg- gravity, F=mg
\/

using newton''s second law to model the situation, and taking the down direction to be positive:

F=ma
-Ft+Fg=ma
-kx+mg=ma
a=(-kx/m)+g

from the value of the acceleration, it should be apparent to you that the acceleration isn''t simply g, and obviously varies as a function of the stretch in the bungee cord. If anyone needs help with any type of physics questions from classical through QM (possibly in exchange for information on game programming(trying my hand at it)) please feel free to email me at
test2269@yahoo.com

p - Homework assitance will be given, but I won''t give full solutions to problems, i''m a TA, I have ethics you know!

David White
UC Berkeley