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Name: John
Status: student
Grade: N/A
Location: N/A
Country: N/A
Date: 5/9/2005


Question:
Conservation of momentum - Bungee jumps How is momentum conserved when a Bungee jumper's velocity drops to zero at the end of a stretched Bungee rope?


Replies:
Whenever some object's momentum changes, the momentum goes to whatever was supplying the force that caused the momentum change. In bungee jumping, that object is usually the Earth. Nobody ever notices the difference because the Earth is so massive.

Tim Mooney


Kinetic energy (K.E.=1/2*M*V^2)and potential energy (U = -K*X, assuming Hooke's law) are exchanged between the jumper and the elastic cord after the "free-fall" part of the jump when the chord begins to stretch. Assume for the sake of simplicity that there is no "free-fall" and the elastic chord begins to stretch the instant the jumper jumps. In that case the initial momentum of the jumper is also zero since the jumper is not moving then either.

Vince Calder


John,

TOTAL momentum is conserved, not the momentum of one object. The jumper and Bungee cable exert force on each other. The Bungee cable and support exert force on each other. The support and Earth exert force on each other. Also, gravity allows the jumper and Earth to exert force each other directly. Most of the momentum is temporarily passed to the Earth, the entire planet. When the cable pulls the jumper back up, the momentum passes back into his body.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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