A person is standing on a bridge, attached to a bungee cord.

Question

A person is standing on a bridge, attached to a bungee cord. The person steps off the bridge and falls down. The isolated system consists of the person, bridge, bungee cord, and the Earth, ignoring friction and air resistance. The amount of energy in the system is 18,000 J when the person is standing on the bridge. At some point during the fall, 6,000 J of energy has been transformed into kinetic energy because the person is moving. Additionally, 3,000 J of energy has been transformed into elastic potential energy because the bungee is stretching. (Air resistance is negligible.) Which choice best describes the amount and form of the rest of the energy at this point? 

 

A. There are exactly 9,000 J of gravitational potential energy in the system because the person is at some position above the ground and the total energy must add up to 18,000 J because energy is always conserved.

 B. There are exactly 9,000 J of thermal energy is the system because the person is heating up as he falls and the total energy must add up to 18,000 J because energy is always conserved.

C. There are slightly more than 9,000 J of gravitational potential energy in the system because the person is at some position above the ground and the total energy must be slightly more than the initial energy because energy increases as it is transformed.

D. There are slightly less than 9,000 J of gravitational potential energy in the system because the person is at some position above the ground and the total energy must be slightly less than the initial energy because energy is lost as it is transformed.

Answer 1

Energy is ALWAYS conserved, so C and D are automatically out. Between A and B, ask yourself if something has ever gotten hotter because it was falling.