By what mechanism is a person injured when he or she falls from a significant height? select one: a. kinetic energy is converted to potential energy; the potential energy is then converted into the work of bringing the body to a stop. b. as the person falls, the amount of kinetic energy is converted into work; work is then converted to kinetic energy upon impact. c. potential energy is created as the person is falling; the potential energy is then converted into kinetic energy upon impact. d. potential energy is converted to kinetic energy; the kinetic energy is then converted into the work of bringing the body to a stop.

d. Potential energy is converted to kinetic energy; the kinetic energy is then converted into the work of bringing the body to a stop.

Explanation:

- At the beginning of the falls, when the person is still at a certain height h, the person has gravitational potential energy:

U = mgh

where m is the mass of the person, g the acceleration due to gravity, h the height above the ground.

- As the person falls down, h decreases, so the potential energy decreases; according to the law of conservation of energy, potential energy is converted into kinetic energy, since the speed of the person increases:

where v is the speed.

- Just before hitting the ground, all the potential energy has been converted into kinetic energy

- When the person hits the ground, he/she comes to a stop: so work is done by the ground on the person, because the ground applied a force required to stop the person, and the kinetic energy "lost" by the person is equal to the work done by the ground to bring the body to a stop.

fgravity=10x10=100n case 1

fgravity=30x20=600n case 2

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