Question about conservation of energy: How is energy conserved if a rock is dropped from the top of a building
kmm4864990 asked:
That is, initially the rock has a high potential energy. Then, its energy is converted into kinetic energy as it falls. So i understand energy is merely changing form.
That is, initially the rock has a high potential energy. Then, its energy is converted into kinetic energy as it falls. So i understand energy is merely changing form.
But then it hits the ground. The rock ceases moving at a relative height of zero. Into what form was the enrgy converted? Is it converted into kinetic enrgy that travels through the particles of the ground? Thanks a lot!
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The energy is passed to the ground in the form of heat. Try this yourself - get two big rocks and keep hitting them together. You will be able to feel a change in the temperature of the areas that are striking each other. Flint is a rock that gives off so much heat when struck that the spark is visible and can be used to start a fire.
Comment by physandchemteach — December 11, 2008 @ 11:10 pm
the engery is transferred into heat and sound
Comment by phdpsychman — December 15, 2008 @ 4:53 am
You are correct in the change from potential energy to kinetic. Once the rock hits the ground, that kinetic energy is also changed into another form. Heat and sound are both correct, but they are relatively weak forms of energy and do not account for all of the energy difference. Much of the energy is indeed transferred through particles as turbulence.
Consider a similar scenario. A rock is dropped from a really REALLY high building. Its potential energy gradually converts to kinetic energy. But what happens when it hits terminal velocity? If its velocity no longer increases and as its mass is constant, kinetic energy no longer increases (recall KE = 0.5mv²). Therefore, in this scenario, it is more obvious that the lost potential energy is being transferred into the particles of the air (which are in fact causing the air resistance).
Comment by cluekoo — December 15, 2008 @ 7:24 am