Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about energy, can't create/destroy etc

  1. Oct 30, 2004 #1

    I just had a thought today and i thought i'd come here and post it up, hopefully i got the right section. Keep in mind i haven't done physics for a year. :uhh:

    Anyways, i know you can't create, or destroy energy, so, i assume i'm correct in assuming there's a finite ammount of energy in the universe.

    Now, say you have a chunk of rock floating through space, it happens to fall under the gravitational pull of a planet, now this rock would gain kinetic energy wouldn't it as it falls towards the planet? when it hits, it disperses into heat etc etc, so that's okay, but the energy where did it come from? Was it transferred from the gravitational pull of the planet? If so, would that mean that gravity has infinite potential energy? Since a gravitational pull never weakens unless the mass of a planet is decreased (which isn't happening) so I doubt that, because that would mean that energy in the universe is not finite, and that energy can be created... i think.

    So then i thought, perhaps the chunk of rock has potential energy already floating in space, as it was predetermined to fall to that planet, but this would only apply to newtons model of the universe and that all particals theoretically have a path laid out for them, but with quantum physics that was disproved.

    So can anyone tell me the real deal or poke holes in my writing?
  2. jcsd
  3. Oct 30, 2004 #2
    I've heard that the universe has a finite amount of matter. However, perhaps it has an infinite amount if the universe does indeed go on forever.

    Reminds me of the Gabrilles horn paradox. Infinite but with finite surface area.
  4. Oct 30, 2004 #3
    I think i can answer what you said, as it doesn't technically go one forever, but it is expanding at such a rate, that it seems to go on forever, i think that's right.

    Anyways, anybody with my question?
  5. Oct 30, 2004 #4


    User Avatar

    Staff: Mentor

    If you want, you can right now calculate the gravitational potential energy between yourself and any other body in the universe. That energy has been there since the Big Bang.
  6. Oct 30, 2004 #5


    User Avatar
    Science Advisor
    Homework Helper

    You're right. The rock already had the potential energy, simply by being a distance away from the planet. (If you want to put the rock from nearby to a distance farther from the planet you have to fight against the gravitational pull, so it would gain potential energy again).

    When dealing with rocks, and macroscopic objects in general (a dust particle is already macroscopic), Newtons laws offer an accurate enough description. When I want to know where I can find a tennisbal at a given time after throwing it, I`m NOT going to use quantummechanics. The uncertainty in determining the position and velocity is neglegible even for a grain of sand, let alone an asteroid or meteorite.
  7. Oct 30, 2004 #6
    ahh, you're right, there should be a little hitting head emoticon, i knew i was wrong i just didn't know why.
  8. Oct 30, 2004 #7
    the energy created by tidal forces

    There are several ways we capture energy by tides and convert it to electrical energy. If energy can't be created nor destroyed, where does the energy come from that we are continually gaining from tidal forces. Does the gap between the earth and moon narrow in proportion to the energy we gain daily through tides?
  9. Oct 30, 2004 #8

    Actually, the gap between the Earth and the Moon is slowly increasing. A few hundred million years ago the Moon was alot closer to the Earth than it is now, and months were alot shorter.

    For the Earth/Moon system, the angular momentun of the Earth's rotation about its axis is transfered to the orbital angular momentun of the Moon about the Earth. The Moon gets an orbital energy boost out of this too. Upshot: The Earth spins slower and slower (days gets longer) while the Moon moves into a larger, slower, but higher energy orbit. Eventually the day on Earth will become synchronized with the lunar month, with huge temperature extremes between night and day.
  10. Oct 30, 2004 #9


    User Avatar
    Science Advisor
    Homework Helper

    The earth is not a closed system, so energy conservation doesn't apply perfectly.
  11. Oct 31, 2004 #10


    User Avatar
    Science Advisor
    Homework Helper

    When my teacher first told me this, I was shocked. :eek:
    Life on earths surface would be impossible if there were only one revolution per day. I didn't calculate the time it will take for this to happen though.
  12. Oct 31, 2004 #11
    Why do you think life wouldn't be able to adapt to those circumstances, especially as the change will occur only very gradually?

    I can imagine the creatures in the early days of the Earth's history talking about how impossible life would be if their puddles dried up....
  13. Nov 1, 2004 #12


    User Avatar
    Science Advisor
    Homework Helper

    Life needs regularity in the environment. I remember something from a docu, that if there were no tidal forces the climate would become utter chaos and life would be unable to adapt.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook