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

Energy use exerting gravity

  1. Aug 15, 2008 #1
    This probably is a really basic question but nobody has ever told me the answer. Does a mass emparting a gravitational force on another mass exert any energy doing this.

  2. jcsd
  3. Aug 15, 2008 #2


    User Avatar

    Staff: Mentor

    Energy is not something that is "exerted". Force is exerted. Energy is the result of force and movement. So the answer is, if there is no movement, there is no energy. Ie, a book sitting on a table is not undergoing any changes in energy. An object in a non-circular orbit, on the other hand, is constantly exchanging energy between potential and kinetic, though the total energy is constant. Gravitational energy is conserved.
  4. Aug 15, 2008 #3
    Thanks russ,

    I dont really understand how something can exerting force on something else without losing any energy in the process. What is it about a mass that pulls another towards it? I cant think of a better way to put this but is there any 'communication' between the two masses.
  5. Aug 15, 2008 #4


    Staff: Mentor

    That is simply the definition of energy. It is f.d and not just f. No movement means d=0 which implies f.0=0 regardless of the amount of force.

    For example, a large rock sitting on the ground is not using any energy, but it is exerting a large force on the ground (and viceversa).
  6. Aug 15, 2008 #5
    Thanks for you help just one more thing. I still really dont understand how gravity works is it a wave or particle that acts on from other masses? Does it tke a finite amount of time before the gravtitional force from one body acts on another? I only ask because things like light and electromagnetism have been explained to me so many times but nobody has ever told me the basics of gravity and why it happens.

  7. Aug 15, 2008 #6


    User Avatar
    Gold Member

    As far as I can recollect, gravity waves have been detected. Graviton particles haven't, but seem to still be a valid theory. Gravity itself propagates at the speed of light. I would guess, then, that gravity shares the same wave/particle duality as EM radiation. I don't know how spacetime curvature fits into that.
    Last edited: Aug 15, 2008
  8. Aug 16, 2008 #7
    Thanks everyone for your help.
  9. Aug 16, 2008 #8
    As far as I can tell, gravity waves haven't been detected--at least not as of Oct. 2007.
  10. Aug 16, 2008 #9


    User Avatar
    Gold Member

    That's quite possibly true, Phrak. My memory isn't too reliable. I was thinking that the experiment in (Australia?) with the massive cylinders had registered something, but I could very well be mistaken.
  11. Aug 17, 2008 #10
    I didn't know either. Gravity waves are spoken about with great certainly, being solutions to Einstein's field equations, so statements can be misleading. So I found this,


    and this

    [PLAIN]http://www.wired.com/science/space/news/2007/10/gravitational_waves" [Broken]
    Last edited by a moderator: May 3, 2017
  12. Aug 17, 2008 #11


    User Avatar

    I think the situation is that two (neutron?) stars in orbit around one another were observed to spiral towards each other in the exact manner predicted by their emission of gravitational waves, but nobody's actually detected them in a lab before.
  13. Aug 17, 2008 #12
    Okay, gravity is not a force.

    A force is something that is exerted upon an object, and this is the classical Newtonian answer to what gravity is.

    What Einstein came up with was something just a little bit different. Einstein states that the universe in which we exist, rests upon something known as spacetime. It is a 4 dimensional entity (or sometimes referred to as 3+1 dimensions, for three spatial dimensions and one temporal) which is not rigid and straight.

    Newton's model of the universe rested upon a straight, non-curved universe.

    Einstein's model states that the universe rests upon a curved, non-linear "spacetime".

    How gravity operates is that it curves the path of objects on spacetime. So, objects are moving on spacetime, and then gravity is the curvature that alters the path that that object will travel (called, formally, a "geodesic"). Now, locally, this will be felt as a force, called the "fictitious force". All accelerated reference frames feel this force.

    What causes gravity? Energy, mass, and electromagnetic fields. They are all, when boiled down, nothing more than energy. And in Einstein's world, mass is energy (hence, E = mc^2).

    Think of a ball sitting on a trampoline. If you're not around, that ball will roll in a straight direction. But if you jump on that trampoline, that ball will curve towards you.

    So, gravity is propagated, in Einstein's model, at the speed of light. Meaning that if there's a change in the position of the source of gravity, the rest of the universe won't feel that change until the moving curvature of spacetime catches up to the rest of the universe.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook