I was prompted, by this thread to recall an old question I’ve wondered about for ages, but never answered.
Is the energy in gravity infinite?
Gravity is a function of mass, so for it to decrease, there would have to be a coresponding decrease in mass.
But a object in synchronous orbit is using energy from gravity to keep from flying off into space. So all those satellites are making terra firma shrink.
Right?
Peace,
mangeorge
Wrong. There is no energy in gravity. The gravity is not using energy to keep things in orbit. The gravity is warping space-time and the bodies are following geodesic paths–those that require no force. The gravitional energy is what bodies acquire by falling. But that cannot be recovered except by expending energy. That is why a gravity shield is inconsistent with conservation of mass-energy.
No. There is no “energy” in gravity to begin with, it’s just a force which eists between any two pieces of matter. Objects in orbit aren’t using up any energy to stay there, they’re freely falling in a path which happens to take them in a stable elliptical path around the earth.
Okay. If you drop a bowling ball on your toe, what squishes your toe?
And spacecraft always use the gravity of bodies they pass to accelerate (slingshot) them on their way.
Force isn’t energy? I can use gravity to generate electricity. As in Hoover dam. If gravity has no energy we’re getting “free” power, which ain’t possible.
Gravity tries to make everything “one”, and infinitely dense. How can that be without energy.
I’m not challenging you guys, I’m just curious.
Hydroelectric power isn’t gravity energy, it’s solar energy. The Sun evaporates water that eventually condenses as rain which falls in the lake that empties in the river that falls over the cataract, releasing its energy in the form of electricity.
The energy that was imparted to the bowling by whatever forced lifted it.
You’re indirectly using the ebergy imparted to the water by the sun. The sun causes the water to evaporate. The vapor rises into the atmosphere. It eventually condenses and falls, turning the turbines in the dam on its way back to where it originally was.
Force is not energy. Force X distance = energy.
Or in laymans terms, you can use a field(which can exhibit a force) to store energy, as in your bowling ball example, but the force it’s self needs something to act on to say the system has energy.
In your bowling ball example, when you picked up the ball, you gave the ball:gravity system energy. Take the ball away and no energy, take the gravity away and no energy.
To your orginal post, gravity is not just a function of mass, but also of distance. The things orbiting the earth are not using energy. They are using the force(f=m X a), which acts toward the earth, but because they do not get any closer, they don’t actually use energy(E=F x d). In this case d = 0.
This is just something to consider about gravity; I don’t know that it will help but it may give you a clearer picture about what gravity is.
Suppose a beam of light passes close to a massive object, massive enough so that it causes the beam of light to curve in response to the gravity field. The beam of light did not reflect off of anything, in fact it came into contact with nothing. So why did it change direction? The answer is that the beam of light did not change direction at all, it kept going in a straight line. It was space that curved.
it occurs to me that gravity is the perfectly ironic companion to all of our lives… it’s our most physically limiting phenomenon… yet, it really shouldn’t exist… much like ourselves. but we just ignore that and wonder why our lawns are so imperfect.
I may be way off on this, but I believe that the ‘slingshot effect’ is actually making use of a planet’s motion around the sun, not it’s gravity. The spacecraft gains some of the planet’s kinetic energy, causing the craft to speed up and the planet to (imperceptibly) slow down. This can also be done in reverse, slowing down the craft and speeding up the planet.
You have the right idea, it’s just not worded correctly. The slingshot effect does make use of gravity, in that gravity is the force involved. The energy that the craft gains or loses comes from or contributes to the planet’s orbital kinetic energy.
Now I get it. Thanks all. 
I’m glad that you are satisfied with that explanation; however I think it is only half the story;
if you want to know where that kinetic energy comes from,
the energy that you feel with your foot when you drop a bowling ball on it- it comes from the distance that the ball was dropped.
Any two separated massive objects exert gravity on each other, and this force of gravity that brings them together produces energy.
So where does the distance that separates the massive objects of the universe come from?
It comes from the expansion of the universe.
It is the expansion of the universe that stops everything being in the same place, and from having the same potential energy.
That is where the kinetic energy comes from-
and unless the whole universe collapses in upon itself at a future date,
an eventuality which seems unlikely at the moment,
there will always be masses separated by distance which have the potential to come together under their mutual gravity and release energy.
Kinda like temporarily attaching yourself to a passing truck using bungee cords (don’t try this).
Wait a minit.
If I set a BB on my toe very gently it hurts anyway, even after it’s compressed my toe all it’s going to. No movement, no “d”.
.
.
I know, it’s moving very slowly.
From m-w;
